Sony SPP-930, Руководство по обслуживанию

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A passive loop filter is employed to connect the synthesizer to the VCO. This tuning voltage may be observed at the ATE test point
connector. This voltage should nominally be set to 2.0V for the base and 1.2V for the handset by adjusting the oscillator frequency
with the variable capacitor.
The loop filter cutoff frequency is set to about 1kHz to allow relatively fast power-up times.
2.2.5 IF Amplifier Stage
There is only one stage of discrete IF amplification. Transistor Q6 is used as an amplifier with 330
Ω
input and output impedance. The
rest of the IF gain is provided by the FM demod IC discussed below.
2.2.6 IF Filtering
The choice of l0.7MHz as an IF frequency, allows the use of relatively inexpensive filters. Two ceramic filters are used to achieve the
desired adjacent channel suppression. Two different bandwidth filters are used, 230kHz and 150kHz, so that any shifting in the
passband does not reduce the bandwidth excessively.
2.2.7 Demodulator, Data comparator, RSSI Comparator
This RF board uses a MC13156 FM demodulator. It incorporates all three of the above functions into a single IC.
The Quadrature circuit is made up of L13, C87, C136 and C48, C87 is a variable capacitor and allows tuning of the circuit. The
Quadrature voltage may be observed at the ATE test point connector. This voltage should nominally be 2.2V for the base and 1.2V for
the handset when a signal is center tuned.
The recovered audio signal from the demodulator has a peak to peak amplitude of approx. 0.5V (for 50kHz p-p modulation). One
path from the recovered audio port is filtered through a lowpass data filter and passed back into the data comparator. The output of the
data comparator is fed into the MPU for processing.
The RSSI current from the demodulator is fed through a variable resistor which sets the threshold voltage for the RSSI comparator
inside the IC.
2.3 Transmit Section
2.3.1 TX Amps and Attenuator
There is one transistor which provides the necessary gain for the transmit section. Transistor Q1 amplifies the signal from the TX
VCO to apply the correct output power into the antenna. (-5dBm for both handset and base)
Transistor Q3 provides a switchable attenuator to control the output power during power up or channel change before the PLL locks
onto the correct channel.
2.3.2 TX VCO
The basic operation of the TX VCO is the same as the RX VCO, except for one detail. The TX VCO is also modulated by the transmit
voice and data through a second varactor in the tank. The audio deviation is adjusted to a nominal value of 50kHz p-p by adjusting
R119.
2.3.3 TX Synthesizer/PLL
The TX PLL is combined into one IC with the RX PLL. See above.
The loop filter cutoff frequency is about 100Hz. This allows the data and audio modulation to include frequencies down to about 100
Hz. The power-up time of the TX PLL is not critical.