Rx Squelch; Transmitter; Figure 2-2. Transmitter Block Diagram - Motorola CP185 Basic Service Manual

Theory Of Operation: Major Assemblies

2.2.1.3 RX Squelch

The mute (squelch) circuitry switches off the audio amplifier when no audio is detected from the
recovered audio. The squelch circuit main components are U202 & U201.
U202 will adjust the squelch circuit sensitivity depending on Noise level from recovered audio. Noise
level is amplified by internal amplifier of U201 to help U202 decide the squelch circuit sensitivity. If
the noise level is over the set threshold, the microprocessor mutes the radio.
Note Perform squelch tuning after any RX part replacement. Refer Chapter 5.6: Receiver Tuning
on page 5-8.
2.2.2

Transmitter

The radio's TX Power Amplifier system is a three stage amplifier which is able to amplify the VCO
output up to the permitted maximum transmit power levels (UHF2: 4W, VHF: 5W).
LPF
TX VCO output signal passes thru a 3 dB, pie style resistor, attenuator before going into the TX
power stage acting as isolation between the low power VCO and high power amps. The next stage
consists of a pre-driver (Q401) and a driver amplifier (Q402). The TX RF signal (UHF2 : -4 dBm, VHF
: -3 dBm) from the attenuator is amplified to +25 dBm (UHF2) or +28 dBm (VHF) by the pre-driver
and driver amp. This is followed by the final PA, an enhancement-mode N-channel MOSFET device
(Q403), which provides a 12 dB gain.
The fnal PA draws current directly from the DC battery supply voltage input via L413.
The PA matching network consists of C416 – C422 (UHF2) or C417, C418, C420, C455 (VHF) and a
strip line, which matches the TX Power impedance to approximately 50 ohm. Antenna switch is
shared between TX and RX circuit. In TX mode, PIN diodes (CR401, CR301) are forward biased
which enable the High Power RF signal to pass through the antenna. In RX mode, both diodes are
off. Signals applied to the antenna jack are routed, via the Harmonics LPF in to the RX circuit. The
High Power RF Signal finally passes through a TX Low Pass Filter, a 7th order Chebyshev filter
(L409 – L411, C426 – C429, C445 – C446).
Strip
CR401
Line
ANT-SW
BATT.
CURRENT
DETECT

Figure 2-2. Transmitter Block Diagram

FINAL DRIVER P.DRIVER
AMP AMP
Q403 Q402
R417 U401
APC
AMP
3dB
Q401
Atten.
2-3
From VCO