Larcon FMT-24 Technical Manual Download

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FMT- 25 VHF FM TRANSLATOR 862-09725

Issue 1A:

100kHz

(IC13/B).

The total filter is flat (

0.25dB) to 95kHz and gives

≥

20dB attenuation at

150kHz.

No adjustment

of the filter is necessary.

The baseband output from buffer IC15 pin 6 is routed via

R112

SK5

(Rx baseband

output) this output, whose

level

is adjustable via

R111

, is used for external use. The

output is also routed to

LK4/A

, which allows connection to

RL5/B

for internal loopback

purposes to the Tx modulator. When internal loopback is used

LK5

can be inserted and

SK6

(Tx baseband input/diversity) can be used for ’standby’ receiver baseband input.

LK4/B can be used to switch the outputs for different switching configurations.

Stereo Decoder circuit. (Audio monitor and Pilot detection).

The output from

IC15

is routed via

R113 (Decoder level adjust),

which sets the optimal

input level to

IC16

(Stereo decoder). The Stereo decoder chip provides some interesting

internal features such as

Image low pass filtering

on input at

114kHz

(3x38kHz),

Ceramic resonator

(XL1) tuning

(no adjustment of oscillators),

Pilot detection

with

pilot

cancel

circuit.

The stereo pilot signal, when present at the input, forces pin 18 ‘high’ via

R119

which in

turn switches on

TR8

and provides a Telemetry signal for external use on

SK17

pin 13.

The Audio outputs are provided at

pin 11 (Right)

and

pin 10 (Left)

IC16

. The

outputs are de-emphasized via

C129

and

C130

and are routed to stereo headphone

buffers

IC17/A

and

IC17/B

. the outputs are routed to

SK7

(Headphone output).

♦

Transmitter Circuits operation.

Synthesizer and modulator

(87.5MHz-108MHz)

The synthesizer consists of a single modulus loop with a divide by

pre-scaler

(IC6).

12.8MHz TCXO

(IC7)

provides an accurate and stable reference for the synthesizer. The

reference is then, divided by two by

IC30/A

6.4MHz

. This reference is applied

direct

the fixed divider in the synthesizer (

IC18).

This divides by

8192

to obtain a

comparison

frequency

781.25Hz

. This mode of operation provides

frequency steps of 50kHz

via

the 12 way DIP switch

SW3

. (64 x 781.25Hz = 50kHz).

The

output on

IC18

provides a lock detect function and is routed to

IC21/A

for

Telemetry and Metering purposes.

LED D17

provides the

Tx Lock function

indication.

The modulator/oscillator

(TR11

and Buffer/amplifier

IC23)

is routed via the

pre-scaler

(IC22)

to the programmable

divider in the synthesizer

(IC18),

whose division ratio is set by

DIP switch

(SW3).

The DIP switches set the

Oscillator Frequency (87.5MHz-108MHz).

The Phase comparator output of

IC18

(PD-OUT) has a 3-state output and is applied to a

Loop integrator

(IC19)

and

Loop filter (IC20),

whose dc output in turn, controls the

frequency of the

oscillator

via Varactor diodes

D23

and

D24.

(Control sensitivity is

approximately 2MHz/Volt).
Diodes

D25

and

D26

provide the modulation function and the dc control voltage tuning the

oscillator is also applied to these diodes. This dc voltage is offset via R160 and R161,
which equalizes the deviation level across the 87.5MHz to 108MHz band.
The Loop filter provides

≥

40dB of attenuation at frequencies greater than 30Hz. This is

done to ensure that the Stereo modulation, phase and amplitude are uninfected by the
Synthesizer loop to provide excellent stereo separation, even at 30Hz. The modulation is
inserted via

IC24

and

R167.

SK6, SK8 and SK9

provide the input to the modulator.