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Reducing Noise in Gain & Phase Measurement of DC-DC Converter's Control Loop
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bendras:
I am trying to measure gain and phase of the control loop of a DC-DC buck converter (see below for images depicting the setup).
The problem is that DC-DC converter's switching frequency (around 500kHz) gets coupled into the input and output sine waves which I am trying to measure (see below for images of the affected signals). 
I have tried moving measurement points away from the main inductor and using differential probes but that does not seem to help much if at all.

Does anybody know of a way to eliminate or at least reduce pickup of such noise?


Sketch of the Setup:



The Actual Setup:







Noisy Signal:


Noisy Signal Zoomed In:
boB:
I think that a real gain-phase analyzer will have a swept filter as well as tracking generator so that would I think help a lot.

That's most likely a good reason that an oscilloscope is not used for this purpose.  Not saying that it can't be done though.

boB

Edit:   For example.....

http://www.ridleyengineering.com/hardware/ap310-analyzer/intro-ap-analyzer.html

The AP310 is a frequency response analyzer designed specifically to make sensitive gain-phase measurements in high noise environments, and is used to measure the following:

Transfer Functions
Loop Gain and Phase
Output Impedance
Audiosusceptibility or PSRR
Input Impedance
Measure large and small signals with its advanced analog and digital signal processing. Receive rugged and reliable results in all environments, including challenging power supply applications.

Measure individual passive power components to accelerate your design process, including:

Power capacitors
Power Inductors
Power Transformers
Leakage Inductance
Winding capacitance

HOW IT WORKS
The Frequency Response Analyzer is a sophisticated instrument that injects a sinusoidal test signal and measures the response of a system to that frequency on two return signals. It makes swept frequency response measurements that give magnitude and phase data plotted verses frequency. The frequency sweeps can be either logarithmic (for Bode-type plots) or linear.

A rugged and easy-to-use Windows interface allows the plotting of relative or absolute signal amplitudes. The analyzer uses the latest in analog and digital signal processing technology that provides an affordable, high-performance, high-productivity measurement instrument.
KIKi:
Did you try soldering GND wire on the bottom layer (put GND wire on other side of the PCB)? You should move GND wire as far as you can from the SW pin (inductor).
capt bullshot:
The probing technique looks quite inappropriate.
You should use the usual grounding method for the scope probes: Remove the ground clip, connect the probe GND directly to the circuit boards GND plane and the probe tip as short as possible to the measurement points.
Often done by winding a short piece of stiff wire around the GND sleeve and soldering that to the GND plane while pointing the tip to the test point. That should reduce the coupled switching noise significantly.
David Hess:
The switching frequency should be asynchronous to the swept frequency measurement so trigger the DSO on the swept frequency and use averaging to remove the switching noise.
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