Ramsey Electronics FZ-146 Specifications download pdf (Page 19)

FX-146 • 19

FX-146 CIRCUIT OVERVIEW

The FX-146 VHF FM Transceiver circuit theory is explained in progressive

stages and in some detail as part of our "Learn As You Build" approach to

electronic kits. Builders are encouraged to study and learn about a stage or

section, build it and then test it before going to the next stage. The circuit

explanations are necessarily written for people with all levels of experience,

starting with and FAVORING beginners.

Following is a straight and "minimally chatty" synopsis or overview of FX-

series technical information provided in the building stages. However, we'll

still follow the same stage-by-stage designations of the building process.

A: DC Power Input

Much of the circuitry operates on the regulated 8 volts supplied by voltage

regulator VR1. "+8R" or "+8T" are points where the regulated 8V output is

switched for Receive or Transmit by the PTT circuitry (Q12, U4c, U4d, Q13,

Q14).

The Receiver IC (U1) and the digital frequency synthesis circuit are powered

by +5 volts regulated by VR2. The op amps used in the circuit (U4 and U5)

operate from this single supply through the use of voltage divider networks

at the respective IC's. The full 12-15 volt input is supplied to the transmitter

RF output section and to the audio amplifier (U2).

Components L20 and C42 provide ignition noise filtering. The 5-pin DIN

Packet I/O Jack (J1) has pinouts corresponding to current conventions for

TNC's. Receiver audio to pin 4 may be taken from the amplified speaker

output, or from the FM discriminator output or from the true FSK data output

of U1. Selection is by a jumper wire on the PC-board.

B: Receiver Audio Amplifier

The LM380 is a self-contained general purpose audio amplifier capable of

over 2 watts audio output with a voltage gain of 50. Audio from from the FM

discriminator (U1) is fed through C7 through the 10K volume control (R7) to

pin 2, the amplifier input. The amplified output at pin 8 is available through

C34 to both the speaker jack and pin 4 of the Packet I/O Jack. C41 in series

with R108 across this amplified output are good practice recommended to

prevent self-oscillation of the IC. Pin 1 is bypassed to ground through C48 in

normal operation.

If pin 1 is grounded directly, the internal bias of the LM380 is upset, and the

amplifier is silenced. Q6 is a simple switch. When 8 volts is applied through

R107 and D22 to the base of Q6, the transistor collector grounds pin 1 of

U2, thus silencing the receiver during transmit. The COR output of U1 (pin

16) also mutes the amplifier.

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Ramsey Electronics FZ-146 Specifications Page 19