Larcan FM-250, Technical Manual

Page: 45 / 86

FM-250 FM BROADCASTING EXCITER
Issue Date: 21 Jan 2003
d) High pass filter
The output from the Limiter circuit is connected to IC20/a and IC23/a , which form the High
pass filters, removing any low frequencies below 10Hz . This is done to remove any dc
switching transients contained in the incoming audio program.
e)
Pre emphasis
The outputs of the High pass filters are routed to the pre emphasis circuits IC20/b and IC23/b
which have a time constant of 50uS (LK2 and LK3 In) or 75uS (Links out ). The pre
emphasis is selected by operating Relay RL2, which requires an external s/c between PL3
pins 1 and 2 . R104 and R129 are provided for setting the required input level to the
Stereo coder section (see stereo coding section).
f)
15kHz Low pass filters
The Left and Right 15kHz filters are identical and only the Left will be discussed.
The filter has three distinct sections and was designed for minimum ripple and group delay .
•
The first section, IC21/a and IC21/d , is configured as a notch filter centered at 23.8kHz .
The notch frequency can be adjusted by R108 .
•
The second section, IC21/b and IC21/c , is configured as a notch filter centered at 19kHz .
The notch frequency can be adjusted by R113 .
•
The third section, IC22/a and IC22/b is configured as a fifth order 19kHz low pass filter.
Adjustment is provided by C101 .
The filter is adjusted by monitoring TP4 , which is the output of a differential amplifier IC29 ,
with very high common mode rejection. This unique feature enables very simple set up of the
15kHz filters by allowing the
Left or Right filter to be measured individually and also
allowing simple differential measurement with both Left and Right together . This is
very important when setting up the Mono into Stereo calibration in the final Exciter.
The adjustment is made simple by setting the notches as described and adjusting the trimming
capacitor for a flat response at 15kHz (with respect to 1kHz). The response of the filter
should be as follows when adjusted:
1kHz 8kHz 12kHz 15kHz 19kHz
≥
23kHz
0dBm +0.15dBm +0.1dBm 0dBm -40dBm -40dBm
The amplitude variance of this filter is very small and should not present any problems in set up,
unlike other filters found in competitive equipment.
g)
Stereo coding and switching
IC6/a and crystal XL1 form the reference oscillator . F ine frequency adjustment is via C38 . The
oscillator operates at 4.256MHz and is buffered by IC6/b .
The square wave output is then divided by 8 (via IC7/a) to give a frequency of 532kHz .
The 532kHz clock signal is applied to IC8/a and IC8/b (dual 4 bit shift register) which forms a
14 step multiplexer via IC9 and IC10/a , allowing a count of 7 (532khz/7 = 76kHz) . Each