RF Technology R50
Page 11
components between FIL405 and U401 form the matching network for the
filter.
U401 is an IF receiver IC which includes mixer, oscillator, amplifiers and
discriminator. It down-converts the 2
nd
IF to the 3
rd
IF at 455kHz, which is
filtered by ceramic filters FIL402, and FIL403, before the signal is
demodulated(discriminated). The discriminator uses ceramic resonator X401
as its 455kHz reference.
X402 is a crystal used to create a 21.855MHz oscillator that becomes the 3
rd
LO. The
frequency can be varied slightly by varactor D406, and thus it can be adjusted
by the 3rd_LO_ADJ voltage. This is done as part of calibration.
Pin9 of U401 is the recovered audio output. C411, C412, C413, R441, R444 and
U402A form a 5
th
order ellyptical high pass filter with 3dB cut-off at 7.5kHz.
Any “signal” at such frequencies is assumed to be noise, as exciters sharply
attenuate all signals above 3kHz. U402B amplifies this FM “noise” signal, and
D403 and C438 act as a peak detector. The voltage on C438 is proportional to
the square root of the noise energy. U402C works as a comparator which
asserts the low active digital signal, N_DET, if the noise voltage is higher than
the D/A output, NSQ_SET.
Pin8 of U401 is the other recovered audio output which can be muted by pin5.
Capacitor C484, and D407, ensure that the audio remains muted until the
+10V is at the right voltage. The discriminated audio signal is amplified by
U605A and then fed into the Audio Section (Sheet 6).
Pin7 of U401 is the Radio Signal Strength Indicator (RSSI) output. U402D is a buffer.
The RSSI voltage is converted to a digital value by one of the A/D inputs of the
MicroController.
5.5 Voltage Contr olled Oscillator (Sheet 5)
There are two similar VCO circuits in this sheet, the difference between these two
VCO is component values, so only one VCO circuit is illustrated in this
section.
Q501, C532, C533, C534 and L501 form a Colpitts oscillator, the frequency of the
oscillator is decided by the tuning voltage of varactor D501. The capacitance
of D501 is in series with C531, and this total capacitance is in parallel with
C501 and also with the series connection of the feedback capacitors C533 and
C534. The net capacitance forms a tank resonance with the, high Q, air coil
inductor, L501.
The bias for varactor D501 comes from the loop filter of the 1st PLL. Q503 forms a
gyrator circuit which reduces noise that might be injected from the power
supply.
The parameter, Vgs
(off)
, of Q501 can vary from 1 to 4V. D503, R517 and C519
provide some AGC, in order to reduce the dependence of the output level on
this parameter.
MMIC amplifier MA501 provides about 20dB gain for the VCO signal and feeds it to
the PLL chip.
MA502 and MA503 provide about 30dB gain of the VCO signal to drive the LO port
of the double balanced mixer MX401 at a nominal level of +7dBm.
NB that Q506, R547, R531, L508, C510, C540, R537, and R536 are not fitted. Place
for these components has been provided on the printed circuit board to allow
