Operating instructions

Page 10 RF Technology R50
U303A, U303D are buffers that isolate the bias voltage. The Microcontroller,
U204, has both of these buffered voltages connected to two of it’s A/D ports.
U304A, U304B divide the FoLD
of the PLL chip by 100, and send it to a Timer input
pin of the MicroController, U204.
5.4 Receiver Section (Sheet 4)
The RF input signal (25MHz-50MHz) is first filtered with a band-pass filter. This is
implemented with two filters, firstly a high pass filter (C401, C402, C403,
C407, C415, C416, C417, C419, C420 and L401, L404, L405, L406) and then
a low pass filter (C404, C405, C406, C421, C422 and L402, L403, L407,
L408).
After band-pass filtering, the signal is amplified. Q401 is the front-end amplifier.
After amplification, unwanted products generated by the amplifier are filtered.
The capacitors and coils between Q401 and C457 provide this additional
filtering for the RF signal amplified by Q401.
This amplified RF input signal is referred to on the schematic as RF_AMP. It is up-
converted by mixer MX401 to create the 1
st
IF.
It is also amplified by Q405,
and Q406, which are the front-end amplifiers for the Noise Blanker circuit.
The output of Q406 is “detected” by D401, and if the waveform “envelope” exceeds
approximately 3.3V, the output of U403 will go high. U404 is a retriggerable
monostable multivibrator, which produces a pulse at least 2usecs long after
each rising edge detected on the output of U403. If rising edges re-occur
during each pulse, the pulse period keeps extending until they stop.
The output of U404 is level shifted and inverted by Q408 to create the blanking signal
N_BLK. C470, R416, and R414, act to slew the negation of N_BLK, without
affecting the slew rate of the assertion of N_BLK.
U303C provides some AGC for the RF amplifier formed by Q405 and Q406. They
also reduce the intermodulation which is produced by Q405 and Q406.
MX401 is a double balanced mixer which converts the RF signal to the 1st IF. The
frequency of the 1st IF is at least three times higher than the highest RF
frequency to obtain good image and spurious rejection. In the R50 receiver, the
1st IF frequency is approximately 246MHz. The actual frequency depends on
the frequency tuned to, and may vary by as much as +/- 120kHz. The 1
st
LO is
set to be approximately 264MHz above the frequency being tuned to.
Q402 is used to amplify the 1st IF signal before it is filtered by FIL401.
FIL401 is a SAW filter, which provides an excellent spurious rejection for the 1st IF.
The filtered IF signal is then fed to double balanced mixer MX402. The output of
MX402 produces the 2nd IF, which is then amplified by Q403. The 2nd IF
frequency is set at exactly 21.400MHz, to avoid interference from any in-band
RF signals. The 2
nd
LO is set to approximately 224.6MHz. Again the LO
frequency may vary from 224.6MHz by as much as +/-120kHz depending on
which frequency the receiver is tuned to.
Q404 and Q409 gate the 2
nd
IF signal. If the N_BLK signal is high, then the 2
nd
IF is
quenched. N_BLK is generated by the impulse noise detector.
FIL404 and FIL405 are two 21.4MHz crystal filters. Combined they form a four pole
crystal filter. The filter has a minimum of 40dB attenuation at the adjacent
channel frequencies (25kHz channel spacing). C448, C477, L423 and