A couple of comments:
1. This is a good DIY solution to measure passives. +/-10% is good enough as I typically want to be in the ballpark (sanity check).
2. There are several bumps in the transient response - this would indicate low phase margin, but this behavior is not observed on the voltage waveform. Are we exceeding the capability of the current probe? This is me really asking for less of a 'ripply' response.
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A couple of comments:
I can compile a BOM and ultimately compare costs to PicoTest.
The combination of Q1-3 + Q5 is a normal cascode circuit, nothing so unusual. It lets the collectors of Q1-3 see a low impedance for high speed. Just the choice of Q5 is odd to me: the MJE182 does not look like especially fast. My choice would be more like BD135, 2N2219, 2N3019.
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What are your trying to measure ?
Hello Jay_Diddy,
Picotest has an application note on using the output impedance and Q of the response over frequency. I believe the purpose when measuring output impedance over frequency, one wishes to find the Q of the response and this is related to phase margin. Picotest pushes this as non-invasive measurement technique and frankly would be very helpful with customer designs when trying to determine stability in a production prototype or released system. Who wants to cut traces, which one may not even have access to in a 10 layer board, when one can measure output impedance.
I am simple person so Bode and load transients are my tools. When I grow up, I will try the output impedance method and/or Nyquist.
Hello Jay_Diddy,
Picotest has an application note on using the output impedance and Q of the response over frequency. I believe the purpose when measuring output impedance over frequency, one wishes to find the Q of the response and this is related to phase margin. Picotest pushes this as non-invasive measurement technique and frankly would be very helpful with customer designs when trying to determine stability in a production prototype or released system. Who wants to cut traces, which one may not even have access to in a 10 layer board, when one can measure output impedance.
I am simple person so Bode and load transients are my tools. When I grow up, I will try the output impedance method and/or Nyquist.
This is really my point. If you have a power supply that that has a very dominant single pole, it doesn't have a measurable Q so you can't determine very much about the stability.
If you have a two pole system, the smaller the phase margin, the higher the Q and easier it is to determine stability with the current injector. But that isn't a good design anyway.
I am interested in trying to measure the stability non-invasively. I will try one day …
Regards,
Jay_Diddy_B
Hi,
two comments about this:
- the NISM method is, strictly mathematically speaking, *only* correct if you have a second-order system, i.e. *one* dominant pole.
Picotests has mentioned this (not very prominently) in their product description. Complex multipole systems - watch out
IIRC, the method works on Q(Tg), i.e. the stepness of phase change at a pole.
As long the poles are well apart, and steep enough, OK. If not - see above.
IIRC, Sandler said the method is exact for 2nd order systems (which is correct). In return, it is *not* exact for higher order.