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 bit.
OSCILLATOR/MODULATOR
OSC/MOD PCB Top view
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 ferrite
core.
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 available.
Wav file of my first test of the transmitter: test.wav
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