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Frequency response using 2-ch Scope
If you have an oscilloscope and a sine generator but the generator output amplitude is not will regulated, you can stillĀ do frequency response tests if the scope has two input channels. The key to this is simply knowing that when the 3dB-down point is reached, the phase shift of the output signal compared to the input signal will be 45-degrees.

Connect the sinewave generator to the amplifier input and connect one probe from the scope to the amplifier input, as well. Connect the bench load to the amplifier output along with the second scope probe. It does not matter what the gain of the amplifier is, just set the channel sensitivity for the scope to be appropriate for the end of the amplifier that channel is monitoring. ideally, both waves should be as close to full-scale on the display as possible.

Turn on the amp and see the waves be essentially overlapped at low-frequencies. As you sweep the frequency of the generator higher, the waves will eventually begin to spread apart.

If the scope has an X-Y function, then you can generate a LissajousĀ figure, commonly exploited for RF work. The problem with this technique is that the shape of the resulting ellipse depends on BOTH the amplitude of the two waves and their phase difference, so it works best if the generator output is highly stable, which we assumed was questionable at the beginning oft his discussion. if it is stable, checking output amplitude of the amplifier is much simpler, as described in another post here. The other problem with the Lissajous method is that when 45-degrees phase difference is achieved, the ellipse is a very specific ratio of diameters rather than being, say a straight line, as it is for 0-degrees or 180, or as a perfect circle at 90-degrees and 270.

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