Analyzer/tracer for the output signal phase of an amplifier - diyAudio
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Old 17th March 2011, 04:04 AM   #1
tryalx is offline tryalx  United States
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Default Analyzer/tracer for the output signal phase of an amplifier

Hi everybody,

I am looking for an analyzer or curve tracer that can plot the phase/gain (up to MHz range) at the output of an amplifier. I would very much appreciate any recommendations! How would such a device be called, maybe I can google it?

Thank you.
Alexandru
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Old 17th March 2011, 02:03 PM   #2
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try looking for "Network Analyser"

Something perhaps like an HP3585A as you are asking for 1MHz + bandwidth, these are 20Hz to 40MHz Spectrum and Network Analysers.
Manual from: http://www.home.agilent.com/agilent/...ect=SR.GENERAL


Mik

Last edited by audiomik; 17th March 2011 at 02:23 PM. Reason: add possible instrument type
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Old 17th March 2011, 10:51 PM   #3
tryalx is offline tryalx  United States
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Hi Mik,
Thank you for your reply. I will look into the network analyzers. I thought there would exist some equipment specialized for audio amplifiers that can plot the gain/phase curve at the output. I would like to see how the phase of the signal looks close to and beyond the unitary gain frequency.

Alexandru
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Old 22nd March 2011, 01:14 PM   #4
cliffyk is offline cliffyk  United States
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Velleman's PCSU1000 and PCGU1000 pair, or their PCSGU250 combo instrument, can produce automated bode plots of the response and i/o phase relationship of nearly any device.

Here's the response (solid lines) and phase (dashed lines, vertical center is 0) of a small 15Wpc amplifier I designed, at 3 level settings (100%, 75%, 30%) with loudness contour enabled:

Click the image to open in full size.

The PCSU1000/PCGU1000 combo can plot over several spans from 1kHz to 5MHz, with a number of start frequencies (1Hz to 100kHz depending on span); and has a special 10Hz to 30kHz audio range. The PCSGU250 has similar bode plotting capacity with a 1MHz upper limit IIRC.

The system can automatically scale the oscilloscope's gain to obtain a maximum dynamic range is 75dB, not spectacular but not bad. This is limited by the 'scope's vertical gain range (5mV to 2V/div) and sensitivity, and the DUT's signal capabilities.
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Old 23rd March 2011, 11:00 PM   #5
tryalx is offline tryalx  United States
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Thank you! The PCSU1000/PCGU1000 combo is a nice tool. I wish the frequency generator would go up to the 30Mhz range. I am mainly interested at looking at the phase of the output signal from 1.5Mhz up when the gain becomes unitary.
Would the software that comes with the PCSU1000 oscilloscope give you numeric values of the difference in phase between the two inputs of the oscilloscope? In that case I could use a "manual" frequency generator that I have and combine it with the PCSU1000.

Thank you!
Alexandru
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Old 24th March 2011, 06:55 PM   #6
cliffyk is offline cliffyk  United States
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The PCSU1000 can do that (as can many DSOs).

Here's a PCSU1000 screenshot of a 3kHz signal on CH1, and the same signal passed through a simple RC filter to create a phase shift on CH2:

Click the image to open in full size.

The phase display is on the left, showing CH1 32.8 "ahead" of CH2, and CH2 being 32.8 "behind" CH1. The peak-peak and RMS levels of each channel are also shown; these could be used to determine when you have achieved unity gain.

Note that the RC filter has reduced the magnitude of the filtered signal (CH2), this would of course not be the case with your unity gain scenario.

Here are the same signals displayed on a Lecroy WaveJet 322, with the period of CH1 and the "skew" of the two signals being measured:

Click the image to open in full size.

The WaveJet displays the skew of 35.3μs, and period of 333.3μs. To convert skew and period to phase in degrees = 35.3/333.3*360 = 38.13.

In the last here's what these signals look like on a Lecroy 9354L:

Click the image to open in full size.

With phase displayed directly in degrees.

These were created at 3kHz as that was convenient to the RC filter I had at hand--there would be no difference in methodology at 3MHz...
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Old 24th March 2011, 09:03 PM   #7
tryalx is offline tryalx  United States
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Thank you for taking the time to explain me in such a detail, and for the pictures that very nicely demonstrate the approach!
I looked on my Rigol DS1102 oscilloscope and it doesn't provide the automatic measure of the skew parameter. Yet, I found the option to enable two pairs of cross hairs which I can position on each of the channels. This way I can derive the skew value and compute the phase with the simple formula you mentioned.
I am happy with this solution for moment, although it would be nice to have a plot as you posted previously.

Thank you,
Alexandru
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Old 24th March 2011, 11:01 PM   #8
cliffyk is offline cliffyk  United States
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Yup, using cursors to measure the skew will work nicely...

Also I just looked at the Rigol DS1102 specs, it looks as though you have CH1->CH2 Delay measurements for both the rising and falling edges--those are the same as "skew":

Click the image to open in full size.
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Old 25th March 2011, 06:40 AM   #9
tryalx is offline tryalx  United States
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You are a genius, the Delay1 -> 2 gives me exactly what I need! I guess I haven't fully explored what my Rigol DS1102 can do.
I have measured a delay of 80ns at 2Mhz when the gain becomes unitary, which gave me 57.6 degrees ... that is pretty good.

Thank you so much for your support!
Alexandru
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