Measuring Audio Phase Shift in Audio Amps or Modulators with a Oscilloscope and audio tone generator

1. 1. Set up the oscilloscope for horizontal versus vertical display, i.e. no time base sweep. Hook your audio signal generator output to the horizontal input channel of your scope, and through a resistive divider,  to the stage of speech amplifier at which you will be applying feedback.

2. Hook your vertical scope input to the modulator audio output (modulation transformer loaded secondary).  Audio only into the scope, no High Voltage DC, please. As the audio is a very high level use a resistive divider to scope. Transmitter into a dummy load, please.

3. Set audio generator frequency to mid band, IE  1 kilocycle and increase level to about 50% amplifier or modulator output. Play with scope input gain and phase reversal switches until you get a long straight diagonal line, with the top towards the right and the bottom towards the left.   like this            /

4. Now slowly lower the generator frequency. At some frequency you will notice the scope pattern changing until the straight line opens up into an ellipse. Continue along, reducing the frequency further until eventually you see a  horizontal ellipse.   This is the 90 degree phase shift frequency.       Like this          

5. As the modulator gain will have dropped with this phase shift, if you increase the scope input gain from the modulator output enough, you will see a circle displayed on the scope. Note that this is the low 90 degree phase shift frequency (corner).   Like this      O

6. Lower the frequency more until you eventually see a straight diagonal line, BUT shifted to the opposite axis.  With the top towards the left and the bottom towards the right. This is the 180 degree phase shift frequency. Note this lower 180 degree phase shift frequency.    Like this     \

7. Next, return back to 1 Kilocycle ( mid band ) frequency to reset gain and then increase the frequency noting the 90 and 180 degree phase shift frequencies at the high end.     

Written by Paul, W1ECO        WA2CYT and  K1DEU

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05/20/2003