Quote:
Originally Posted by HowardB
Instead of a symmetrical amplitude modulated wave, only the bottom half of the wave is modulated. The top half is flat.
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The Windsor/Taylor 65c has a 1000 pF capacitor between the slider of the fine attenuator (VR2 in the Taylor 65b) and the top of the switched attenuator (R9 in the Taylor 65b). With the attenuator resistance, this capacitor acts as a high pass filter that makes the waveform appear more symmetrical by removing the 400 Hz component.
You could try connecting a resistor (say about 2 kΩ) between the RF Output and ground so that C7 acts as a high pass filter. This might make the waveform look more symmetrical but is unlikely to make any difference to a receiver under test because the circuits are tuned to attenuate frequencies other than the one to which it is being tuned.
The photographs show examples of the modulated waveforms on my Windsor 65c. There is a big variation depending on the frequency with the amplitude decreasing at higher frequencies.
The upper channel is the slider of the fine attenuator and the lower channel is the RF output with the attenuators at maximum. Both channels were set to 0.2 V/div. The last photo shows both inputs at GND so that the negative bias mentioned in post #12 can be judged in the other photographs.
David