Service Manual

Page 4.1
3 ~ TECHNICAL DESCRIPTION
4. TECHNICAL DESCRIPTION
4.1 RECEIVER
Refer to Figure 4-1. Description based on UW band
4.1.1
Front-End Filters and RF Amplifier
The receiver input signal from the antenna passes through the harmonic filter and antenna switch. With the
portable in receive mode, diodes D580, D540a and D541a in the antenna switch are reverse biased allowing
the receiver input signal to be coupled through to the receiver front-end with minimal loss. The overall insertion
loss of the harmonic filter and switch is approximately 0.8dB.
A noise blanker is also fitted to E0 band radios. The noise blanker samples the received signal and gates the
45MHz signal in the IF stage in the event that high level noise transients are received. Due to inherent time
delays in the bandpass filters prior to the blanking gate, gating synchronisation occurs before the transients can
adversely affect the following stages.
Varactor-tuned bandpass filters at the input and output of the RF amplifier provide receiver front-end selectivity.
Varactor tuning voltages are derived from the alignment data stored in the radio. The DSP processes this data
to optimise front end tuning relative to the programmed channel frequencies, which may be changed at any
time without re-aligning the radio.
To achieve the required varactor tuning range an arrangement of positive and negative bias power supplies is
used to provide a total bias across the varactors of up to 14.0VDC. A fixed 2.5V positive bias derived from the
5V0 supply using voltage divider R429/R430 is applied to the cathodes of the varactor diodes. The negative
bias supply originates at the DSP/PLA as a PWM signal (FE TUNE) for the four front-end tuning voltages
TUNE1 to 4, for the particular channel frequency selected. The PWM signal is dependent on channel
frequency and tuning and passes through level shifting transistors Q404 to Q411 where it is converted to a
negative voltage in the range -0.5V to -11.5V. The -12.0V rail of the level translators is generated by U904D/E
with D903 to D906 providing the required voltage multiplication.
The RF amplifier stage comprises a low noise transistor amplifier Q400 that is compensated to maintain good
linearity and low noise matching. This provides excellent intermodulation and blocking performance across the
full operating range. The overall gain of the front-end is typically 14dB for all bands.
4.1.2
First Mixer and IF Section
The output of the last front-end bandpass filter is coupled into single balanced mixer T400/D415 which converts
the RF signal to an IF frequency of 45MHz. The local oscillator injection level is typically +8dBm at T400 pin 1
with low side injection used for UHF bands and high side for VHF bands.
Following the mixer is IF amplifier Q401 that provides approximately 15dB of gain and in association with its
output circuitry, presents the required load conditions to the 4 pole 45MHz crystal filters Z401A/Z401B.
The crystal filters provide part the required selectivity for adjacent channel operation with the remaining
selectivity provided by a DSP bandpass filter algorithm.
4.1.3
Quadrature Demodulator
Additional IF gain of approximately 45dB occurs at U400 which is a dedicated IF AGC amplifier/Quadrature
Demodulator configured for single ended input and output operation. The AGC voltage for U400 is derived from
the RSSI function of the DSP. The onset of AGC operation occurs when RF input signal level at the antenna
connector exceeds -90dBm and can reduce the gain by approximately 100dB for strong signals.
Conversion of the 45MHz IF signal to I and Q baseband signals is carried out by the demodulator section of
U400. The 90MHz local oscillator signal is generated by VCO Q402 which is phase locked by the auxiliary PLL
output of U701 via feedback signal AUX_LO2.