Larcan FM-250, Technical Manual

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FM-250 FM BROADCASTING EXCITER
Issue Date: 21 Jan 2003
operate Leds D46, D45, D44 and D43 , which indicate which switch selection is in
operation.
The four inputs to the switch are as follows:
•
Temperature. R47 is used to calibrate this input function.
•
Current. R52 is used to calibrate this input while R51 sets the offset null (0 Amps).
•
Forward power. R55 is used to calibrate this input which is converted from a Square law
scale to a Linear scale via IC7/b and temperature compensated diode arrangement D78
and D79 .
•
Reflected power. R62 is used to calibrate this input which is converted from a Square
law scale to a Linear scale via IC7/c and temperature compensated diode arrangement
D80 and D81 .
d) Stereo decoder
The Stereo decoder input is derived from the MPX input on PL1 pin 3 , which is decoded via
IC14 (Stereo decoder chip). The IC provides both Left and Right decoded outputs on pins
11 and 10 respectively. The IC operates from a 456kHz ceramic resonator, which provides
very good stability while requiring no adjustments. Stereo separation of 40 dB (Typical) is
obtained.
The 19kHz Pilot detection is obtained via pin 18 of the Stereo decoder chip, and is a open
collector output (open when pilot detected). This output is inverted by TR3 to activate Led
D67 (Pilot) and is also routed to the MPX VU Meter via TR4/LK1 . The detected pilot
signal is also routed via D83 to PL1 pin 6.
The Left and Right outputs are then filtered by Fourth order Low pass networks IC13/a,
IC13/b, IC13/c and IC13/d , which remove the 19kHz and 38kHz products produced in
the Decoding process and prevent errors in the VU meter operation.
e)
VU Meters and peak detection circuits.
The Left and Right outputs from the Stereo decoder are routed via R28 (Left meter level)
and R37 (Right meter level) which are used to calibrate the VU meters.
Two identical circuits are used to convert the incoming audio signals to a peak dc level. This is
done by IC4/a, IC4/b, IC4/c and IC4/d. Taking one of the sections only for description
purposes the circuit operates as a
precision full wave peak detector which has a very fast
attack time (1.7mS) and decay time (650mS), conforming to the DIN 45406 Specification .
This calls for a response 1dB down from steady state for a 10mS tone burst and 4dB down
for a 3mS -tone burst. The decay time is 20dB in 1.5 sec’s .
The VU meters operate on a standard VU scale and the bargraph driver used to do this is
calibrated internally to achieve this. IC5 and IC6 form the Left and Right bargraph drivers,
which are operated in dot mode but configured in bar mode by connecting the Leds in series.
This is done to reduce current consumption. The sensitivity of the bargraph driver is 1.2V.
Leds
D23 to D32 form the display for the Left channel with Leds D33 to D42 for the Right
channel.
The Right VU meter bargraph driver has a inhibit function at the input. This is provided such
that when the unit is operated in Mono the customer can choose to have the Left meter