Rigol MSO5000 - Bode Plot curiosity

[6SN7WGTB]

I am looking at a single transistor gain stage - BC546C.

10mV RMS signal in at 1kHz initially, 9V single supply.

Gain is 18dB. Nowhere near any sort of clipping. As good as flat in audio range (1µ decoupling front and rear). Inverting as you would expect.

Thought I would try a Bode Plot of gain and freq response - initially just 20Hz - 20kHz. "in" is CH1, "Out" is CH2, and G1 SG is enabled when I do the sweep.

The gain 'curve' reports correctly as flat and indeed 18dB (and FWIW if I do sweep up to e.g. 2MHz, gain drops off as I'd expect).

The phase angle plot has multiple spikes where it changes from -180˚ to +180˚. Which is not correct. As I watch the sweep of CH1 an dCH2 happening behind the Bode plot they confirm the output remains inverted.

What am I doing wrong?!

[Martin72]

Pics please...

[6SN7WGTB]

Here you go...

[mawyatt]

See if the DSO has Phase Wrap or Unwrap feature.

Best

[6SN7WGTB]

See if the DSO has Phase Wrap or Unwrap feature.

Best

Sorry to appear ignorant, but I have no idea what that is?

[Martin72]

See if the DSO has Phase Wrap or Unwrap feature.

It doesn´t, looked in the manual.

[mawyatt]

Your DUT is an inverting amplifier, meaning the AC response is delayed by 180 degrees. What you are seeing on your DSO is the flipping back and forth between +-180 degrees, because there is no difference between +180 and -180 degrees this is called Phase Wrapping (recycling between + and - 180 degrees). Some Bode functions have the ability to Un-Wrap the phase, so it's a continuous function going thru 180 degrees and onto 360 degrees. Very annoying to not have the feature as you can see with your Phase Plots.

Best,

[6SN7WGTB]

Thank you - that makes complete sense, and yes it’s a very distracting artefact.

[BRZ.tech]

See if the DSO has Phase Wrap or Unwrap feature.

Best

Hello.
I have an MSO5074.
And when using BODE PLOT, a small green square appears on the PHASE line, which is the GM(0º).
In the case of the photo, it gives GM(0º): -17.86dB | 1.002kHz.
The question is: what is it for and how to use this GM(0º)?

All the best.
BRZ.tech

[mawyatt]

We don't have this DSO, have a SDS2000X+.

So best to check manual, but a "guess" is this means Phase Margin.

Google Phase Margin, rather than trying to explain here.

Edit: As TopQuart mentioned, likely it's Gain Margin rather than Phase Margin!!

Best,

[TopQuark]

See if the DSO has Phase Wrap or Unwrap feature.

Best

Hello.
I have an MSO5074.
And when using BODE PLOT, a small green square appears on the PHASE line, which is the GM(0º).
In the case of the photo, it gives GM(0º): -17.86dB | 1.002kHz.
The question is: what is it for and how to use this GM(0º)?

All the best.
BRZ.tech

My guess is Gain Margin (GM), the brother to Phase Margin(PM). GM(0º) of -17.86dB | 1.002kHz probably means the gain curve is 17.86dB above unity gain (0dB) when phase plot crosses 0 deg at 1.002kHz, you can see if that's true in your plots.

No idea what your circuit under test looks like, but for loop response of a closed loop system, like a power supply, phase margin (PM) above 45 deg (some may argue you should have above 50-60 deg) , GM below -15dB (ideally -20dB), gain plot crossing 0dB line at roughly -20dB/decade means you have a stable system.

Edit: I realise you were referring to the photo a few posts above, looks about right. Though the GM measurement don't really tell you anything in this situation.

Edit Edit: Actually I suspect the scope is confused by the phase wrapping and thinks the point when the phase curve "crosses" 0 deg is the GM frequency, when in fact there's not a real crossing occuring at 1kHz, but just an artifact of how the phase plot was plotted. Slightly misleading in my opinion.

[BRZ.tech]

Hi.
Thanks to mawyatt and TopQuark for their input.

I looked at the MSO5000 user manual and it just mentions GM(0º) but doesn't say what it means or what to do with it...

My question was from reply #2. Where 6SN7WGTB placed the photos of his curiosity about a signal amplifier with the Transistor BC546C... The 10mV signal enters through the Base and is removed amplified by the Collector of the transistor.
Although 6SN7WGTB didn't put the schematic circuit, it should be very simple...

I suspect that the Output Signal Phase should be 180° for the entire measured frequency band. And don't make that green W shape on the stage.

I suspect this is an MSO5000 Firmware Failure.

TopQuark said:
...GM(0º) of -17.86dB | 1.002kHz probably means the gain curve is 17.86dB above unity gain (0dB) when phase plot crosses 0 deg at 1.002kHz, you can see if that's true in your plots....

But that W in the green color doesn't exist in theory... because the Gain is 17.86dB in the whole passband, not just at that point of 1.002kHz.. The little green square crosses the Gain Straight at 45 degrees of Phase, and at 5 different points.

All the best,
BRZ.Ttech

[mawyatt]

Should note that Phase and Gain margin refer to a Closed Loop System when referencing the Open Loop Response. For a classic Common Emitter amplifier this PM & GM doesn't generally apply.

Best,