Ramsey Electronics FX-440 Manual page 126

_Stage
M:
Microphone Amplifier and PTT Circuit
If you have studied all preceding circuit explanations, you have a good " '
idea of what the Microphone and PTT circuitry is supposed to
accomplish. Understanding our design clearly and assembling it correctly
will save many headaches and will ensure reliable FX transceiver
operation.
U4 is a "quad op amp" which means 4 operational amplifiers in one DIP
package. Two are used as a conventional microphone gain amplifier, and
the other two are used in the PTT (push to talk) circuit.
Capacitor C83 couples microphone audio to U4A and isolates the audio
(AC) from the PTT circuitry (DC). Op amps are designed to run from both
a positive and a negative voltage source. U4 is powered by a single
+
av
supply through the use of a voltage divider network (R59, R40). The gain
of the amplifier is established by the ratio of R56 to R58. A passive low
pass filter is formed by R51 and C89. The B section of U4 and its
associated components form an active low pass audio filter. The output
of U4B is fed through C62 to modulate the VCO control voltage as
explained in Stage E-F. Trimmer R46 permits adjustment of modulation
level.
I
The purpose of 011 is to shunt the microphone circuit straight to ground
during receive, so that it cannot possibly disturb the VCO. An accessory
modulation input is provided at PC-board point •PL• for direct injection of
DTMF or CTCSS tones, etc.
The PTT circuit is designed to accommodate the popular ICOM-
compatible speaker-mikes. Notice that a single line at J4 serves both
audio and PTT functions. The one shielded wire into the microphone
. takes care of not two but THREE functions which could involve three
conductors and a more complex jack
Three functions? First, we need to supply audio output from the
microphone element to the amplifer. Next, we need some kind of PTT
switching connection. Third, the electret microphone itself needs a small
amount of voltage to operate its internal FET source follower transistor.
Here's how we do it with one mike line. Pushing the button simply
connects the microphone element to the line. About 2 volts through R60
and R57 operate the microphone element which sends audio through
C83 to U4A. PNP transistor 012 senses the tiny current draw of the
microphone element and switches the 8 volts at the emitter through to
the collector. To state it very simply, the output of U4 turns off PNP 013
which had been supplying 8 volts to all •
+
BR" points of the circuit. And
the output of U4C switches on PN P 014 to supply all •
+
BT" points.
Zener diodes 011 and 012 assure positive action, that 013 and 014 are
fully on or fully off when the op amp outputs swing. Releasing the mike
Assm. 64
· FX-440