Bode plot ( 1Hz to 10 MHz) - Page 1
Forum section: electronics / projects

author: Sariel
author_rank: Regular Contributor
timestamp: March 20, 2025, 01:37:48 pm
content: Hello,I've built a preamplifier (10Hz - 1MHz) designed for low-noise, low-frequency measurements, serving as a pre-gain stage before connecting to an oscilloscope or signal analyzer.Now, I want to evaluate its performance and obtain a gain vs. frequency plot.However, many oscilloscopes with Bode plot functionality are limited to a minimum frequency of 10Hz, and VNAs don't operate at such low frequencies.What would be the best approach to measure it?[ Specified attachment is not available ]

author: AnalogTodd
author_rank: Regular Contributor
timestamp: March 20, 2025, 02:29:24 pm
content: I used an older HP4195A to do these measurements (I know what circuit you built based on seeing it on LinkedIn, hello from the writer of that App Note!). While that piece of equipment says it only does 10Hz, it can actually go down to 3Hz with reasonable fidelity.If you're wanting to just get stuff in the 1Hz to 10Hz range, you can also just take a few discrete points and add them to your data, I would not expect anything to be nonlinear down there.

author: mawyatt
author_rank: Super Contributor
timestamp: March 20, 2025, 02:50:36 pm
content: Believe the Siglent DSOs (SDS800XHD and 2000X+ do) Bode Function goes down to 10Hz and up to 120MHz (with capable external AWG). As Todd said you can extrapolate below 10Hz if needed.Best

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 20, 2025, 03:21:43 pm
content: There are two approaches I would recommend. The faster and easiest is to collect data points. Use a signal generator that covers the frequency range. At the very least you can use Audacity to generate the tones and play them with whatever. Note that to do the high end the Audacity approach is going to be pushed at or beyond its limits, but you can use your scope internal capability to do that end of the job. Measure the output of your generator with a dvm or your scope and use the same instrument to measure your devices output. You only need two or three points per decade if your preamp is reasonably well behaved. You can add samples if you find evidence of resonances or other peculiarities. Plot using your preferred plotting software (Excel, Libre Office Calc, Octave or others).The second method is a bit harder. Get a white noise source. Audacity is an option. Use your scope to collect a sample long enough to get your low frequencies (a second is plenty) at a sampling rate high enough to overlap your scopes internal Bode capability. Export this data and do a Fourier transform on it to verify whiteness. Feed that to your preamp and take a similar sample of the output. If your input was white the magnitude of the Fourier transform is the Bode plot. While there are ways to do Fourier transforms in Excel and Calc, it will be easier to use Octave or similar.

author: Sariel
author_rank: Regular Contributor
timestamp: March 20, 2025, 03:42:26 pm
content: Quote from: AnalogTodd on March 20, 2025, 02:29:24 pmThank you, Todd! Nice to meet you here as wellI have access to SDS2000X plus, SDG6022 to perform the Bode plot.I don't yet have access to the SDS2000X plus internal AWG ( need to open this option).I must say that the manual for these devices lacks enough depth on how to connect the devices and perform well the sweep.I also have access to Magnova scope. But its AWG/Bode plotter will be available in July.I do want to try the white noise option. It looks very fun. The SDG6022 has Gaussian noise output which can be frequency-limited.

author: mawyatt
author_rank: Super Contributor
timestamp: March 20, 2025, 03:55:16 pm
content: The SDG6022X works well with the SDS2000X+, we know as we have both and have used them together for Bode Plotting on a number of occasions.If the DSO and AWG are networked together you just need to set the AWG address in the Bode Config menu. If they are not networked, then you can connect the AWG to the DSO with USB and setup in the Bode config menu.The DSO Bode function is quite good and has many useful features, we've used it to plot Open Loop/Close Loop system responses, Non-Linear Oscillator Injection Locking Characteristics, various Filter responses, mixer frequency translation Bode plots, and so on.Here's a few links that folks might find interesting/useful.https://www.eevblog.com/forum/testgear/injection-transformers-bode-plots-application/https://www.eevblog.com/forum/testgear/diy-transformer-for-use-with-bode-plots/https://www.eevblog.com/forum/projects/injection-locked-peltz-oscillator-with-bode-analysis/msg4424434/#msg4424434https://www.eevblog.com/forum/projects/injection-locked-frequency-divider/msg5555107/#msg5555107https://www.eevblog.com/forum/projects/when-a-precision-pot-is-not!!/https://www.eevblog.com/forum/testgear/capacitive-impedance-plots-with-sds2104x-plus-bode-function/msg4335745/#msg4335745https://www.eevblog.com/forum/testgear/admittance-measurements-with-dso-awg-with-bode-function/msg4491952/#msg4491952https://www.eevblog.com/forum/projects/ceramic-capacitor-behavior/msg5535855/#msg5535855https://www.eevblog.com/forum/testgear/bode-plot-torture-test/msg4509301/#msg4509301Best

author: Sariel
author_rank: Regular Contributor
timestamp: March 20, 2025, 04:09:48 pm
content: Quote from: mawyatt on March 20, 2025, 03:55:16 pmHow should I connect them using a USB cable?From the DSO device output (on the back) to the DSG front USB connector? I get an error when I connect as this. Testing config tell me there is no connection.

author: mawyatt
author_rank: Super Contributor
timestamp: March 20, 2025, 04:21:57 pm
content: Quote from: Sariel on March 20, 2025, 04:09:48 pmConnect USB cable to AWG back and to DSO USB front. Set DSO Bode Config Menu Interface to USB and press Test. Just did so and our DSO/AWG (SDS2000X+/SDG6000X) connected.BTW one of the nice benefits of having Siglent gear is the depth of on-line real time help you get here from folks like tautech, 2N3055, Performa01, rf-loop and a few others. These folks have extensive knowledge of various test equipment and general electronics, and provide help for all sorts of issues, even non-Siglent equipment on many occasions.We know, they've helped us a few timesBest

author: 2N3055
author_rank: Super Contributor
timestamp: March 20, 2025, 04:55:45 pm
content: Quote from: Sariel on March 20, 2025, 03:42:26 pmYou can perform Bode plot with internal AWG even without AWG license.But if you have SDG6000X that use it, it is better.Mike already said everything about Bode plot. Nothing to add.As for frequencies lower than 10 Hz....You keep wiring as for Bode plot.In measurements, you have both amplitude related measurements and phase too.You set measurements to measure P-P and Stdev on input(CH1) and output (CH2) and Phase between CH1 and CH2. Enable statistics. Even histicons. Mini histograms will be useful to judge quality of data.Decide how many points per decade you need between 0.1Hz and 10Hz.Set SDG6000X to 0.1Hz, set timebase to 100ms/div and let it run so it gathers measurements and stats.Stop scope. Write down data. Or do screen shot.Set next frequency on AWG. Adjust timebase on scope. Run scope again.Wait.. Rinse and repeat.Enter data in spreadsheet, calculate amplitude ratios and plot.For calculations, use Mean value of the measurement from stats, that is averaged value.It is half manual method but all results are in same place so easy to collect.It is pretty quick as it is, for one time measurement.If you need to repeat measurement often then some SCPI/Python scripting might save time long term.With both scope and AWB on Ethernet, it can be scripted to be fully automatic.Normally, expect this process to be slow. Like minutes per point at lowest end. Capturing only 12 periods at 0.1 Hz is already two minutes. You want to capture at least 10-20 for averaging to do its thing.

author: 2N3055
author_rank: Super Contributor
timestamp: March 20, 2025, 04:57:36 pm
content: Quote from: mawyatt on March 20, 2025, 04:21:57 pmThank you Mike for your kind words.Means a world coming from you!All the bestSiniša

author: mawyatt
author_rank: Super Contributor
timestamp: March 20, 2025, 05:16:19 pm
content: Quote from: 2N3055 on March 20, 2025, 04:57:36 pmWell you deserve this, as you provide considerable help while a few "others" continue to create chaos with no value added!!BTW the new released firmware for the SDG6000X has a Frequency Sweep Mode which is Stepped rather than Linear or Log, which might prove useful in creating the lower under 10Hz results. We haven't used this Mode just yet so can't comment.Best

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: March 20, 2025, 06:45:46 pm
content: Hi,I assume that LNA is similar to the one described in Linear Technology AN-159.I have built a behavioral model based on the schematic and block diagram.The simulation can be used to check that the intermediate stages are not being overloaded.The gain of the circuit is 80dB in the passband. At 1Hz the expected gain is 12dB.I have attached the model.Regards,Jay_Diddy_Ban159 behavioral.asc(2.85 kB - downloaded 26 times.)

author: mawyatt
author_rank: Super Contributor
timestamp: March 20, 2025, 07:00:10 pm
content: That's a considerable range of 160dB between 0.1Hz and ~10Hz, which might prove difficult to measure. Likely requiring an attenuator (or variable attenuator) on the input, as the AWG might not be able to supply a low enough input at 10Hz and above.Best

author: Sariel
author_rank: Regular Contributor
timestamp: March 20, 2025, 07:36:48 pm
content: Attached are my full schematic and the simulated frequency response of the preamplifier.What input levels would you recommend?Looking again at the frequency response, I think that measuring down to 1Hz will be enough. And perhaps up to 5MHz (?)Low_frequency_LNA.pdf(521.95 kB - downloaded 77 times.)

author: mawyatt
author_rank: Super Contributor
timestamp: March 20, 2025, 08:11:19 pm
content: Looks like you have +-5V supplies, maybe stay under 5V with signal peak by 1V on each end, so limit output of each stage to 8VPP (+-4Vp) or less. Then work your way from the output to the input with that limit in mind.You will likely need to increase the input signal amplitude as you move down below 10Hz as the overall gain drops. You will need sufficient output amplitude to get a good SNR at the output at all frequencies, and this will drop considerably below 10Hz as the overall gain decreases.So adapt your input signal level below 10Hz to keep a good level signal at the output.Did you get the DSO talking to the AWG for the Bode Function?BTW make sure your DSO inputs are DC coupled.They are fixed as AC Coupled.Best

author: AnalogTodd
author_rank: Regular Contributor
timestamp: March 20, 2025, 08:37:18 pm
content: When I calibrated the original circuit, I used a 60dB attenuator for the input signal. That was actually created using a 1kOhm:1Ohm resistor divider. With 1% resistors, the worst case you can get for variation in the 60dB attenuation is +/-0.2dB. It's not a great idea for doing something that would be RF but works fine for frequencies up to several MHz.Jay_Diddy_B knows about that LNA as well, he would come to the factory a couple times a year and we would showcase the latest and greatest low noise parts using it. We would take and switch between an older middle ground LDO that ran about 20uV of noise and our new stuff. Ran the output of the LNA to both an oscilloscope and into a guitar amp at the same time. Big difference when you could not only see the change in noise on the scope but hear the difference as well!

author: Sariel
author_rank: Regular Contributor
timestamp: March 20, 2025, 11:49:13 pm
content: Quote from: mawyatt on March 20, 2025, 08:11:19 pmDo I need to configure the DSO inputs (impedance, BW, coupling, voltage level), before initiating the Bode analysis feature of the DSO? Or does the Bode analysis overwrite this configuration when initiated?

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 20, 2025, 11:51:33 pm
content: If your Bode matches the simulation down to 1 Hz, particularly the phase response, it is pretty safe to bet that the next decade down will be fine. No need to try to achieve 120dB dynamic range in your measurements.

author: mawyatt
author_rank: Super Contributor
timestamp: March 20, 2025, 11:54:26 pm
content: Quote from: AnalogTodd on March 20, 2025, 08:37:18 pm60dB attenuation seems about right for above 10Hz since the OP schematic shows mid-band gain of 60dB. So 1VPP AWG input signal level should keep all the stages well below clipping and provide an acceptable signal level at the output with good SNR for the Bode Plotting.Recall standard 1% 1/8 watt leaded resistors have ~ 0.4pF shunt capacitance so should be good well above 10MHz for use with the attenuator. If the OP wants better precision then 0.1% resistors could be used, but probably overkill IMO.At 1Hz the AWG probably could be increased to 10VPP with 60dB attenuator without any stage clipping and the max AWG level (20VPP) at 0.1Hz.Overall would expect a nice Bode Plot throughout the range from 0.1Hz to 10MHz.Best

author: Sariel
author_rank: Regular Contributor
timestamp: March 20, 2025, 11:54:56 pm
content: Quote from: AnalogTodd on March 20, 2025, 08:37:18 pmI see. that the lowest signal the DSO can source in Bode mode is 5mVpp. Probably good enough for testing the filter behavior in the range of 0dB to 60 dB.The input impedance of the preamplifier is 250 ohm. So there is already some attenuation with the 50ohm output of the internal AWG of the DSO.There are several low frequencies HPF stages inside the preamplifier, so a long stabilizing time has to wait. I hope the bode analysis awaits enough time from point to point in its sweep.Otherwise, I will do it manually. I just need to verify it once.

author: mawyatt
author_rank: Super Contributor
timestamp: March 21, 2025, 12:06:44 am
content: Quote from: Sariel on March 20, 2025, 11:49:13 pmYou set things up in the DSO Bode Config Menu. Things like Impedance, Voltage Level, Start/Stop Freq, Sweep Type Lin/Log, Number of Points, Sweep Keyed Variable Amplitude, Display/Plot Parameters and so on. Can even use all 4 DSO channels simultaneously for 3 independent plots, handy for plotting the gain/phase of a cascaded system each stage like your amplifierPlan on spending some time with the Bode Function, it's quite useful/powerfull in capable hands and you will be rewarded for your effortsBest

author: mawyatt
author_rank: Super Contributor
timestamp: March 21, 2025, 12:15:49 am
content: Quote from: Sariel on March 20, 2025, 11:54:56 pmThe Siglent Bode Function utilizes a Frequency Selective measurement capability which is quite good at rejecting unwanted and out of band signals. The function stops at 10Hz and has sufficient time to allow accurate measurements at that frequency with the Frequency Selective feature, you'll have to manually access the lower frequencies and allocate enough settling time for acceptable measurements below 10Hz.Best

author: mawyatt
author_rank: Super Contributor
timestamp: March 21, 2025, 12:30:52 am
content: Quote from: CatalinaWOW on March 20, 2025, 11:51:33 pmAltho the design looks like it has acouplethree 1st Order High Passes and a cascaded Sallen-Key 2nd Order High Pass, the simulation does look like it's following ~80dB/Dec High Pass from 0.1Hz and seems "straight line" amplitude to above 1Hz but below 10Hz, so one could measure a couple points below 10Hz and 1Hz and likely be "good enough". If the OP utilizes the Siglent Bode function for 10Hz to 10MHz then that range is covered and no additional effort/time required.Best

author: AnalogTodd
author_rank: Regular Contributor
timestamp: March 21, 2025, 01:30:41 am
content: Quote from: mawyatt on March 21, 2025, 12:30:52 amYeah, the design was originally done with one very low frequency (0.5Hz) high pass to do DC blocking from what was being measured (ultra-low noise LDOs) and then one internal high pass (~10Hz) to block DC before a gain stage, the internal 2nd order Sallen-Key High Pass and a final high pass (~10Hz again) to block DC to center the output signal around zero. The internal Sallen-Key tweaked the Q a bit in order to try and make the two 10Hz filters and the Sallen-Key all together be as equivalent as possible to a 4th order Butterworth response.Nice thing about creating that cheap and cheesy 60dB attenautor was that I could drop a measurable signal on it and put the attenuated version into the LNA and know that I was within the operating range of the LNA and have measurable signal levels at input and output. You can even calibrate the attenuator by putting a signal larger than the LNA might be able to handle but is within the capabilities of your Bode analyzer.If you want, I can point you to the Application Note I wrote for all of it.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: March 21, 2025, 01:40:59 am
content: Hi,I would set the signal amplitude at Vin to be 0.1mV at 10Hz. You can increase the signal amplitude at a rate of 40dB per decade below 10Hz. Most network analyzer allow for the amplitude to be adjusted during the sweep.I have implement that here with a lop sweep generator and a low pass filter.The signal amplitudes at different stages in the signal chain can be observed.I have attached the model.Regards,Jay_Diddy_Ban159 behavioral.asc(3.61 kB - downloaded 16 times.)

author: Sariel
author_rank: Regular Contributor
timestamp: March 21, 2025, 10:29:07 am
content: Quote from: AnalogTodd on March 21, 2025, 01:30:41 amYes. Please point me to this application note. I know you wrote several

author: AnalogTodd
author_rank: Regular Contributor
timestamp: March 21, 2025, 12:10:12 pm
content: Just two that have my name on them.LT's AN-83:https://www.analog.com/media/en/technical-documentation/app-notes/an-83.pdfLT's AN-159:https://www.analog.com/media/en/technical-documentation/app-notes/an-159.pdfI'm glad I could help you out with this project, it's fun seeing it come to fruition!

author: Sariel
author_rank: Regular Contributor
timestamp: March 21, 2025, 01:27:47 pm
content: Quote from: AnalogTodd on March 21, 2025, 12:10:12 pmThank you!Another issue I still need to tackle is shielding the entire measurement setup. I’m not sure yet how noisy the environment will be. At work, it's terrible—an industrial setting close to a military base. But in my home lab, out in the suburbs, it should be much quieter.I’ll probably need to get one of those Danish cookie tin cans for this experiment. In all app notes, it is always a Danish cookie can.Regarding the measurement equipment: I’ll start by measuring the noise with an oscilloscope (using my Magnova).You mentioned that it can also be done with an RMS multimeter—do you have any recommendations? Also, how can I limit the bandwidth of the RMS measurement to reduce the multimeter’s internal noise? Are there any RMS meters that actually measure up to 100 kHz or even 1 MHz?

author: mawyatt
author_rank: Super Contributor
timestamp: March 21, 2025, 04:35:46 pm
content: The mentioned DSOs support an RMS (SD in measurements menu) reading. This is quite good over a large frequency range (we know we've tested such) and covers a large amplitude range of mv to 10s of volts (DSO input scale factor).Just for fun we looked at our SDS814X HD with and AWG signal source (SDG6022X) set to 0.1vrms. Set CH1 to DC coupled at 100mv/div. Used the Math Function F4 (2nd Display Position) to activate the FIR Filter Function (Low Pass) set to 10KHz, Adaptive Sample Rate, and adjusted DSO Sweep for acceptable number of sample points vs input AWG frequency.Then we employed the Measurements Menu and using the Advanced Menu within creating a list with SD(F4) RMS(F4) Mean(F4) Pk-Pk(F4) Cycle SD(F4) and also engage the Histograms for each.So this DSO is producing the RMS (SD) Measurement of a Low Pass Filtered Signal where the Low Pass FIR Filter Corner can be set and the Number of Data Points and Sample Rate for the Measurements can be set. Not bad for a ~$500 "Entry Level" DSOHere you can see the results, AWG Input 100mvrms @ Frequency Indicated, F4 LPF set to 10KHz, judge for yourselfEdit: Having trouble as usual with getting these images in order, anyway 131.png at bottom is with AWG 100mvrms @ 1KHz, note the Stdve(F4) Measurement of 99.143mv and RMS(F4) of 99.149mv. BTW the results are equally as good for a HPF as we were just playing around with a 10Hz HPF with a 10, 5, and 1Hz AWG Frequency, takes awhile as necessary at this low a frequency.Best

author: AnalogTodd
author_rank: Regular Contributor
timestamp: March 21, 2025, 05:32:49 pm
content: Quote from: Sariel on March 21, 2025, 01:27:47 pmYeah, just need to make sure the Danish cookie tin isn't being used for sewing supplies instead, that's what always seems to happen with them...Shielding and dealing with fields nearby will always be an issue with a circuit like this. Aluminum and copper work great for higher frequencies, but at low frequency the wavelengths are too long and the best way to handle it is with shielding that has high magnetic permeability (steel is decent, MuMetal is a lot better, this is where the book from Henry Ott is useful).For an RMS voltmeter, look at Appendix C in AN-83. Jim Williams took a lot of time to review RMS voltmeters to see which ones gave good results. Personally, I have an old HP3400A that I got (either from a flea market or eBay) that is thermally responding to be most accurate. Most rectify and average RMS meters can give significant errors, logarithmic ones tend to be better with some able to give results that are dead on for 100kHz bandwidth.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 21, 2025, 05:39:46 pm
content: Many of the comments here reflect concerns about accuracy of results. How much accuracy is required is defined by how you intend to use your pre-amplifier? How does that accuracy requirement change over frequency and amplitude? If this preamp is to be used to calibrate other instruments trying to cobble together a measurement with less than optimum instruments may end up in misleading results. If you are trying to determine the accuracy of your simulation you also need to plan a strategy to determine the source of any discrepancies between model and measurement. The errors could result from either or both.An answer of "the best I can do" is a first step down the path followed by Volt Nuts and Time Nuts.

author: mawyatt
author_rank: Super Contributor
timestamp: March 21, 2025, 06:43:51 pm
content: Quote from: AnalogTodd on March 21, 2025, 05:32:49 pmThe DSO we mentioned (SDS800X HD) is reasonably good wrt to rms measurements, obviously achieved with a computational method. Many of the higher end modern DMMs like the KS34465A and DMM6500 also employ computational methods rather than old school analog methods (AD636 for example).We've verified the DSO RMS measurement performance before, but decided to do a quick evaluation before taking care of "other to do" things. Set AWG to 10KHz at 10mvrms and did an FFT to show the DSO noise floor as well as determine the signal level performance. Then let our KS34465A and our new Keithley DAQ6510 take a AC RMS reading just for comparisons.The DSO FFT is shown below with a marker at 10KHz and -39.998dBv (10.0023mv), the KS is reading 10.0167mv and the DAQ is 10.0142mv.This is usually good enough for our needs, as we often check things wrt to RMS readings using our custom Precision RMS Squarewave Generator.Best

author: AnalogTodd
author_rank: Regular Contributor
timestamp: March 21, 2025, 07:55:21 pm
content: Quote from: CatalinaWOW on March 21, 2025, 05:39:46 pmAlways a good question!The circuit that @Sariel based this off of was targeted to measure the voltage noise as close to +/-1% at input noise of 2nV/rt. Hz wideband and to keep the 1/f component as low as possible. You can get nice calibrated RMS voltmeters so that you know those measurements are good, but verification of the LNA has to be done to ensure its gain is right as well as verify the input referred noise of the LNA to ensure it is low enough to not add in and swamp the measured results. The original circuit actually got just above 400pV/rt. Hz input referred noise in the wideband range with a 1/f corner at 100Hz.Quote from: mawyatt on March 21, 2025, 06:43:51 pmThose are some pretty tight results. Given the random nature of noise and that it will fluctuate (after all, it's random!) results that are within 1% would meet the requirements we set up for the original design.

author: Sariel
author_rank: Regular Contributor
timestamp: March 21, 2025, 08:36:48 pm
content: I can also measure the noise (RMS) with DMM6500 and compare to the scope measurements.As Todd said correctly, I want to check that the gain 60dB across the frequency band and find the -3dB points. Also to check if the input refered noise is ~1uVrms integrated in the frequency band.Another possibility is to build my own RMS to DC device based on LTC1968.

author: Someone
author_rank: Super Contributor
timestamp: March 21, 2025, 11:20:05 pm
content: Quote from: Sariel on March 21, 2025, 08:36:48 pmSounds wildly complicated given your stated goal:Quote from: Sariel on March 21, 2025, 08:36:48 pmAs mentioned above by 2N3055 an oscilloscopes measurements will already get the information for you. Where is your problem?The built in "bode" measurement applications on scopes are rudimentary and have limited options so you're almost always better off collecting the points yourself (likely automated), using your knowledge of the signals and noise sources to pick the ranges and acquisition methods to maximise what parameters are important to you. If they added a 0.1Hz measurement range people would complain it takes too long!

author: Sariel
author_rank: Regular Contributor
timestamp: March 22, 2025, 02:28:41 pm
content: An update:I used your guidance for the Bode analysis. Apparently, I have limited license time with the internal AWG of the SDS2000x PLUS, So I am taking usage of this.I measure 50dB of gain (instead of 60dB).Where are the missing dBs?Before I debug every point in my circuit:- Is it due to load mismatch? Assuming the internal AWG output is 50 ohm and my input impedance is 250 ohm (-1.5 dB).- BNC splitter at the AWG output. How many dBs can I lose with it?I am attaching some pictures of my setup and SDS bode configuration.Also, I am attaching again my circuit schematic.

author: Sariel
author_rank: Regular Contributor
timestamp: March 22, 2025, 02:30:06 pm
content: One more picture and the PDF of the schematic.

author: mawyatt
author_rank: Super Contributor
timestamp: March 22, 2025, 02:44:11 pm
content: Quote from: Someone on March 21, 2025, 11:20:05 pmHave you used the Siglent version? We've been using Bode Plots over 60 years starting as a kid designing/repairing/modding guitar/audio amps, have used just about every HP and other instrument available over our career, and for an instrument at this price point it's quite good IMO. It sure beats doing it by hand (which we've done before) or kludging up some spaghetti code (done that too)!!The Siglent implementation has a number of useful features, and our only real complaint is it doesn't have a "Calibration" capability (hint Siglent if your are listening). The Frequency Selective measurement capability is good at rejecting unwanted out-of-band signals which can corrupt a plot.It's on the slow side but somewhat acceptable (suspect the Frequency Selectivity Filtering takes some DSP power and requires a number of "samples"). Here Siglent might consider adding user ability to configure the "Frequency Selective Filtering Function", where the user can trade Selectivity vs Speed (another Siglent hint).Agree at 0.1Hz the measurement would take some serious time, and many would complain (not understanding what's required for such), however Siglent could allow below 10Hz use with a Warning for the required measurement time, maybe allowing user "Selectivity vs Speed" mentioned above.Anyway, we're happy with this Bode feature on this inexpensive DSO, same with FFT, and for us this covers our needs for lower frequency measurements at an attractive price, as always YMMYBest

author: mawyatt
author_rank: Super Contributor
timestamp: March 22, 2025, 02:55:47 pm
content: Quote from: Sariel on March 22, 2025, 02:28:41 pmHere are a few suggestions to consider!First off, did your let the DSO warm up and run Auto Cal?After that, connect DSO CH1 to CH2 and run the Bode Function. You should see ~ 0dB across the plot.Check the amplitude of the DSO Internal AWG output with your DMM6500.Make sure your input signal to the LNA is low enough to not cause any saturation within the LNA stages, are you using an attenuator as suggested?Check the attenuator accuracy (use your DMM6500). As Todd suggested a 1KΩ and 1Ω 1% (or 0.1%) make a good 60dB attenuator which you can rely on in these frequency ranges if the resistors are accurate (check with your DMM6500).Best

author: Sariel
author_rank: Regular Contributor
timestamp: March 22, 2025, 02:59:35 pm
content: I figured that by increasing the voltage level, I got less and less measured gain. So some stage is saturated.I lowered the amplitude to the minimum SDS could (2mV peak) and managed to measure only 56dB of gain.Now in order to reduce even more the input voltage into the amplifier, I have added externally a 6dB SMA attenuator.I measure now 57dB of gain in the mid-band.This 6 dB attenuator, together with the 50ohm output of the AWG, and the circuit input of 250ohm, should have reduced the measured gain by 8.5 dB.So what is going on?I will try now to measure it point by point using a separate AWG and the Magnova scope (which has 12-bit resolution and quite low noise).

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: March 22, 2025, 03:00:13 pm
content: Sariel,The 60dB divider proposed by AnalogTodd consists of 1kresistor and 1. This lowers the source impedance to 1in parallel with 1k, effective 1.Looking at your schematic I make a couple of observations:1) The low frequency characteristic are dependent on the values of large value capacitors, some are electrolytic capacitors, these will have a temperature coefficient so the low frequency transmission will be influenced by temperature.2) The 1MHz filter has some very low values of capacitors. For example C34, only 4.7pF. This is in parallel with the input capacitance of the U9B. This is similar to the input capacitance of the op-amp. You may more predictable behavior if you use large values of capacitance.I think that the gain is well defined in the pass-band, and no so well defined outside of the pass-band.Accurate characterization of the LNA outside of the pass-band may be of limited used.Just my observations.I based my earlier analysis on AN-159. You may have to adjust my analysis.Jay_Diddy_B

author: Sariel
author_rank: Regular Contributor
timestamp: March 22, 2025, 03:08:54 pm
content: Quote from: Jay_Diddy_B on March 22, 2025, 03:00:13 pmThank you!It is a good observation regarding the input capacitors.But it should be mostly affecting the last stage of the 1MHz LPF Sallen key.At the moment I have put the jumper to measure the 100KHz branch.I have also simulated the entire circuit using the OPA221 spice model, which hopefully has the input capacitance inside it.I am attaching my simulation schematic and results (together with the 6dB attenuator pad).

author: mawyatt
author_rank: Super Contributor
timestamp: March 22, 2025, 03:29:34 pm
content: Quote from: Sariel on March 22, 2025, 02:59:35 pmAre you sure your 6dB attenuators are producing what you expect wrt the LNA Input. Remember it's AC coupled and input Z rises at lower frequencies and you are not working in a 50Ω environment!!As suggested by Todd, myself and now Jay. Use the 60dB attenuator based upon 1KΩ and 1Ω, then you don't need to worry about impedance mismatches and whatnot. You can now reliably get a 1mvrms signal at the input using the AWG level of 1vrms. Of course your Bode Plot will be scaled by 60dB, so the midband output will be ~1vrms and the Bode Plot will show the "Variation from 60dB Gain"!!Best

author: mawyatt
author_rank: Super Contributor
timestamp: March 22, 2025, 03:53:09 pm
content: Quote from: Sariel on March 22, 2025, 03:08:54 pmIn your simulations are you assuming ideal components? Did you measure the components utilized in the LNA assembly, and consider tolerances?Also agree with Jay, 4.7pF in a Sallen-Key Low Pass is too low and with PCB and component parasitics likely to produce something far from the intended response. Consider scaling C9-12 up by 10 and R27-30 down to 1K.Best

author: Sariel
author_rank: Regular Contributor
timestamp: March 22, 2025, 04:48:42 pm
content: Quote from: mawyatt on March 22, 2025, 03:53:09 pmThank you.I can definitely scale the 4.7 pF capacitor.As for the rest of my components:Resistor tolerance: 1% or 0.1%Capacitor tolerance: 0.1 pF for low-value capacitors, and 1% for higher-value onesElectrolytic capacitor tolerance (used in the HPF): 20%Right now, my main focus is achieving 60 dB in-band gain with ±0.5 dB flatness/error. This will confirm both the design's performance and the accuracy of my measurement setup.I'll add the 60 dB attenuator as suggested and also perform independent measurements using an AWG and the Magnova scope with sine waves of 1 kHz, 10 kHz, and 50 kHz.After that, I’ll verify the -3 dB points, followed by measuring the integrated noise output of the LNA.

author: tszaboo
author_rank: Super Contributor
timestamp: March 22, 2025, 05:20:00 pm
content: Quote from: Sariel on March 22, 2025, 02:59:35 pmI think a few things. You are splitting the output signal of the AWG, and routing it to a scope and DUT input.Are your scope channels set to 50 Ohm or probe input? Both have issues, different way. If it's 50 Ohm, then your load is not 250 Ohm. If it's regular probe input, then remove the BNC cable, and place there your regular scope probe. At the end add a BNC to scope probe adapter (buy one if you don't have one, they are cheap).Also my experience: When I was trying to make bode plot measurements with my Siglent AWG and scope, the setup was always kind-of falling apart above 10-20MHz, even though both these instruments support higher frequencies. I never tracked down what really causes that, could be ground currents. Or maybe a proper RF splitter needs to be used.

author: Sariel
author_rank: Regular Contributor
timestamp: March 22, 2025, 05:43:08 pm
content: Quote from: tszaboo on March 22, 2025, 05:20:00 pmIn Bode mode, the input impedance of the scope is 1Meg.I connect using coaxial cables directly.The LNA input impedance is 250ohm, which can be defined in the bode analysis menu.

author: mawyatt
author_rank: Super Contributor
timestamp: March 22, 2025, 06:04:36 pm
content: Quote from: Sariel on March 22, 2025, 04:48:42 pmWe grabbed a couple resistors from the scrap parts bin, a 1Ω 1% 3W and a 1K 1% 1W. Didn't bother to measure them but you can see the inductive lead/body effects beginning to show above ~100KHz. Attenuation is ~59.59dB @ 0 degrees at 10Hz and rises to ~59.37dB @ 1.96 degrees at ~100KHz.Error from ideal 60dB (actually 60.00868dB) attenuation could be due to DSO and/or unmeasured resistors, if we were doing this would measure each resistor and select pair that produces 60dB attenuation (or tweak with parallel resistor to achieve ideal 60dB).Also, might be better to use SMD rather than leaded resistors for the 60dB attenuator as they have less parasitic inductance (and capacitance).Best

author: mawyatt
author_rank: Super Contributor
timestamp: March 22, 2025, 06:29:22 pm
content: Quote from: tszaboo on March 22, 2025, 05:20:00 pmUsually best to go into the 50Ω world above ~10MHz with 50Ω DSO terminations. This usually isn't a problem as most impedances are lower in the higher frequency ranges, altho we've used 10X Scope Probes (with ~20pF shunt capacitance which moves the effective probe Z down below 1KΩ) with the BNC adapter with some success above 10MHz.Best

author: Sariel
author_rank: Regular Contributor
timestamp: March 22, 2025, 09:25:17 pm
content: Just to emphasize.This design of LNA is a full SMD design. Full ground plane in 2nd layer. All components are on the Topside.Two 9V batteries are supplying the +-5V voltages to all amplifiers (after an LDO).1% or less tolerance for resistors and capacitors. OPA221 is the amplifier used for all stages.[ Specified attachment is not available ]

author: mawyatt
author_rank: Super Contributor
timestamp: March 22, 2025, 10:48:53 pm
content: If you roll one of these again consider including the 60dB attenuator (jumper) on the PCB, makes testing and characterization much easier. We used to do this on most of our sensitive stuff even including custom ICs.Best

author: Someone
author_rank: Super Contributor
timestamp: March 23, 2025, 12:04:24 am
content: Quote from: mawyatt on March 22, 2025, 02:44:11 pmYes, and you then going on to list all the improvements you would like is evidence that they are relatively fixed function. If that function does exactly what you want then great! but the possible ways to measure a transfer function are vast and time is generally saved by using some more flexible framework.Quote from: mawyatt on March 22, 2025, 02:44:11 pmJust because you can't code well doesnt mean others can't, and there are ready to use examples out there to work from so its not some barrier for all.

author: mawyatt
author_rank: Super Contributor
timestamp: March 23, 2025, 01:11:18 am
content: Quote from: Someone on March 23, 2025, 12:04:24 amWe also have the Pico 4262 and AD2 (both of which have Bode Functions) and still prefer the Siglent for most things. This is not from a junior technical or hobby level reference but with hands on experience with these actual instruments. Since you've used the Siglent, care to comment in detail on what you found??Sure there are a few things we would like, but for a low cost DSO it's pretty damn good as it stands now (same for FFT). Care to post something better anywhere in this price range??QuoteAnd these "examples" are easier to use, setup and apply than the built-in Siglent DSO Bode Function, care to post some of these "examples" that don't require any computer, extra hardware, or plotting software??Best

author: jonpaul
author_rank: Super Contributor
timestamp: March 23, 2025, 01:11:39 am
content: no reason to go ynder 100 H=Hz unless a mechanical servo systemSee breakpoint fo opamp/system gain.J

author: mawyatt
author_rank: Super Contributor
timestamp: March 23, 2025, 01:19:23 am
content: Normally yes below 100Hz isn't necessary, however OP is interested in noise levels which presumed includes 1/f regions.The LNA design has three 1st order high passes and a Sallen-Key 2nd Order 10Hz high pass, so relatively complex low frequency high pass behavior that "looks" like 80dB/decade below 1Hz.Best

author: trobbins
author_rank: Frequent Contributor
timestamp: March 23, 2025, 06:05:44 am
content: I regularly use the Bode plot FRA tool with Pico 4224A down to 1-2Hz for valve audio amp assessment of feedback, and have gone below 1Hz for choke assessment. Yes the plotting steps take longer the lower one goes, so it depends what measurement bandwidth is of interest. I chose the 4224A specifically for the 12-bit native resolution, and the 1MHz AWG. This is a high signal level application, with probes at 10:1, so haven't yet had low level concerns.

author: mawyatt
author_rank: Super Contributor
timestamp: March 23, 2025, 12:12:20 pm
content: Agree the Pico is a good tool, our 4262 was acquired for the 16 bit ADC DSO performance and good spectral quality internal AWG for lower frequency work (below 5MHz). However it seldom gets utilized as we don't like the tethered Laptop as bench space is limited and prefer stand alone instruments.If available to the OP, both the Pico 4224 or 4262 would be good choice for the LNA tasks.Best

author: Sariel
author_rank: Regular Contributor
timestamp: March 23, 2025, 12:58:17 pm
content: As suggested, I added the -60dB attenuator to the setup.I configured the scope's AWG for 0.4V (Peak to peak value).Attenuator measured values (with DMM6500): 850ohm (series resistor) and 1.1ohm (shunt resistor)=> -59dB attenuation.Adding these values to the Spice simulation suggests that the total gain should be +1.8dB.I measure now using the setup: +0.5dB.Therefore I am missing 1.3dB. Now it is the hard part. Where is this missing dB?Some more notes:1. I upgraded the SDS to the latest firmware.2. Did self-calibration.3. Measured a through cable alone and got 0.02dB insertion loss.

author: gf
author_rank: Super Contributor
timestamp: March 23, 2025, 01:15:45 pm
content: Quote from: Sariel on March 23, 2025, 12:58:17 pm20*log10(1.1/(850+1.1)) = -57.77 dB

author: Sariel
author_rank: Regular Contributor
timestamp: March 23, 2025, 01:21:29 pm
content: Quote from: gf on March 23, 2025, 01:15:45 pmAlso need to add the 50 series output of the AWG.Then it is -58.25 dB.Instead of measuring +1.8dB of gain , I measure 0.5dB.Therefore 1.3dB of gain is missing.

author: mawyatt
author_rank: Super Contributor
timestamp: March 23, 2025, 01:22:38 pm
content: And the input Z of the LNA.Also measure the Bode Plot of your attenuator.Best

author: Sariel
author_rank: Regular Contributor
timestamp: March 23, 2025, 01:42:56 pm
content: Ok.I replaced the attenuation resistors. Remeasured them.Ran the simulation again.Series resistor: 985 ohm. Shunt resistor: 1.09 ohm.The simulation with the attenuator showed a total gain of +0.55 dB.I measured +0.52dB using the SDS Bode analysis.Magnificent.Thank you guys!I can throw a party tonight

author: gf
author_rank: Super Contributor
timestamp: March 23, 2025, 01:54:22 pm
content: Quote from: Sariel on March 23, 2025, 01:21:29 pmNo. Zout of the AWG isnot relevantif the scope measures the voltage at the divider input.Or how is the scope connected? Tee at the generator output with one leg to the scope and the other leg to the divider?Quote from: mawyatt on March 23, 2025, 01:22:38 pm250 Ohms in parallel to 1.1 Ohms should be virtually negligible (1/2 %).And if the scope would directly measure the amp input voltage (after the divider), then Zin would not matter at all.(Just wonder if SNR would be sufficient in the latter case, but measuring 1mV or maybe even 100µV may well be feasible with a narrow-band measurement).

author: mawyatt
author_rank: Super Contributor
timestamp: March 23, 2025, 02:16:59 pm
content: Quote from: gf on March 23, 2025, 01:54:22 pmThe Bode Function is a ratio of Vout over Vin and the AWG source Z shouldn't matter as a proper setup measures Vin at the input to the attenuator.If the LNA input Z is 250 ohms agree that's a small change wrt to ~1.1Ω (0.04dB), however the OP is looking for absolute accuracy, so thought we would mention such!!Measuring at the LNA input would not be a good idea, as the scope probe/connection alone would introduce significant Mains Noise and other unwanted signals/noise.Anyway, seems OP has now achieved acceptable results using the input attenuator!!Best

author: Sariel
author_rank: Regular Contributor
timestamp: March 23, 2025, 02:44:49 pm
content: Regarding the PCB parasitic capacitance.You were right. it indeed affected the 1MHz Sallen key LPF.Instead of 1MHz (-3dB) I got -3dB at 612KHz and -9dB attenuation at 1MHz.I am used to designing quite a lot of high-frequency circuits (GHz stuff) and there I take into account every 0.05pF of capacitance.When designing this product, I was like "It is just DC. I can do whatever I want".I was wrong. My arrogance cost me the -3dB point location.As suggested I can scale differently the SK filter of the 1MHz LPF accordingly. Great lesson.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 23, 2025, 03:13:59 pm
content: Might I ask OP why 1 dB of discrepancy was unacceptable, but now 0.3 dB is celebration time?

author: mawyatt
author_rank: Super Contributor
timestamp: March 23, 2025, 03:33:49 pm
content: Quote from: Sariel on March 23, 2025, 02:44:49 pmGlad things are working out for youWe've worked from DC to ~200GHz, nanovolts to kilovolts over our career, so appreciate how parasitics can effect results and have the teeth marks to prove itNice LNA, keep us posted on your measurementsBest

author: AnalogTodd
author_rank: Regular Contributor
timestamp: March 23, 2025, 05:09:23 pm
content: Quote from: Sariel on March 23, 2025, 01:42:56 pmGlad to hear it's working!

author: Someone
author_rank: Super Contributor
timestamp: March 23, 2025, 10:47:50 pm
content: Quote from: mawyatt on March 23, 2025, 01:11:18 amWe get it, you're a massive fanboy/grl of the Siglent scope.Who was suggesting a zero effort, zero external resource solution? Strawman.The built in "bode" plot is substantially fixed in its operation and limited in what can be manipulated. Thats fine if it makes the measurement desired, but even if it does I mentioned that often it could be done quicker using other techniques/methods. Time saved from adding some external control/automation could be significant rather than doing it by hand (as the on board range didn't span what the OP wanted). When people discover those limitations are getting in the way of measuring what they want (the OP) then suggesting ways around that is entirely appropriate. Rather than trying to argue the alternative would need some effort or additional resource, which if you take that line leaves the OP with.... not making the measurements they want.

author: mawyatt
author_rank: Super Contributor
timestamp: March 24, 2025, 01:51:23 am
content: Quote from: Someone on March 23, 2025, 10:47:50 pmNot a Siglent fan or any equipment fan, just like a well designed instrument at an affordable price. Of course this assumes one is skilled enough to appreciate and utilize the instruments performance and features, of course YMMVWe've highlighted our question above, where's the examples??BTW Strawman seems to fit your "examples" not the Siglent DSO Bode Function under discussion, as it's certainly easier and quicker to setup which is how this discussion started. The Pico 4262 we have has an excellent Bode Function Feature, and tends to be faster. However during some of our tests with acquiring oscillator Gain/Phase performance and Injection Locking Characteristics it didn't perform as well as the Siglent DSO as the out-of-band sampling rejection didn't seem as good and this showed in corrupted results. This was attributed to the Siglent using a more sophisticated frequency selective sampling method which explains the slower performance, altho not confirmed. However our main issue with the Pico is it requires a dedicated Laptop and why it's seldom used, just takes too much space and time to get setup and running.The Siglent Bode Function is offered across their entire DSO line, and has been updated a number of times and no reason to expect this won't continue, so not "Fixed" as you state, more a continual expansion and refinement of an already good and versatile feature in skilled hands.Since you'veclaimedto have used the Siglent Bode Function and know all about it's performance & limitations, what DSO/AWG did you use, and when? What were the detailed setup parameters and DUT results, and the shortcomings?Best

author: 2N3055
author_rank: Super Contributor
timestamp: March 24, 2025, 07:25:19 am
content: Quote from: Someone on March 23, 2025, 10:47:50 pmBode plot is substantially fixed in its definition to start with.It is an AC phase/magnitude response plot of a 2-port network device.There is input,output. You apply stimulus to input, measure at the output.You need to be able to control input stimulus amplitude to ensure DUT is working in designed dynamic range.You sweep through frequencies, save results.You display results in a table and graphical format.Bode plot in Siglent and Keysight is as close to actual zero effort to use. Connect, enable, set few parameters and off you go. It has automatic vari levels to deal with devices with large dynamic range and complex behaviour. You can draw stimulus level curves yourself.You can export measurement data for later or for further analysis.I can code well enough. Not top class but I manage. But I refuse to reinvent the wheel if I don't have to.Only limitation, in this case, was that OP wanted to go below 10 Hz, and like Jon said, it is very niche applications where you are tuning PID controller with time constants of 10 sec or something.In that case this Bode plot method is very slow anyways.In practice, both Keysight and Siglent implementation work extremely well (Frequency being limited to 10Hz and 20 MHz (Keysight) and 120 MHz (or generator) for Siglent (with limited results on the high end, honestly. Stay below 20-30 MHz for best results. Higher than that is domain of VNA anyways, IMHO). You have cursor measurements, and Siglent also have some automated measurements.Siglent is also only one on the market (regardless of the manufacturer or price) that supports 3 outputs.So you can actually plot same device in several places in signal path getting corelated plots inside signal path.Or you can plot a 3-way filter (like active X-over with low/mid/high cuts) all at the same time on single graph.Recreating Bode plot in code, by hand, of that level of sophistication is not that easy.I would code one myself only as a last resort.I would speculate that you, at one point, had to create such code, and now you have it ready made and by just tweaking it a bit here and there you can go by with it.But those that don't have anything ready to use, why would they embark to write something from the scratch when very good, well working implementation already sits in their scope.And if there is some occasional need for something very specific, it is easy enough to grab few points manually.If you specifically work with tuning PID on slow servo systems (mechanical, thermal) you then need to use something else, of course. Sample with DMM, analyse on PC, write your own code.In those system Nyquist diagrams are also used a lot, for establishing stability criteria..And then also other methods like Lyapunov stability criteria...With these types of work you are much deeper down the rabbit hole than just plotting Bode plot of 2-port device.But for general purpose Bode plotting, what is in the scope right now covers very wide area very well.You should really try it once without prejudice. It can do a lot.Of course nothing is perfect.

author: mawyatt
author_rank: Super Contributor
timestamp: March 24, 2025, 03:06:32 pm
content: Great synopsis ^^^ as usual 2N3055Since "Someone" hasclaimedto have used the Siglent DSO Bode Function, one would think that he or she would know some of these features and characteristics! This was why we asked what DSO/AWG used and Bode setup details, thinking maybe we could help he or she understand this Bode feature and how to apply such with good results.Anyway, OP had access to Siglent DSO and employed this to achieve acceptable results for their LNA (with some help), and don't think they had to write any code to do such altho likely had to collect a few points below 10Hz. Was mentioned using the unique 3 Output Feature to plot the Gain/Phase of each stage in the LNA cascade, which should produce very interesting result and helpful in understanding where the dynamic range bottleneck within the LNA chain resides.Many may not realize that the Bode Dynamic Range is not limited to just the 12 bit ADC DR, but the ability to do Input Signal Level Scaling and Output Range Scaling (believe this is independent for each output!!) extends the Bode DR nicely, and having a true 500uv/div DSO Low Noise Range is icing on the cake!! Vaguely recall rf-loop showing this with a Stepped Attenuator some time ago.By applying R&D efforts across the entire DSO lineup, Siglent spreads the firmware/feature development costs which all DSOs benefit, even the low end SDS800X HD series. The Bode Function, Math Functions, and FFT reflect thisBest

author: mawyatt
author_rank: Super Contributor
timestamp: March 24, 2025, 03:16:40 pm
content: Quote from: Sariel on March 23, 2025, 02:44:49 pmBTW you can create more viewer friendly scaling by using the manual scaling of the Gain/Phase Display, makes the graphs look more uniformBest

author: gf
author_rank: Super Contributor
timestamp: March 24, 2025, 03:34:40 pm
content: Regarding your mesage in another thread:Quote from: Sariel on March 24, 2025, 09:29:43 amThe Bode application may be limited to 10Hz, but what prevents you from using your SDS2000X Plus and its AWG to manually measure down to 0.1 Hz?

author: Someone
author_rank: Super Contributor
timestamp: March 25, 2025, 12:13:55 am
content: Quote from: 2N3055 on March 24, 2025, 07:25:19 amI have used the built in "bode" plots on both the Keysight and Siglent scopes. Do I need to say this more times? They do what they do and if that is all the user needs there is no problem.They have their own functional limitations, very basic ones like frequency range (OPs problem) or number of points per decade (oh no can't measure narrow filters!), or more complex ones such as having a fixed tradeoff between accuracy and speed, or method of selectivity, or excitation waveforms (can make significant differences), or new dimensions such as varying input amplitude through a sweep.As soon as something outside the built in function is needed, there are various options to resolve that. Instead we get fanboi/grl talking endlessly to the hills about there is no problem and people should not consider alternatives because the built in thing is unquestionably fantastic. Yes it does what it does and you can find all sorts of neat things to use it for, but blindly sticking with a tool and not understanding (or distracting from) its methods and limitations is a dead end.Then you end up like mawyatt:Quote from: mawyatt on March 24, 2025, 01:51:23 amSwapping between different capture hardware not because the hardware is the limitation but the bundled analysis is limiting, thats exactly the failing in your methods which I am pointing out. Other users would make better use of the hardware they have and do the analysis with 3rd party tools, or write them themselves, rather than buy some other shiny new thing that mostly duplicates what they already have. Many users have shared examples of frequency analysis automation code on this forum for people to use so it is not some huge barrier as you make out.The OP is still hunting for a solution to measure their frequency response outside the (artificially) limited range of the built in analysis...... but do continue to insist no-one may suggest ways to solve that. I'll instead try and answer the OPs question:Quote from: Sariel on March 20, 2025, 01:37:48 pmThe OP is correct that it can't be done with the inbuilt function, and requires some alternative. By hand? sure, can do. Or it can be automated with little to no effort by taking existing examples.

author: mawyatt
author_rank: Super Contributor
timestamp: March 25, 2025, 02:04:38 am
content: Quote from: Someone on March 25, 2025, 12:13:55 amNone of our methods fail Mr or Mrs Someone, seems you fail and can't come up with what Siglent DSO/AWG you "claimed" to have used and how you setup things up, what parameters and performed any measurements (we've asked a few times now). Nor can you come up with any "examples" you "claimed" that are "out there". Seems you are avoiding these questions, because you really don't have any backup to your "claims"??Assuming we acquired the Pico 4262 for the Bode Capability, Mr or Mrs Someone is wrong again!! The Pico 4262 was recommended (by folks much more knowledgeable than Mr or Mrs Someone!) and acquired for the development and verification of a Unique Bi-Phase Modulated 128 Independent Channel 16 Bit AWG (scalable to 256 channels) for controlling a new type Electronically Steerable Phased Array Radar System. Later after the successful AWG development we began to use the Pico for other things which included the built-in Bode Capability since it was readily available. If this Pico was available to the OP he could certainly use it, but he indicated access to the Siglent and why the discussion went that way.Here's our quote from the 23rd:Agree the Pico is a good tool, our 4262 was acquired for the 16 bit ADC DSO performance and good spectral quality internal AWG for lower frequency work (below 5MHz). However it seldom gets utilized as we don't like the tethered Laptop as bench space is limited and prefer stand alone instruments.If available to the OP, both the Pico 4224 or 4262 would be good choice for the LNA tasks.BestNo one has said the Siglent is the only solution for the OP, only "Someone" with limited understanding and knowledge has repeatedly attempted to steer the discussion with no valued added.Best

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: March 25, 2025, 02:32:04 am
content: Hi group,In my lab I would reach for my HP3577A network analyzer. This has a range of 5Hz to 200MHz. It offers 50and 1Minput impedance. It has a resolution bandwidth down to 1Hz.These are the accuracy specifications:Quote from: gf on March 24, 2025, 03:34:40 pmThe performance outside of the passband is relatively unimportant. It is dependent on some large value capacitors which could change with temperature. At 0.1Hz the signal should be -160dB relative to the passband.It only requires a check that out of band signals will be rejected.Jay_Diddy_B

author: Someone
author_rank: Super Contributor
timestamp: March 25, 2025, 06:06:48 am
content: Quote from: mawyatt on March 25, 2025, 02:04:38 amOP asked for how to make the measurements at lower frequencies than the built in app can be set to. Thats this thread. You claimed automating that with external control was too hard (for you), yet myself and others do not see that as a difficult solution.You can continue adding voluminous "argument" and noise about all the things you can do and try to claim I'm somehow unknowledable or misleading with your fake concern and sealioning. Which is not addressing the really simple question the OP had: how to make the measurement at a lower frequency.Buying some other equipment to make a trivial measurement that the equipment the OP already has can make? Thats ridiculous. They have equipment which can make the measurement they are seeking. There are several ways to achieve that which the OP can trade off for their particular circumstance.It's entirely possible the OP needs more information on how to measure bode plots as they seem to understand the information it provides, but not necessarily how it is collected/measured. But any chance of having such a plain and educational discussion seems to have gone.

author: tszaboo
author_rank: Super Contributor
timestamp: March 25, 2025, 12:39:34 pm
content: Quote from: gf on March 24, 2025, 03:34:40 pmSome extra info: It's limited to 10Hz, because it's using the AC coupling on the input. Also, near DC there might be some more errors because of the tolerance of the AC coupling.Quote from: 2N3055 on March 24, 2025, 07:25:19 amActually you can setup multiple outputs, but minor detail.Quote from: Someone on March 25, 2025, 12:13:55 amYou are right, but then it depends why OP wants the Bode plot. If to verify a filter, you can put together a pass criteria out of 3 measurements. If you only want to do that once, take the tools you have and put it together from those. If you need to do it in production testing, or more often, requirements are different.

author: mawyatt
author_rank: Super Contributor
timestamp: March 25, 2025, 01:22:08 pm
content: Quote from: Someone on March 25, 2025, 06:06:48 amSee you are still avoiding answering the questions about which Siglent DSO/AWG you used, and details, seem the "unknowledgeable" fits, as you've demonstrated such!! Same for the plentiful coding "examples' you claim!!Here's the 4th post on the thread by OP (his 2nd post), note the mention of access to Siglent DSO and AWG, also the mention of the Magnova.Thank you, Todd! Nice to meet you here as wellI have access to SDS2000X plus, SDG6022 to perform the Bode plot.I don't yet have access to the SDS2000X plus internal AWG ( need to open this option).I must say that the manual for these devices lacks enough depth on how to connect the devices and perform well the sweep.I also have access to Magnova scope. But its AWG/Bode plotter will be available in July.I do want to try the white noise option. It looks very fun. The SDG6022 has Gaussian noise output which can be frequency-limited.So what equipment did we suggest the OP buy? Can't find a single statement in this entire thread where we suggested any equipment purchase!! We did say if the OP had access to the Pico DSOs, they would be a good option. Here's our quote from 3/23.If available to the OP, both the Pico 4224 or 4262 would be good choice for the LNA tasks.Since the OP stated access to the Siglent DSO/AWG we helped the OP get things up and running with the SDS2000X Plus and SDG6022. Think the OP knows how to use the Bode results and what they represent, which the Siglent provides & displays with minimal effort (no coding required), but had some trouble with the setup and attenuator which was quickly resolved. Here's the OP 3/23 response.Ok.I replaced the attenuation resistors. Remeasured them.Ran the simulation again.Series resistor: 985 ohm. Shunt resistor: 1.09 ohm.The simulation with the attenuator showed a total gain of +0.55 dB.I measured +0.52dB using the SDS Bode analysis.Magnificent.Thank you guys!I can throw a party tonightAlso OP had issues due to parasitics effecting active Sallen-Key Low Pass Filter which the Bode Plot revealed, here's the OP response.Regarding the PCB parasitic capacitance.You were right. it indeed affected the 1MHz Sallen key LPF.Instead of 1MHz (-3dB) I got -3dB at 612KHz and -9dB attenuation at 1MHz.I am used to designing quite a lot of high-frequency circuits (GHz stuff) and there I take into account every 0.05pF of capacitance.When designing this product, I was like "It is just DC. I can do whatever I want".I was wrong. My arrogance cost me the -3dB point location.As suggested I can scale differently the SK filter of the 1MHz LPF accordingly. Great lesson.So instead of your hearsay, straw-man tactics, diversion, and unsupported claims/understanding/knowledge about the Siglent DSO/AWG Bode Function, you have no value added, and continue to contribute no value!!Best

author: mawyatt
author_rank: Super Contributor
timestamp: March 25, 2025, 01:53:19 pm
content: Quote from: gf on March 24, 2025, 03:34:40 pmActually nothing for the OPFrom 2N3055 Reply #8.As for frequencies lower than 10 Hz....You keep wiring as for Bode plot.In measurements, you have both amplitude related measurements and phase too.You set measurements to measure P-P and Stdev on input(CH1) and output (CH2) and Phase between CH1 and CH2. Enable statistics. Even histicons. Mini histograms will be useful to judge quality of data.Decide how many points per decade you need between 0.1Hz and 10Hz.Set SDG6000X to 0.1Hz, set timebase to 100ms/div and let it run so it gathers measurements and stats.Stop scope. Write down data. Or do screen shot.Set next frequency on AWG. Adjust timebase on scope. Run scope again.Wait.. Rinse and repeat.Best