Ramsey Electronics FZ-146 Specifications download pdf (Page 26)

FX 146 • 26

Stage M: Microphone Amplifier and PTT Circuit

U4 is a LM324 quad op amp: 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). 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 adjusts 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. One shielded wire into the microphone

handles THREE functions. First, we need to supply audio output from the

microphone element to the amplifer. Secondly, 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. Zener 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 "TX" indicator D17 to a safe level.

The PTT circuit may also be activated at pin 3 of the Packet I/O jack. A

direct short to ground is not necessary. The author noted very positive PTT

action with resistance as high as 100K from pin 3 to ground.

Stage TX: Transmitter Buffer, Driver and Final

The transmitter section, Q10, Q9 and Q8, is conventional VHF RF circuitry

that has proven quite reliable in Ramsey FM transceivers. Just a few circuit

notes are in order.

Transistor Q10, the transmit buffer, amplifies the VCO output from C56 to

about 10 milliwatts, quite sufficient for checking modulation and PLL

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Ramsey Electronics FZ-146 Specifications Page 26