Multitone FLA 872, Technical Manual

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TLA 852/FLA 872
TM1205 Issue 2 Page 3 - 5
The output from the RF amplifier passes through an image filter which provides these
functions:
a) Attenuation of signals outside the required band.
b) Attenuation of the first image frequency.
c) Matches the output impedance of the RF amplifier to the input impedance of the
mixer stage.
L401 and CV407 form a parallel tuned circuit with the resonant frequency set to the
carrier frequency.
L407 forms a series resonant circuit which provides a notch filter at the image
frequency.
The local oscillator comprises transistor TR410, crystal XL402 and associated
components. Frequency adjustment is by CV405. The local oscillator frequency is
calculated as follows:
fx = carrier frequency minus 21.4MHz
where fx is the crystal frequency in MHz.
C452, C453, C494 and R439 provide decoupling of the supply rail to give good
adjacent channel performance. The local oscillator signal is coupled to the mixer
circuit by C465.
The mixer circuit, TR403 and associated components, receives the RF output of the
image filter. The RF signal is mixed with the local oscillator output to produce a
21.4MHz product. This is matched to the filter, FL401, by L406, C495 and R412.
FL401 has a bandwidth of ±7.5kHz at the 3dB points.
The output from FL401 is matched by C485, R444 and L410 to IC401 pin 24. IC401
is a combined IF mixer / local oscillator / detector.
The second oscillator runs at 20.945MHz and, with the exception of XL401 and its
tuning capacitors C407, C408, is built into the IC. As the second oscillator is not
trimmed, small frequency errors are absorbed when tuning the first local oscillator.
The input at pin 24 is applied to the internal second mixer, and is mixed with the
second oscillator output to produce a 455kHz 2nd IF signal at pin 3. The IF signal
then passes through two ceramic filters FL403 and FL402 which remove unwanted
mixer products and provide further selectivity.
The output from FL403 is split: one output connects through C444 to pin 6 of IC401
and the IF amplifier; the other optionally connects to the Received Signal Strength
Indicator (RSSI) circuit in IC401 through C454, R426 and pin 5.