Operations Manual

RF Technology R50 Page 15
5 CIRCUIT DESCRIPTION
5.4 Receiver Section (Sheet 4)
voltage. OUTD (pin15) is used to set the LCD bias voltage, this feature is not used in
this version. Communication between U301 and MicroController U204 is via the
serial bus.
U302 is a dual channels PLL chip, X301 is the reference for both PLL channels, PLL
channel 1 is for the 1st VCO, channel 2 is for the 2nd VCO. C315, C317, C327,
R316 and R317 are components of loop filter for the 1st VCO, C316, C328, C329,
R330 and R331 are for loop filter of the 2nd VCO. U303A, U303D are buffers that
send the bias voltage of two VCOs to MicroController U204.
U304A, U304B convert the F0LD of the PLL chip to a square wave by a division of
100, and send the signal to MicroController U204.
5.4 Receiver Section (Sheet 4)
RF input signal (25MHz-50MHz) is filtered by a high pass filter (C401, C402, C403,
C407, C415, C416, C417, C419, C420 and L401, L404, L405, L406) and a low pass
filter (C404, C405, C406, C421, C422 and L402, L403, L407, L408). Q401 is the
front-end amplifier, capacitors and coils between Q401 and C440 provide additional
filtering for RF signal amplified by Q401. 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 a good image and spurious rejection. In
R50 receiver, the 1st IF frequency is 246MHz.
Q402 is used to amplify the 1st IF signal, FIL401 is a SAW filter which provides an
excellent spurious rejection for the 1st IF. The filtered IF signal is then fed to a double
balanced mixer MX402, the output of MX402 (the 2nd IF) is amplified by Q403, the
2nd IF frequency is selected at 21.4MHz to avoid the interference from in band RF
signal.
Q404 is a gate of the noise blanker, the gate is controlled by the impulse noise
detector.
FIL404 and FIL405 are two 21.5MHz crystal filters which can be equivalent as a four
pole crystal filter, the filer provides minimum 40dB attenuation at adjacent channel
frequency. C448, C477, L423 and components between FIL405 and U401 are
matching network for the filter.
U401 is IF receiver IC which includes mixer, oscillator, amplifiers and discriminator.
The frequency of oscillator is 21.855MHz, it can be tuned by the 3rd_LO_ADJ
voltage.
The product of mixer is 455KHz, build in amplifiers of U401, FIL402 and FIL403
provide sufficient gain and adjacent channel rejection for 455KHz IF signal to be
discriminated.
Pin9 of U401 is recovered audio output, C411, C412, C413 and R441, R444 and
U402A are working as a high pass filter to eliminate the audio frequency, U402B
amplifies the FM noise signal, D403 and C438 convert the noise level to a DC voltage.
U402C works as a comparator, the offset voltage can be set by the OUTC of U301.