Guardian G25AMK005, Technical Manual

Page: 22 / 85

2-12 G U A R D I A N V H F 1 1 0 W M O B I L E
U22 is the reference oscillator temperature sensor used to monitor the temperature near Y2. Its output is labeled
XTALTEMP on the schematic diagram. This line is normally monitored by the microprocessor so the reference
oscillator can be adjusted for drift due to changes in temperature.
2.5.2.2 Receive/Transmit VCOs and Buffer Amplifiers
The receive VCO operates from 181 to 219 MHz since high side LO injection is used and the first IF is
45 MHz. The transmit VCO operates from 136 to 174 MHz. Each VCO is a Colpitts type design utilizing a low
noise, bipolar transistor as the active device. The receive VCO uses Q24 and the transmit VCO uses Q21, each in
the common collector configuration. The Colpitts capacitors are C169/C180 (receive VCO), and C137/C142
(transmit VCO). These capacitors enable Q24 to oscillate in the 181 to 219 MHz frequency range and Q21 to
oscillate in the 136 to 174 MHz frequency range. L53 is the resonating inductor for the receive VCO and L45/L46
are the resonating inductors for the transmit VCO. CR20/CR21, and CR16/CR18 are the coarse tuning varactor
diodes for the receive and transmit VCO respectively. These diodes are used to coarse tune the VCO such that the
LPF, phase detector output voltage (from U19) at TP10 equals 1.65 Vdc. The receive and transmit VCOs share the
coarse tuning DAC controlled line labeled CTUNE
. Coarse tune dc voltage swings from nominal 1.8 to 22 Vdc.
CR19 (receive VCO) and CR17 (transmit VCO) are the fine-tuning varactor diodes controlled by U19 as was
explained previously. CR15 is the modulation varactor diode for the transmit VCO. The output from the receive
VCO is coupled off Q24-E using C174. The output from the transmit VCO is coupled off Q21-E using C139. The
signal is measured at the C174/R146 node (receive) and the C139/R107 node (transmit), and measures -15 dBm
typically.
Q26 and Q22, and associated components, form the first VCO receive and transmit buffer amplifiers respectively.
These amplifiers buffer the VCO output from changing-output voltage standing wave ratios (VSWR) that could pull
the VCO off frequency. The output from each measures -5 dBm typically. The buffer is measured at the
C170/R141 node (receive) and the C138/R131 node (transmit). Q25 is the second buffer amplifier. This amplifier is
common to both the receive and the transmit VCOs. R131, R141, and R142 are the combining elements used to
make this possible. This buffer outputs a signal large enough, after subsequent attenuation and filtering, to properly
drive the RF IN pin of the synthesizer (U19-5). The output from this buffer is measured at the C167/C53/L52/C175
node and measures 0 dBm typically
.
The output from the Q25 buffer is filtered by C175, C53, L52, and C176. This LPF prevents the synthesizer IC
(U19) from locking on to harmonics of the desired frequency. The insertion loss of the LPF is 0.4 dB typically.
The signal is then split by R138, R139, and R144, and sent on to the appropriate receive or transmit final buffer
amplifier. The signal measured at R139/C150/ C186 node is -6 dBm, and the signal measured at R144/R143/C168
node is -6 dBm.
Q23 (receive) and Q27 (transmit) is the final buffer amplifier. Q23 amplifies the signal up to the level needed to
properly drive the LO port of the DBM (discussed previously). Q27 amplifies the signal up to the level needed to
properly drive the PA pre-driver (previously discussed). The signal measured at RXLO is +7 dBm typically. The
signal measured at TXLO is +7 dBm typically.
U17, Q20, etc. form the voltage regulator for the receive and transmit VCO/buffer amplifiers. R67, C124, and Q20
form a super filter, which attenuates voltage regulator noise that may otherwise degrade the synthesizer phase noise
performance.
U20, U21, Q29, Q30, CR23, CR24, CR35, C199, C202, C203, etc. create a voltage multiplier. The circuit is
configured as a voltage quadrupler. Circuit losses and output loading lowers the voltage down from 24 Vdc to about
22 Vdc. The driver circuit (U21) switches at about 192 kHz. This frequency was selected so harmonics would not
land at or near the second IF frequency of 455 kHz. The 22 Vdc supply is used to power the DAC supporting quad
op-amp U18.
The shift register (U24) is used to control transceiver modes of operation and functions. The line labeled STD/SIDE
selects the desired antenna port of the transceiver. The line labeled TX/RX selects either the transmit or receive
mode of operation. +3.3V RXEN turns the receiver on and off (previously discussed). Q34/Q32 and Q33/Q31
enable and disable the receive and transmit VCOs and buffers respectively (discussed previously). U16 is the
voltage regulator that supplies all 5V digital circuitry on the transceiver.