Midland Syn-Tech III P25 - Page 17

SYN-TECH III P25 PORTABLE RADIO
SERVICE MANUAL
3-3
receiver-transmitter switch and the 7 pole harmonic filter. Harmonics are suppressed more than 45 dB by
the filter.
The power control circuits monitor the current drawn by the driver and power amplifier by the voltage drop
across R503 (R506). This voltage is compared to the SQUELCH_POWER_TUNE (S_G_ADJUST) and
the difference signal is applied to the transistors’ gates to regulate the output power.
3.2.1.3 VCO Circuits
The Voltage Controlled Oscillator (VCO) circuits are designed separately as a receiver VCO (RX VCO)
and a transmitter VCO (TX VCO). The TX VCO circuit operates at the channel frequency, 136-174 MHz
(380-470 MHz). Since the first IF is selected as 45 MHz, the RX VCO circuit operates at F
chan
+ 45 MHz,
181-219 MHz (F
chan
– 45 MHz, 335-425 MHz).
The Colpits oscillators are supplied approximately 3.5V which is filtered from the REG5V source. The
3.5V is switched on by the RX_ON or TX_5V (sourced from TX_ON) signal such that only one oscillator is
running.
When RX_ON switches to 3V the RX VCO circuits begin to operate. The oscillating signal is amplified by
TR301 and low pass filtered to suppress unwanted signals. The signal is applied to the LO terminal of the
first mixer and the R_VCO signal is also fed back to the synthesizer IC. The synthesizer IC controls the
RX VCO frequency by changing RX_VCO_TUNE (CONTROL) until the desired frequency is achieved,
F
chan
+ 45 MHz (F
chan
- 45 MHz).
When TX_ON switches to 3V the TX VCO circuits begin to operate. The oscillating signal is amplified by
TD402 and TD401. The signal is now the proper amplitude and is applied to the amplifier circuits. The
T_VCO signal is also fed back to the synthesizer IC. The synthesizer IC controls the TX VCO by changing
TX_VCO_TUNE (CONTROL) until the desired frequency is achieved, F
chan
.
3.2.1.4 Synthesizer Circuits
The synthesizer circuits are composed of the Temperature Controlled Crystal Oscillator (TCXO), dual PLL
synthesizer integrated circuit, loop filter, 5V and 3V voltage regulators. The frequency synthesizer
includes a phase detector, a current mode charge pump, as well as a programmable reference divider
and a feedback dual modulus frequency divider.
The VCO frequency is established by dividing the crystal reference signal down via the reference divider
to obtain a frequency that sets the comparison frequency to 2.5 kHz or 3.125 kHz depending on whether
the VCO frequency is a multiple of 2.5 kHz or 3.125 kHz. This reference signal is then presented to the
input of a phase/frequency detector and compared with the divided VCO signal, which was obtained by
dividing the VCO frequency down by way of the feedback counter.
The phase/frequency detector measures the phase error between the reference signal and the divided
VCO signal and outputs control signals that are directly proportional to the phase error. The charge pump
then pumps charge into or out of the loop filter based on the magnitude and direction of the phase error.
The loop filter converts the charge into a stable control voltage for VCO. The phase/frequency detector’s
function is to adjust the voltage presented to the VCO until the feedback signal’s frequency and phase
match that of the reference signal. When this “phase-locked” condition exists, the VCO frequency will be
N times that of the comparison frequency, where N is the feedback divider ratio.
The loop filter is optimized for fast locking low VCO noise, such that typical lock times are 10ms and 20ms
for receiver and transmitter synthesizers, respectively, at full range frequency jumps.
A two-point modulation technique is used to obtain a flat modulation response (±0.2dB amplitude ripple at
0-3kHz AF range). The TX_AUDIO_1 and TX_AUDIO_2 signals, which are applied to TXVCO and TCXO
simultaneously, are compensated by means of digital potentiometers to maintain the flat modulation
response across the RF range.
The TCXO provides the 14.850 MHz reference frequency used by the synthesizer and also the third
harmonic 44.550 MHz second LO frequency used by the demodulator.