Quote:
Originally Posted by crackle
The TP301 shows a nice wave form of about 100mV albeit very condensed, (tight)
The test on Q520 was not so good, can I confirm the collector is the single pin on the left in this instance.
All I get there when I key on USB is a slightly fuzzy trace, but so sign of modulation with my voice. see trace here
Mike |
U503 appears to be the balanced modulator. Q523 manufactures the carrier signal using the crystal Y500 and a varactor diode D507 to pull the frequency.
This is the first step in making SSB.
Q520 should be amplifying/buffering the output of the balanced modulator. At this point there should ideally be NO carrier. U503's balanced acton should null it by cancellation. Talk into the mike and the waveform on Q520 should look to be a right mess. Put in a pure tone to the microphone input and look with a spectrum analyser and you should see two tones, 1kHz above and 1kHz below the carrier frequency, and a little bit of carrier leaking through.
This is a DSB-SC signal.... Double sideband, suppressed carrier.
It now needs to go to a narrow crystal filter to remove the unwanted sideband to turn it into a single sideband signal, then it needs to pass through stages of linear amplification on the way to the antenna.
David