Ramsey Electronics FZ-146 Specifications download pdf (Page 122)

FX 146 • 122

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 +8V

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.

The purpose of Q11 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 Q12 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 Q13 which had been supplying 8

volts to all "+8R" points of the circuit. And the output of U4C switches on

PNP Q14 to supply all "+8T" points.

Diodes D11 and D12 assure positive action, that Q13 and Q14 are fully on

or fully off when the op amp outputs swing. Releasing the mike button

instantly reverses the status of Q12, Q13 and Q14 to return to receive mode.

R70 limits the current drawn by D17 to a safe level. This LED usefully

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Ramsey Electronics FZ-146 Specifications Page 122