MICROPHONE AMPLIFIER, LPF AND BALANCED MODULATOR
MIC AMPLIFIER AND LPF
The microphone amplifier has a gain of more than 20dB. The amplifier is followed by an
2.7KHz active low pass filter. A dual op-amp is used for both stages. The choice of op-amp
is not critical. I use an LF353 but most dual op-amps should work in this circuit.
MIC AMP PCB TOP VIEW
Like all the other circuit boards in this transceiver, the PCB was drawn by hand. I used double-sided
PCB. All ground connections are made to the top PCB foil (Ground plane). The holes for any component
leads that are not grounded, must be reamed out (countersunk?) with a suitable tool. I used a 3mm HSS drill
OSC/MOD PCB Top view
The carrier oscillator/balanced modulator unit is very similar to the circuit used in the 80M SSB rig.
All transistors are 2SC710's. Any general purpose NPN transistor with an ft of a couple of hundred MHz should be
suitable. I used BC547's in the 80M rig. The transformer is 6 turns, trifilar wound on a medium to high permeability
Details of transformer phasing.
I tried several types of diode in the balanced modulator. OA91 and 1N34 point contact diodes worked well.
1N4148 silicon diodes also gave rood results. I decided to use the 1N4148's, as they are more readily
Wav file of my first test of the transmitter:
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