TM1215 Iss. 6.0 RPR750
.2.1 Radio Board RF Section (VHF/UHF)
he RPR 750 Series VHF/UHF radio is a dual conversion superhet configuration. The
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he output of the RF amplifier passes to the Image filter, which attenuates the 1
st
Image
408, L401 and CV403 form a further parallel tuned circuit, which is again adjusted for
0dB
he local oscillator (LO) is formed by XL401, TR406 and associated components. C417
s
he mixer circuit, comprising TR405 and associated components, receives the output
it
he output of FL401 is fed into IC401, to be mixed with the second local oscillator. This
are
f
his signal is then passed through filter FL403, to remove any unwanted products and
.2.2 Radio Board RF Section (HF)
he RPR 750 Series HF radio is a single conversion configuration. The RF signal is
erial AE1 is tuned and matched by CV401 and banded components, C434, C453 and
s
he local oscillator is formed by crystal XL401 and TR406, plus associated components.
he crystal frequency is (Channel Frequency - 455kHz)/2.
3
T
incoming RF signal is detected by the loop antenna AE401/402 and is finely tuned by
capacitor CV401. C433 and C435 provide matching to the RF amplifier formed by TR4
and TR404, which provides in the order of 10dB gain.
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frequency, signals outside of the required band and also matches the output of the RF
amplifier to the mixer stage. L405/406 and CV402 form a parallel tuned circuit, which is
adjusted to have minimum loss at the frequency of operation.
C
minimum loss at the frequency of interest. Additionally L401 and C408 form a series
resonant circuit, which is close to the 1
st
Image frequency. This gives approximately 4
attenuation at the Image frequency.
T
and L403 tune the collector of TR406, to the third harmonic of XL401. This circuit provide
the correct Local Oscillator frequency and reduces the level of other harmonics. For the
UHF receiver, further harmonic reduction is achieved by tuned circuits L408, C440 and
L402, C410. Fine frequency adjustment is achieved by CV404.
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from the Image filter and mixes it with the output from the local oscillator, to produce a
45MHz (UHF) or 21.4MHz (VHF) product signal, which is selected by the resonant circu
of L407 and C439. The resultant signal is then fed into xtal. filter FL401.
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runs at 44.545MHz (20.945MHz for VHF) and comprises XL402, associated tuning
components and IC401. The frequency of this oscillator is not variable, so any errors
compensate for by the tuning of the first LO. The output from FL401 is fed into pin 20 of
IC401 and mixed with the LO output, to produce a second IF signal at 455kHz on Pin 3 o
IC401.
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provide further selectivity. The output of FL403 is then fed back into the IF amplifier of
IC401, on Pin 5.
3
T
received by the ferrite + loop aerial AE1, which is mounted on the Decoder board and
connected to the Radio board via PL1/SK401.
A
C435. The incoming signal is matched to the base of TR403, which in conjunction with
TR404 forms a cascade rf amplifier with a gain of 15 - 20dB. Tuning and matching of thi
amplifier is provided by L405/406, CV402, C455, C450, C405, C452 and C406. The
output of this stage is coupled to the mixer stage of IC401, via C419.
T
This circuit doubles the frequency generated by XL401 and the circuit formed by L403 and
C417 selects the second harmonic. The resultant signal is fed into IC401 via C451.
T
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