MIC AMP, LPF, MODULATOR

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 bit.

OSCILLATOR/MODULATOR

OSC/MOD TOP VIEW

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.

Phasing

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 17KB

BACK TO HF TRANSCEIVER PAGE