Bode plot with Siglent scope - Page 1
Forum section: electronics / testgear

author: Mike99
author_rank: Frequent Contributor
timestamp: September 01, 2025, 04:52:36 pm
content: Sorry if this is a daft question ...I'm using an SDG2202X-E with an SDG2042X AWG to measure the frequency response of the line input on an audio amplifier.The Bode function gives two options for display: Vout and Vout/Vin. The results are quite different (see photos).When I connected the AWG directly to the scope the trace was flat across the frequency range apart from the small bump below 100Hz. If the input level to the amplifier doesn't change, why isn't Vout/Vin flat across the range?Mike

author: D2236
author_rank: Regular Contributor
timestamp: September 01, 2025, 05:52:33 pm
content: “Vout/Vin” mode is the true transfer function from the amp’s input node to its output, and that’s where the amplifier’s own frequency-dependent design choices show up.In other words, the lack of flatness isn’t an artifact of the scope, it’s evidence of the actual input/output transfer function. That’s the trace you want if your goal is to characterize the amplifier itself.

author: Mike99
author_rank: Frequent Contributor
timestamp: September 01, 2025, 07:34:40 pm
content: Thank you. I was trying to verify the amplifier spec which simply says "10Hz to 50kHz +/- 0.5%".Mike

author: Martin72
author_rank: Super Contributor
timestamp: September 01, 2025, 08:55:36 pm
content: I once quickly and roughly measured a digital amplifier (Noobsound), once at idle and then with a 4-ohm load.The Bode plot function is well suited for this purpose.

author: Mike99
author_rank: Frequent Contributor
timestamp: September 02, 2025, 09:18:18 am
content: Similar shape to mine. That's encouraging!Mike

author: tszaboo
author_rank: Super Contributor
timestamp: September 02, 2025, 11:47:38 am
content: To answer this, you have to tell us a bit about your settings and cabling.Sometimes it's the output impedance, and input impedance of things (even at these frequencies) can cause issues. Best is to make a drawing with the setup, and write down how much input or output impedance you expect at each nodes. And make sure you include your scope probe because it's not 10MOhm in AC>

author: mawyatt
author_rank: Super Contributor
timestamp: September 02, 2025, 03:08:39 pm
content: Siglent has done a good job with their Bode (FRA) implementation, and hopefully will continue with ungraded functions and features. Spending some time to understand Bode (FRA) theory of operation, characteristics, and features can pay long term dividends for the user. For the best Bode results, especially at higher frequencies (120MHz with proper AWG), the signal source should be measured directly or as close as possible to the DUT input. Since Bode is a plot of Vout/Vin, where Vin is the DUT input and Vout the output, getting Vin as close as possible wrt to the true DUT is important for quality results.Remember that most BNC cables and DSO probes have a delay of 5~10 ns/meter. A typical 1000mm BNC cable (RG58) at 10MHz has a phase delay of ~20 degrees (if we did our math correctly). This effects both Vin and Vout, so using equal cable types/length/probes for sensing Vin and Vout can help offset this delay effect and improve results.Best

author: tszaboo
author_rank: Super Contributor
timestamp: September 02, 2025, 06:22:47 pm
content: 120MHz?? No matter what test setup I was trying, it always fell apart above ~20 MHz, giving more than 1dB error. I tried 50 Ohm terminators, scope probes, T pieces, BNC to scope adapter, nothing really works.I think it's the ground loops.If you have tricks, please share it.

author: mawyatt
author_rank: Super Contributor
timestamp: September 02, 2025, 08:11:34 pm
content: Quote from: tszaboo on September 02, 2025, 06:22:47 pmNo "tricks" we can disclose!!Here' what we get with a SDG2000X AWG and SDS824X HD and 3104X HD DSOs. Connections were just BNC cables with a "T" at the DSO, short BNC cable to other channel with one channel of 3104X set to 50Ω, other High Z. With SDS824X HD we used a cheap 50Ω Terminator on CH1 which likely is some of the reason for the peaking around ~50MHz (note the scale differences).This illustrates one of the advantages of having a precise 50Ω DSO termination which is available on the higher end DSOs. We're sure the SDS800 would perform better with a quality 50Ω terminator....if we had one!! Also a proper 50Ω splitter would be better than the BNC "T", however we don't have one of these eitherAnyway, we're hoping Siglent is "Listening" and may consider a "Calibration" where the cabling, probes, setup fixture, AWG, and DSO artifacts can be compensated by proper calibrations before measurements. The hardware seems good enough to extend the range/performance it seemsBest

author: TimFox
author_rank: Super Contributor
timestamp: September 02, 2025, 08:56:14 pm
content: Quote from: Mike99 on September 01, 2025, 07:34:40 pmDid that spec read “+/- 0.5%” or “+/- 0.5 dB” ?

author: tszaboo
author_rank: Super Contributor
timestamp: September 02, 2025, 09:23:18 pm
content: Quote from: mawyatt on September 02, 2025, 08:11:34 pmBy the looks of it, your measurement also suffers from the same phenomena at high frequency. At 50 MHz you have the same 1dB error in the transfer function, and at around 20MHz it starts to deviate from the ideal curve. Maybe my memory was failing me with the" 20MHz 1dB"I have a pair of Phillips 9585 terminators for the scope, maybe the next one will have it inside... it's a good terminator, when I measure S11 from the scope I get flat response to 100+ MHz, so it's not that. And let's remark that the best terminators are scope specific.Anyway, thanks a setup like this is good for audio for sure.

author: Mike99
author_rank: Frequent Contributor
timestamp: September 03, 2025, 08:43:39 am
content: Quote from: TimFox on September 02, 2025, 08:56:14 pmIt was +/-0.5%. I'm using cables, not probes, so the scope input was set to X1.Mike

author: Grandchuck
author_rank: Super Contributor
timestamp: September 03, 2025, 03:36:02 pm
content: @tszaboo tried 50 ohm terminators, so I can't help. However, thought I'd post two results from a Siglent 2000X HD scope. Notice the difference between Hi Z and 50 ohms. I also threw in a sweep from a Rigol DSA815.

author: Performa01
author_rank: Super Contributor
timestamp: September 07, 2025, 09:48:42 am
content: Even though the Bode Plot has been covered in my review already (Reply #9, 3rd topic “Bode Plot at a glance”) and frequency response as well as accuracy have been checkedhttps://www.eevblog.com/forum/testgear/sds800x-hd-review-demonstration-thread/msg5293771/#msg5293771I still want to provide another measurement, this time with external 50 ohms termination at the scope inputs.I should remind everyone, that we cannot simply get 50 ohms inputs from a cheap scope that doesn’t provide it right from the start. Even with the very best external termination, the input impedance will still be 50 ohms in parallel with 17 pF. By contrast, serious scopes with true 50-ohm inputs provide a separate input path for that and there is no significant input capacitance. As a consequence, these more serious instruments, starting at the SDS1000X HD, will have an input VSWR <1.5 up to their upper bandwidth limit and even above.Yet for low frequencies up to some 100 MHz, the external termination can be good enough in many cases. Here is the Bode Plot from 1 kHz up to 120 MHz with ancient hp10100C terminators at the DSO inputs.SDS824X_HD_Bode_0dBm_120MHzAs can be seen, Amplitude plot is flat within ±0.1 dB and phase error is less than 5 degree up to 100 MHz.Of course, these are the results of a proper measurement, which also requires a proper setup. In this particular case, it is very simple: a quality 50 ohm sma power splitter Suhner 4901.19.A, fed by a 60 cm sma microwave cable with a quality sma/BNC adapter for connection to the SDG6052X AWG, which served as a source here. The two outputs are connected to the DSO inputs via two identical (except for manufacturing tolerances) 20 cm sma microwave cables with sma/BNC adapters and said ancient hp10100C through terminators.Btw, the AWG doesn’t play a role in this scenario, as long as it can provide the 120 MHz sine waves with sufficient amplitude.

author: Performa01
author_rank: Super Contributor
timestamp: September 10, 2025, 09:06:50 am
content: I thought I should show how the Bode Plot behaves on a scope with proper 50 ohms inputs.For this, I’ve grabbed the trusty old SDS2504X HD and used the exact same setup as above (for the SDS824X HD), except for the external through terminators. I’ve used the internal 50 ohms path instead.SDS2504X_HD_FRA_0dBm_120MHzOf course the SDS2504X HD is a much more serious DSO than the SDS800X HD, which also means, among other things, that it can provide true 50 ohms inputs. And it shows.S21 drops by less than 0.05 dB at 120 MHz and phase error is less than 2.5 degree.As a verdict, this measurement proves that the only limitation to the usable bandwidth is the probing. With inadequate, sloppy probing, the usefulness of the Bode Plot will be limited to low frequencies.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 10, 2025, 09:01:41 pm
content: Could you show me the setup screen for the SDS824X HD for the post above the last?For fun, here's a plot of a resonant piezoelectric device I've been experimenting with. Note that I'm using a current probe for channel 2.

author: Performa01
author_rank: Super Contributor
timestamp: September 11, 2025, 07:44:50 am
content: Quote from: Conrad Hoffman on September 10, 2025, 09:01:41 pmSure, see attachment. Please note that I had to set the generator level to +6 dBm to compensate for the 6 dB loss in the resitive power splitter.Quote from: Conrad Hoffman on September 10, 2025, 09:01:41 pmYes - it's essential to be able to easily analyse narrowband structures nicely like this, isn't it?