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| Rigol MSO5000 - Bode Plot curiosity |
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| TopQuark:
--- Quote from: BRZ.tech on January 29, 2023, 01:47:16 pm --- --- Quote from: mawyatt on January 02, 2023, 08:28:29 pm ---See if the DSO has Phase Wrap or Unwrap feature. Best --- End quote --- 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 --- End quote --- 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, |
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