Rigol DS1052E Flatness Revisited Using Peak Detector - Page 1
Forum section: electronics / projects

author: Mechatrommer
author_rank: Super Contributor
timestamp: June 07, 2011, 02:47:03 pm
content: I've made a little experiment to observe my Rigol DS1052E flatness using a simple Peak Detector. But result unsuccessfull. I cannot observe any attenuation happening. The result and test setup are attached below (pictures and xls report). The result is graphed in "result.jpg", but the expected result should be like the sketching in the last graph of "expected.jpg". Please give idea on how to improve this, or should i just take the result with grain and salt?Hypothesis:DS1052E reading will attenanuate at close to its BW limit (50-100MHz)Method:Vpeak will be assumed as the absolute/real value in reading with theoritically 0% error. By inspecting Vpeak read by DMM, and comparing with DSO reading, attenuation can be estimated from Vdrop (see calculated variable below).Setup:1) Hantek 3x25 Function Generator + PC to control2) DIY Amplifier to boost 3x25 signal (lets just call it 10x25)3) Peak Detector embedded in 10x25, 1N4148 diode + 1uF SMD capacitor4) Cheapo ATX PSU to power 10x255) some BNC connectors and alligator clips6) Rigol DS1052E Oscilloscope7) Uni-T 71A MultimeterIndependent variable:1) test frequency from 3x25 & 10x25, Sine signal 1Hz - 100MHzDependent variable:1) Vpeak: Peak detect reading on CH2 and DMM (CH2 reading only to confirm DMM reading. DMM reading is to be trusted)2) Vmax, Vmin: Reading from DS1052E (Vmax, Vmin) on CH1Calculated variable:1) Vdrop: voltage drop in peak detector compared to Vmax (Vmax - Vpeak)2) Vpp: just another useless redundant calculation to better see the signal reading/outputResult/Conclusion:Vdrop calculated do not demonstrate any consistent attenuation. Hence DS1052E reading does not attenuate up to the test signal (100MHz).Hypothesis proven wrong!Related links:Dependence of a scope's frequency response on vertical attenuator settingIdea for Super Peak Detector

author: joelby
author_rank: Frequent Contributor
timestamp: June 07, 2011, 04:56:37 pm
content: I'm not entirely sure why you're trying to use a peak detector, but is your function generator's output amplitude flat over its range? If you do a frequency sweep and view the output directly on your oscilloscope, do you see any change in amplitude?It's too late for me to be bothered switching everything on to take screenshots, but I'm quite sure that my DS1102E attenuates signals at the higher frequencies when I sweep using my RF signal generator and a constant amplitude.

author: Mechatrommer
author_rank: Super Contributor
timestamp: June 07, 2011, 06:03:53 pm
content: 1) my signal generator and dso is not flat/constant and there is no hard prove to saying that or otherwise.2) my only hope lies in the peak detector, to bring the balance to the force, to tell the truth, of what the signal generator is outputting. and to compare with what the dso is reading.3) dont tell me about the termination again. its only half the Vmax, half the Vpeak, half the Vdrop half everything. i still cannot see the attenuation.4) maybe i'm just lost in the cloud of myth here.addition:the idea is to convert super mad high frequency in to DC level which is easily measured accurately, with DMM. or maybe i should re-ask1) where the dso attenuation comes from?

author: Unknown
author_rank: Unknown
timestamp: June 09, 2011, 11:04:42 pm
content: Quote from: Mechatrommer on June 07, 2011, 06:03:53 pmHow do you calibrate your peak detector? What's the uncertainty compared to your measured signal level? Do you change the vertical attenuator setting? The large change in amplitude looks like an issue to me. If you change the attenuator setting, you distort your measurements (the attenuator may not be linear at high frequencies). If you don't, the low vertical resolution will kill your accuracy.Quote from: Mechatrommer on June 07, 2011, 06:03:53 pmThere will usually be a deliberate low-pass filter in a DSO to prevent aliasing. Any amplifier will have a roll-off. Bandwidth needs to be limited to limit noise (high bandwidth = high noise). In a perfect design, all components of the system have some roll-off that combined results in the desired result of a Gaussian response with a -3dB point around 100MHz.

author: Mechatrommer
author_rank: Super Contributor
timestamp: June 10, 2011, 06:37:27 am
content: thanx for the reply alm. i thought this is going to be dead.Quote from: alm on June 09, 2011, 11:04:42 pmhow do i calibrate my peak detector?Quote from: alm on June 09, 2011, 11:04:42 pmN/A. the only data i have is the 1n4148 datasheet something likethiswhere forward current vs forward voltage graph (If vs Vf) is the most sensible to me as reference (it says Vf around 0.4V to 1.2V on the extreme If). but thats "current" is difficult for me to measure during the test (and designing the circuit for I measurement without affecting the DUT). looking at my result, the voltage drop range from 0.7 to 1V, so i'm just assuming the current will be somewhere 4-100mA (attached graph from datasheet below)Quote from: alm on June 09, 2011, 11:04:42 pmyes changed. i dont like it but i have to, since the generator will attenuate from ±10V at lo-freq down to ±2V at 100MHz. so in order to get decent range on voltage reading, i have to change vertical scale (from 5V/div to 1V/div)Quote from: alm on June 09, 2011, 11:04:42 pmi was asking this because i concerned that the roll-off is due to the front end circuit, that will in turn affecting the DUT signal. so, is DC signal got attenuated too?let me give an example. let say we are feeding dso with two signal at CH1 and CH2. and let say the dso is rolling off to -3db @ 100MHz. CH1 is 100MHz sine ±2V (true signal from generator) and CH2 is 2V steady DC. from the dso roll-off characteristic (-3db@100MHz), the CH1 will be attenuated and displayed as sine wav ±1V, but how about CH2? will the screen show a flat line at 1V too? or at 2V (true signal un-attenuated)?but as i mentioned, i used both CH2 and a multimeter to measure volt peak (detector) at CH2 and as far as i can see, CH2 reading and the DMM is tally.

author: Mechatrommer
author_rank: Super Contributor
timestamp: June 10, 2011, 07:27:20 am
content: made another test without dso. just to see how much loading a dso gave to the source generator on previous experiment. reading of terminated signal also given, just for curious mind folks out there.from the graph, i can say, the previous test, loading is not much for low freq, and around 30% at 100MHz (1.953 - 1.3159) / 1.953 x 100%. but the point is, even with 30% loading in the previous experiment, the peak detector is there, to monitor the signal output, loaded or not loaded. (and also pls note that this is done today, but the original test is done few days back, the calibration/signal maybe off)since attenuation is not observed in previous test. there are several possible explanation.1) if dso is indeed rolling of at 100MHz, it means the 1n4148 performance also rolling off at the frequency, hence the voltage drop consistency observed.2) if 1n4148 is not rolled off, so does the dso.3) if dso is rolled off, but not 1n4148. the graph on the first post ie "expected.jpg" should be achieved, ie Vmax and Vpeak converge (come close together), ie Vdrop approaching zero at hi-freg, which is not achieved.4) if 1n4148 is rolled off, but not the dso, Vmax and Vpeak should diverge (farther away), ie increasing Vdrop.so... idea?

author: Unknown
author_rank: Unknown
timestamp: June 13, 2011, 05:21:24 pm
content: Quote from: Mechatrommer on June 10, 2011, 06:37:27 amWith something like a scope, spectrum analyzer or power meter with enough bandwidth so roll-off is not an issue. For example, the 1973 HP 11096A HF probe (which is basically just a germanium peak detector with a resistive divider and compensation to compensate for the non-linearity of the diode) recommends a VHF signal generator and a power meter accurate to 1% up to 480MHz. They set the signal generator terminated into 50 ohm to the desired power level with the power meter, verify that it's stable, and check if the DC voltage from the probe to be consistent with the expected voltage for that power level into 50 ohms within +/- 1dB or so. That +/- 1dB would probably not be good enough to determine a scope's frequency response.Quote from: Mechatrommer on June 10, 2011, 06:37:27 amIf the difference between Vmax and Vpeak is less than the uncertainty, the results are meaningless.Quote from: Mechatrommer on June 10, 2011, 06:37:27 amThis will cause error at low Vf, but uncertainty also includes for example frequency-dependence of your detector probe, variation in output amplitude of your generator, noise in your acquired signal (eg. from the generator, RFI or ADC in your scope/DMM), plus whatever I forgot.Quote from: Mechatrommer on June 10, 2011, 06:37:27 amI would try to change the generator's amplitude while keeping the frequency constant, to get a feel for the linearity of your detector as a function of the amplitude. Of course this may also be frequency dependent.Quote from: Mechatrommer on June 10, 2011, 06:37:27 amNot sure what you mean. The input impedance will be reduced at high frequencies due to the capacitive loading, which will change loading by something like 25% at 100MHz with 10pF of capacitive loading and proper 50 ohm termination. This may change the behavior of your DUT.Quote from: Mechatrommer on June 10, 2011, 06:37:27 amCH2 is independent of CH1 in this regard, so it will still show 2VDC. CH1 should show about +/- 1.4V by the way if it was attenuated by -3dB, since these are amplitude ratios, not power.Quote from: Mechatrommer on June 10, 2011, 07:27:20 amIt's hard to tell in my opinion. You're basically dealing with a generator with unknown output amplitude as function of output frequency, a detector with unknown frequency response, and a scope with unknown frequency response. As with any calibration, you need some sort of known quantity for reference. The scope may not roll of by -3dB at 100MHz, it will likely have some extra bandwidth to compensate for manufacturing tolerances, changes with attenuator settings and changes with temperature. For example, for the TDS-220 I measured about -1 to -1.5dB at 100MHz at room temperature, depending on vertical settings.

author: Mechatrommer
author_rank: Super Contributor
timestamp: June 13, 2011, 11:24:10 pm
content: thanx alm. i will re-study later on this.