INS-42037-1: FMC88108-6
User’s Manual
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18
8. Technical Details
8.1 Translator PCB Assembly
a)
Band Pass Filter, Mixer and 10.7MHz IF Filter.
The VHF (87.5
– 108MHz) RF input is routed from
SK1
to a bandpass filter which is tuned using varactor diodes
D1
and
D2
.
L1
and
L2
can be used for fine tuning. The tuning voltage for the varactor diodes is derived from the local
oscillator locking voltage at
IC2/B
pin 7
and can be monitored at
TP1.
The output of the bandpass filter is routed to the mixer (
MX1
). The mixer is a passive double balanced type with a local
oscillator drive level of -76dBm from
IC3
. The output of the mixer is routed to a 10.7MHz IF bandpass filter.
The 10.7MHz bandpass filter consists of three ceramic filters (
FL1, FL2 and FL3
) that have low group delay and
excellent attenuation characteristics. Each filter has tuning capacitors at the input side (
C10, C15 and C20
) to obtain
optimum stereo distortion and separation characteristics. Each filter is buffered by a J-Fet high impedance amplification
stage to recover the insertion loss of the individual filters (approximately 6dB). The filters are terminated with optimum,
220 Ohms resistive loads. The complete filter gives attenuation of approximately 20dB at ± 200kHz offset and 60dB at
± 300kHz offset. Spurious response is better than 75dB. The high Q of the ceramic filters ensures excellent
performance and stability compared to normal LC filters.
b)
Rx Frequency Synthesizer with Local Oscillator.
The Rx synthesizer consists of a single loop with a divide by 65 pre-scaler
IC4
. A 6.4MHz reference crystal is utilized
with a parallel loaded synthesizer chip
IC5
. This reference is divided internally by 8192 to 781.25Hz. This mode of
operation provides frequency steps of 50kHz via the 12 way DIP switch
SW1
. (64 x 781.25Hz = 50kHz). The LD output
on
IC5
provides a lock detect function and is routed to
IC13/A
. LED
D8
provides the Rx lock function indication.
The local oscillator (
TR5
and buffer amplifier
IC3
) is routed via the pre-scaler (
IC4
) to the programmable divider in the
synthesizer (
IC5
), whose division ratio is set by DIP switch (
SW1
). The DIP switches set the local oscillator frequency
between 76.8MHz and 97.3MHz, which is 10.7MHz lower than the required receive frequency (87.5MHz - 108MHz).
The local oscillator output from
IC3
is also routed to mixer
MX1
.
The phase comparator output of
IC5
(PD-OUT) has a 3-state output and is applied to a loop integrator and loop filter
(
IC2
), whose DC output in turn controls the frequency of the local oscillator via varactor diode
D4
. (Control sensitivity is
approximately 2MHz/Volt). A +5V regulator
IC6
is provided for the Rx and Tx synthesizers.
c)
IF Demodulator (10.7MHz).
The 10.7MHz input is routed to pin 15 of
IC7
. The 10.7MHz IF demodulator is a simple, single tuned quadrature type
(
IC7
). The balanced output of the demodulator at pins 8 and 9 contains the demodulated baseband signal. This
provides common mode rejection and good S/N ratio, typically
≥70dB.
C61
is provided to optimally tune the
demodulator for minimum second harmonic distortion. A DC voltage proportional to the field strength is provided on pin
13 of
IC7
and is adjusted via
R41
. This output is routed to comparator
IC16/B
and the switched DC output can be
monitored at
SK2 pin 1
. The balanced baseband output from the demodulator is routed to the signal processing circuits.
d)
Baseband Amplitude and Phase Correction with 100kHz Low Pass Filter.
The baseband signal from the IF modulator is routed to a balanced unity gain amplifier stage (
IC8/A
). This is then
routed to a low pass filter stage (
IC8/B
) in conjunction with a band reject filter (
IC9/B
). This stage also provides the
amplitude correction for the IF filter and is adjusted via
R53
to achieve maximum stereo separation at 1kHz. A group
delay correction circuit (
IC9/A
) is also provided for optimal stereo separation at 15kHz and is adjusted via
R58.
The baseband output from
IC9/A
is routed via
R62
(Rx baseband level adjust) to
RL1/A
and
RL1/B
for audio switching
purposes. Relay
RL1/C
is switched via
SK3 pin 1
(s/c to ground) and this allows the Rx baseband to be switched off
and allow the BIF input, on
SK3 pin 4
, to be activated (to accept an external audio input). These signals are then routed
to the Tx modulator circuits.
