Ramsey Electronics FT146 - Manual (Page 5)

FT146 • 5
and IC chip) give the equivalent of about 130 or more transistors and diodes.
And, in addition to 13 inductors, a crystal and the various plus and jacks, there
are over 60 capacitors and resistors. Surely, all that should result in a decent
transmitter! You could easily spend twice the money plus hours of time trying to
gather the equivalent parts from catalogs and still need to make your own circuit
board.
FT146 CIRCUIT DESCRIPTION
Basic overview: The FT146 is a crystal controlled FM transmitter that uses a
varactor modulated crystal oscillator followed by a 9 times frequency multiplier
and power amplifier. Test points are built-in for easy alignment.
Detailed description: Transistor Q1 functions as a Colpitts crystal oscillator
whose frequency is determined by Y1 and varactor diode D1. Transistor Q2
functions as a buffer amplifier to isolate the crystal oscillator from other portions
of the circuit. The crystal oscillator frequency is multiplied by 3 (tripled) in
transistor Q3. Frequency multipliers are nothing more than amplifiers that
produce lots of distortion! In this case we're interested in having enough
distortion so that the third harmonic is fairly strong. We "pick-off" or filter the
third harmonic with a band pass filter, comprised of L9,13 and capacitors
C28,21,22,16. This allows transistor Q4 to be driven only by the third harmonic
of the crystal frequency - in this case, around 48 MHz. Q4 is another tripler,
multipling up the 48 MHz to 144 MHz. Inductors L5,11 and capacitors
C25,17,18,10 for the band pass filter for the three times output frequency.
From here on out, we're working at the actual carrier frequency and use a
couple of transistors to amplify the signal up to a 4 to 6 watt level. Transistor Q5
boosts the signal to the 250 mW range and Q6 then produces the full power
output. Impedances must be matched between stages to allow for maximum
power transfer, and that's the function of a couple of coils that are hand wound.
A low pass filter follows the final amplifier to limit out of band signals (remember
those multiplier stages?). Modulation is accomplished by varying the
capacitance of varactor diode, D1. This varying capacitance shifts the frequency
of the crystal ever so slightly causing a frequency shift, which is FM or
Frequency Modulation. And yes - this frequency shift does get multiplied as it
travels through the multiplier stages. The signal used to vary the varactor diode
is our desired audio modulation. Op-amp U1 functions as a microphone
amplifier, clipper and low pass filter. We clip the microphone signal to prevent
overmodulation and limit the maximum modulation frequency since either one
could cause our transmitter to "splash" into adjacent channels.
To make our transmitter compatible with standard ICOM/YAESU style
microphones we use transistor Q7, which senses when current is being drawn
by the microphone. When the mike is keyed, the current drawn turns on Q7
which applies bias to transistor Q1, allowing it to operate and thus the rig goes