User Guide

Technical Documentation
NHD–4
System Module
P.A.M.S
Page 4–31
Original 11/97
N704 is biased at pin 3 by approximately 3.40 V, supplied from VRX via
R803. VRX should be about 4.40 V. The RX_LO signal is routed to the
mixer (Z1) through a SAW filter (Z701).
N705 is biased at pin 3 to approximately 2.70 V, from VTXT through R809
and R815. VTXT should be approximately 4.40 V. The TX_LO signal is
delivered to the CDAGCT IC (N100) pin 2, through C795.
The VHF Synthesizer
The second PLL frequency synthesizer is the VHF, generating a constant
180 MHz. The varactor diode also doubles as an FM modulator for AMPS
TX operation. The 180 MHz output of the oscillator is amplified by the
common base buffer amp (V303).
The voltage at junction of R313 and the cathode of the varactor diode
(V301) should always be a constant voltage somewhere between 1.5 V
and 3.0d V. The base, emitter and collector of the oscillator transistor
(V302) should be 2.32 V, 1.73 V, and 2.56 V respectively. The buffer
amplifier (V303) is biased up with 4.30 V at the collector, 3.33 V at the
base, and 2.62 V at the emitter. All these voltages originate from VRX90
which ought to be about 4.40 V at R806.
The ANATX line feeds the AC audio signal from the CDRFI IC (N700) via
C341. When active, this signal can be viewed with an oscilloscope. With
the ST tone active, the ANATX line will have an amplitude of
approximately 250 mVpp at the R317, C341 node. ST (signaling tone)
generates 10 kHz of modulation frequency.