Bode Plot on Siglent SD804X HD - Page 1
Forum section: electronics / testgear

author: rpschultz13
author_rank: Contributor
timestamp: August 27, 2025, 09:31:36 pm
content: I've been learning how to use my scope and an AWG to do bode plots. Couple questions:1) I've connected the USB from AWG to the flat USB in the back of the scope, and I have a USB mouse in the front. But then that only leaves a square USB on the back. Is there a way to connect the AWG, mouse and USB stick at the same time? Now when I want to do an image I have to unplug something to plug in a USB stick.2) Picture shows a HPF at around 300 Hz. Measurement P1 values actually says 338 Hz. How can I use the cursor to look at specific points?!? I've pushed every button on it, I can't figure it out.[ Specified attachment is not available ]

author: mawyatt
author_rank: Super Contributor
timestamp: August 27, 2025, 09:59:17 pm
content: Use a USB Expander on the USB front. We use a cheap Yuanxin 3 port expander.BTW now you are up-and-running check out the State Variable Filter where you can plot 3 variables, LP, HP and BP, might prove useful with your audio work!!https://www.eevblog.com/forum/projects/state-variable-active-filter/Best

author: tautech
author_rank: Super Contributor
timestamp: August 27, 2025, 10:28:17 pm
content: Quote from: rpschultz13 on August 27, 2025, 09:31:36 pmDid you miss the Cursor menu in Bode Plot mode ?

author: rpschultz13
author_rank: Contributor
timestamp: August 27, 2025, 10:54:25 pm
content: Ah, yes I missed the menu. I was pushing the cursor button and it wasn't doing anything.Thanks!

author: rpschultz13
author_rank: Contributor
timestamp: August 28, 2025, 01:02:53 am
content: Yes! The “mid” section and the HPF/notch section are both SVF. I learned it from ESPhttps://sound-au.com/articles/state-variable.htmNotice the HPF/notch switch. It switches from one output to a different output of the same SVF!There’s a picture at the end of this thread, I’ll put one here later.https://www.eevblog.com/forum/testgear/need-signal-generator/Thanks for the link, I’ll dig through that!

author: rpschultz13
author_rank: Contributor
timestamp: September 01, 2025, 02:10:21 pm
content: I'm trying to learn how to use this scope to measurenoiseon this pedal and compare with others. It's 12 bit, but can do 15 bit in ERES mode (I have no idea how that works).When learning about noise, there's peak-to-peak, RMS and a few other details I don't fully understand.Do I need to have a load on the pedal (plug into a guitar pickup) or not? I'm assuming cranking the gain knobs would raise the noise level.How do I setup the scope to read noise?Thanks!

author: tautech
author_rank: Super Contributor
timestamp: September 01, 2025, 08:10:00 pm
content: Quote from: rpschultz13 on September 01, 2025, 02:10:21 pmOversampling.You can see the sampling rate drop when using ERES.QuoteNoise or distortion ?Inject a sinewave through the pedal and analyze the output on the scope.

author: rpschultz13
author_rank: Contributor
timestamp: September 01, 2025, 08:31:09 pm
content: Noise, not distortion. For acoustic guitar, any clipping/distortion is bad.

author: tautech
author_rank: Super Contributor
timestamp: September 01, 2025, 08:47:41 pm
content: Quote from: rpschultz13 on September 01, 2025, 08:31:09 pmWhat are the normal signal levels through the pedal ?I'd be doing any tests near normal levels and maybe push levels to where any clipping/distortion starts.Dunno anything about stringed instruments and amplification of there signals.Only that any checks/tests would wisely be done at levels of where they might operate.@KungFuJosh might be able to advise as he works on this sorta stuff.

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 01, 2025, 09:52:22 pm
content: The scope + AWG is going to feed the signal through the pedal, and you want to have the signal level be similar to what you expect that would be fed to it from the instrument. The frequency range should reach well above that of the guitar to account for harmonics and such.Load simulation is more useful when characterizing things that need it like guitar pickups if you don't think the pickup alone is realistic compared to the complete circuit that will be in the guitar.

author: rpschultz13
author_rank: Contributor
timestamp: September 01, 2025, 10:20:22 pm
content: Here's my 1st measurement of 2 guitar pedals. No load, just plugged straight into the scope. Turning the knobs doesn't change any of this.Ch2 has a peak-peak of 80 mV. Op amps running internally at +/-15v.Ch3 has a peak-peak of 1 mv. Op amps running single sided at 9.5v.Does a higher voltage cause it to pick up more noise?I can't hear any noise when I use them. So I'm wondering if it's higher frequency stuff.Ch3 lists this for it's noise specs:S/N (ref 300mV): >93 dB-A wtd, EQ flat, single channelDynamic Range: >103dB-A wtd, XLR, 2x 48V phantom power, RL>20k, single channelDistortion (THD+N): <0.10% 20Hz to 20KHz, 300mV, nominal gainI'm thinking I need to send in a frequency and measure noise levels at different frequencies?[ Specified attachment is not available ]

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 01, 2025, 10:42:33 pm
content: Quote from: rpschultz13 on September 01, 2025, 10:20:22 pmYes, a Bode plot, aka FRA (Frequency Response Analysis).

author: rpschultz13
author_rank: Contributor
timestamp: September 01, 2025, 11:04:02 pm
content: [ Specified attachment is not available ] [ Specified attachment is not available ]Ch2 at 200 HzCh3 at 200 HzCh2 has noticeably higher ?noise?. How do I quantify this?Is there some function in math or analysis that will calculate this?[ Specified attachment is not available ]

author: tautech
author_rank: Super Contributor
timestamp: September 01, 2025, 11:07:35 pm
content: Please set trigger to the channel being used.

author: rpschultz13
author_rank: Contributor
timestamp: September 01, 2025, 11:35:10 pm
content: Ah thanks. That's why I'm here, to learn.Same thing, Ch2 and Ch3 at 200 hz. Now, how can I figured out how much noise is associated with each device?[ Specified attachment is not available ]

author: rpschultz13
author_rank: Contributor
timestamp: September 02, 2025, 12:01:05 am
content: If I zoom in, it's clear that Ch3 has a cleaner signal.Just based on the plots:Ch2 has a noise level of +/- 20mVCh3 has a noise level of +/- 0.25mV[ Specified attachment is not available ]

author: tautech
author_rank: Super Contributor
timestamp: September 02, 2025, 12:07:22 am
content: To apply KISS principles, Tee the source (AWG ?) into one channel and the DUT.DUT output to another scope channel with settings appropriate to compare resultant waveform to the source.Lunch has arrived, later....

author: rpschultz13
author_rank: Contributor
timestamp: September 02, 2025, 12:12:50 am
content: Yeah I currently only have a pair of BNC-to-1/4" cables. I have 2 more on order. Once I get them I'll be able to plot at the same time.Thanks for the help!

author: tautech
author_rank: Super Contributor
timestamp: September 02, 2025, 12:48:10 am
content: Some Zoom lessons.Source SDG1022X Plus @5mVPress the Zoom button near the s/div encoder to engage or disengage a zoomed window.The window with the dotted border is active to the controls.With the V/div and the V Pos encoders you can magnify any portion any a waveform for close inspection and the unzoomed windows retains the key to your POI.Each tab on the display tells you what's going on.Please disregard I left the input set to 10x....the reason why more noise than expected is showing....

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 02, 2025, 12:53:26 am
content: You should also learn about FFT functions.

author: rpschultz13
author_rank: Contributor
timestamp: September 02, 2025, 10:41:32 pm
content: OK I figured out the zoom function. Wow. Game changer.Also, I think I fixed the noise. I inadvertently changed one of the 100n op amp filter caps to a 22n, thinking it was part of an audio filter. I realized it at the time but didn't fix it. So fixing it NOW and getting rid of the noise confirms to me how important those 100n caps are on the op amps.Ch2 is my preamp I'm designing, running at +/-30v with OPA4134'sCh3 is a commercial preamp running at +9.6v, unknown op ampsBoth zoom plots I think are setup with the same scales.[ Specified attachment is not available ]

author: tautech
author_rank: Super Contributor
timestamp: September 02, 2025, 11:10:45 pm
content: Quote from: rpschultz13 on September 02, 2025, 10:41:32 pmYet the display tabs tell a different story.You have optimised vertical zoom to see what you need but horizontal has been zoomed also.Click/touch any tab to have it respond to the controls and adjust to suit your needs.Measurements of these types are made simpler when the trigger H Pos is at 0s on the display where we can see the blue pointer indicator show the H trigger position to the left of 0s.Press the H Pos encoder to set it back to 0s......allposition controls have this press to zero capability.

author: rpschultz13
author_rank: Contributor
timestamp: September 05, 2025, 12:12:28 am
content: Hmm. OK. Looks like there's something going on every 1.5 us, or 667 kHz. I'm not sure what to make of it.[ Specified attachment is not available ]

author: tautech
author_rank: Super Contributor
timestamp: September 05, 2025, 12:50:05 am
content: Quote from: rpschultz13 on September 05, 2025, 12:12:28 amAs a sanity check to ensure you are not chasing ghosts in your DUT, start turning OFF neighbouring devices one by one.Some might see this incredible sensitivity inconvenient however it trains you to find, know and determine what might be a problem and what might not.A properly RFI/EMI clean workspace is actually quite rare.

author: rpschultz13
author_rank: Contributor
timestamp: September 05, 2025, 09:38:03 pm
content: I redid measurements with only scope and AWG plugged into the wall, DUT on 9v battery. No change, still 667 khz.Here's a current picture of the PCB's in the enclosure.[ Specified attachment is not available ]

author: rpschultz13
author_rank: Contributor
timestamp: September 05, 2025, 09:43:36 pm
content: Schematics:[ Specified attachment is not available ]

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 05, 2025, 10:33:08 pm
content: Random Thoughts:Are you probing at the Output on the pedal?Troubleshooting steps would include:- Probing at different points in the circuit.- Adjusting knobs and/or wire positions to see if they have any effect on the issue to isolate the source; assuming the source is within the pedal.Try eliminating the AWG and use an acoustic guitar or battery powered signal generator (or a battery powered looper pedal).If your scope and AWG are not connected with good EMI/EFI filtering, they could introduce an external issue. It's less likely with the pedal connected with a battery, but not impossible.I was troubleshooting a guitar tube amp once. For over a year (side project). The issue was caused by a power supply for a device on a different floor of my house. I didn't have enough 9V batteries to power that amp.Thanks,Josh

author: tautech
author_rank: Super Contributor
timestamp: September 05, 2025, 10:38:29 pm
content: Quote from: rpschultz13 on September 05, 2025, 09:38:03 pmConsider assigning your measurements to the zoom window and selecting only those types you need.Also, trigger level can influence the frequency measured.Further, never assume you have isolated all interference sources as RFI/EMI doesn't much care about walls.

author: rpschultz13
author_rank: Contributor
timestamp: September 05, 2025, 11:02:05 pm
content: Currently I’m not probing, but using BNC-1/4” cables into the 1/4” jacks.

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 06, 2025, 12:30:43 am
content: Quote from: rpschultz13 on September 05, 2025, 11:02:05 pmTomato, potato.Try what I suggested, and if that doesn't help, switch to a probe, and start moving along the circuit.As Rob and I both pointed out, this might be in vain if the source of the thing is external.

author: tautech
author_rank: Super Contributor
timestamp: September 06, 2025, 01:16:11 am
content: Quote from: KungFuJosh on September 06, 2025, 12:30:43 amAfter some study of the schematic and the TEC 3 switcher I'm not so sure.While RPS has done a good job of bulk, local and decoupling capacitance placement TEC 3 documents show its output ripple and noise as a pretty ordinary ~30mV ripple with a larger spike each 5us !Some additional high speed ceramic capacitance close to the TEC 3 switcher output would be advisable IMHO.https://www.tracopower.com/series/tec-3https://www.tracopower.com/sites/default/files/products/application_notes/tec3_characteristic_curves.pdf

author: rpschultz13
author_rank: Contributor
timestamp: September 06, 2025, 02:44:21 am
content: Wow, good catch. I have recently determined that the 3W is overkill, and that the 2W will probably work. My current measurements suggest it draws 1.14W.Looking at the Tec2, it still has those spikes:https://www.tracopower.com/series/tec-2I'll look at this closer this weekend.

author: rpschultz13
author_rank: Contributor
timestamp: September 07, 2025, 06:02:54 pm
content: Have been learning how to use the cursors.First plot is a HPF. Using the cursors, I calculate 79 Hz, and P1 also calculated 79 Hz.2nd plot is a mid frequency 9.4 dB cut. Q = f0/BW. F0 = sqrt(f1*f2) = 473 hz. BW = 786. Q = 0.6Question: Is there an easy way to switch between the 2 cursors, instead of going through the menus?