26-11-2013, 12:25 AM
I have just given this a little thought.
The demodulator is no more than an AM detector and the RF produces DC as expected with a blank carrier.
The project was designed in the days when a X10 scope probe was beyond the means of the home constructor.
X1 is the setting and the fact that the demodulator is a voltage doubling type is another clue as affordable scopes only had a little bit of gain and the doubler was needed in those days.
With the probe set to X10 first.
So if you modulate the carrier with a signal generator and sweep the audio up and down the carrier will produce a constant DC level with a very high modulation frequency a little more DC will appear and as you sweep down the DC will slowly get what looks like ripple on rectified mains and at a much lower frequency it will become a sine wave.
Set the audio so that it looks like rectified mains again.
Set the sensitivity of the scope down by 10 (0.01V/cm to 0.1V/cm for example) and then set the probe to X1 and see what happens. If what happens makes you very confused. Congratulations you have just got the scope set to the correct input impedance for your probe!
The capacitor across the output of the probe is set for a 1M ohm scope input impedance and there is not enough loading to discharge it, you will just get DC. It looks like they tested it with audio filters connected and this means the capacitor value may be a little high unless the sawtooth rate is quite low and the scope they used had an old radar tube in it.
Now it just needs a little bit of tinkering with the capacitor value and sawtooth frequency to suit a modern scope tube.
The demodulator is no more than an AM detector and the RF produces DC as expected with a blank carrier.
The project was designed in the days when a X10 scope probe was beyond the means of the home constructor.
X1 is the setting and the fact that the demodulator is a voltage doubling type is another clue as affordable scopes only had a little bit of gain and the doubler was needed in those days.
With the probe set to X10 first.
So if you modulate the carrier with a signal generator and sweep the audio up and down the carrier will produce a constant DC level with a very high modulation frequency a little more DC will appear and as you sweep down the DC will slowly get what looks like ripple on rectified mains and at a much lower frequency it will become a sine wave.
Set the audio so that it looks like rectified mains again.
Set the sensitivity of the scope down by 10 (0.01V/cm to 0.1V/cm for example) and then set the probe to X1 and see what happens. If what happens makes you very confused. Congratulations you have just got the scope set to the correct input impedance for your probe!
The capacitor across the output of the probe is set for a 1M ohm scope input impedance and there is not enough loading to discharge it, you will just get DC. It looks like they tested it with audio filters connected and this means the capacitor value may be a little high unless the sawtooth rate is quite low and the scope they used had an old radar tube in it.
Now it just needs a little bit of tinkering with the capacitor value and sawtooth frequency to suit a modern scope tube.
