[ARC5] SS Cathode modulator.

[J Mcvey]

I think there has to be an active carrier for linear am, so the cathode current current would have to be about 50%?
 


     On Tuesday, October 13, 2015 3:08 AM, AKLDGUY . <neilb0627 at gmail.com> wrote:
   

 I'm pretty sure the 4 transistor modulator will work, with two caveats.

Firstly, when the cathode resistor Q4 is lightly turned on, the voltage
at its collector will rise to a high enough value that may destroy Q4.
There really needs to be a high value resistor from Q4's collector to
emitter to keep this voltage down to 50 or so.
Normally there is a 51K ? resistor fitted in the transmitter to prevent
cathode-heater breakdown when the relay opens the cathode circuit.
The article doesn't mention this resistor. Intending constructors
should ensure that a resistor is fitted, and fit one if not present.

Secondly, there's a great likelihood of RF feedback with this circuit
when Q4 is partially conducting since RF flows in the cathode circuit
and Q4 lifts the cathode above ground by at least 0.6 volts (the latter
assumes Q4 is a silicon transistor and turned fully on). The RF will
almost certainly get back into earlier stages of this modulator.
There needs to be something like a 0.005 uF capacitor from cathode
to ground to decouple RF at that point.

The capacitor together with the 51K resistor forms a low pass filter
and the value of capacitance can be chosen to roll the audio off as
a bonus.

73 de Neil ZL1ANM
______________________________________________________________
ARC5 mailing list
Home: http://mailman.qth.net/mailman/listinfo/arc5
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:ARC5 at mailman.qth.net

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html