MT-MR PLATINUM. POINT TO POINT AURAL RADIO RELAY SYSTEM
30
4.4.- Receiver Performance.
This device receives the RF modulated carrier signal incoming from antenna at assigned frequency within VHF or
UHF band, filtering it in first place in order to cancel all spurious RF components whose frequencies are below or
beyond the assigned frequency sub-band, and amplifies the resulting RF carrier signal in order to increase its level
to those required to drive the Mixer stage situated at PLL/VCO unit, keeping Receiver’s Noise figure as low as
possible. This unit is composed in first place by a three-section bandpass filter, followed by an active amplifier stage
composed by a low noise RF transistor working in class C common emitter configuration. Omce carrier is amplified
in this stage, it is filtered again by another three bandpass sections having a slightly norrower bandwidth than first
sections.
Then, signal is amplified again by a twin-stage cascaded pre-amplifier in order to raise its level to the required for
driving the mixer stage.
As said before, after RF signal crosses through output bandpass filter section, it is amplified again by the second
pre-amplifier stage, composed by two cascaded transistors, also working in common emitter, class C configuration.
These two transistors increases signal level to adequately the low-noise double-balanced Mixer stage.
Bandpass filter sections are factory-adjusted. Tuning of these sections are very critical, being its
adjustment very sensitive. Only a slight misadjustment can change the sensitivity , selectivity and noise
characteristics of the whole receiver. It is not advisable to re-touch the tuning capacitors at field under any
circumstances, unless some very slight readjustments that are indicated ahead in this Manual, only in case
of changing the receiver’s operating frequency
.
In case of any problem with pre-selector filter unit, damaged unit must be replaced with a new one, previously
adjusted at Factory for the sub-band employed, and the damaged unit must be sent to the OMB Laboratories for its
analysis and repair.
Once receiving frequency is programmed, as indicated ahead in Microcontroller Unit’s performance description, we
can proceed to energize receiver, adjusting receiving frequency in the RF Generator. If receiving frequency is
exactly adjusted, the frequency (obtained with a frequency counter) at IF TEST terminal of IF Processor &
Demodulator Unit will be exactly 10.7000 MHz. If a slight difference exists, it can be corrected with a little re-touch
of TCXO’s fine frequency adjustment capacitor using a non-inductive adjustment tool, in order to cancel this small
difference. Correct procedure is as follows:
-Connect unmodulated RF Signal Generator (it must be a
precision
device having a frequency error not greater
than 0.9 ppm) at Receiver’s antenna input connector. Adjust level to receive –40 dBm and frequency to the nominal
receiving value.
- Connect the input of a Spectrum Analyzer in the terminal <
IF TEST
> at Mixer & IF Unit. Check the purity of
unmodulated IF signal over the whole frequency spectrum. Now connect the Digital Frequency Meter at the same
test point, reading IF value, and performing the indicated corrections as required.
- Check now if Receiver’s sensitivity is within its normal value, decreasing input RF signal’s amplitude until reach
the noise threshold value. If sensitivity is lower than originally, it’s allowed to re-touch slightly, over the same point,
the variable tuning capacitors of input bandpass filter section at RF Active Pre-selector unit, in order to optimize
both sensitivity and selectivity until its normal values are reached. Once this procedure is finished, re-check IF value
at test point <
IF
TEST
> as before.
