MX800 Transceiver Technical Manual
© SPECTRA ENGINEERING 2006 Revision 4.2.3
68
The receiver uses high side local oscillator injection for bands A to M and
low side injection for all other bands. RF from the VCO main output on SKT-
6 is buffered and amplified to +17dBm by IC4 and injected in the high level
mixer which down converts the signal to the first IF frequency of 90 MHz
(45MHz for bands A, B and 70MHz for bands A4). This IF signal from the
mixer is terminated by a bi-directional constant impedance network and is
then amplified by a bipolar amplifier TR2 with a gain of 15dB and 3rd order
intermodulation intercept +35dB. This provides a high degree of
intermodulation rejection for the receiver. A 4-pole crystal filter FL3A/B
with its associated matching networks follows this stage. The signal is further
amplified and filtered by a transistor amplifier TR3 and its associated 2 pole
crystal filter FL4 before being fed into the main IF demodulator chip IC1
with a second IF frequency of 455kHz. The resulting audio is passed out to
the Micro Controller board on SKD-3. The Received Signal Strength
Indication (RSSI) from IC1 is buffered by IC5A and connected to the Micro
Controller board via SKD-4.
RF from the VCO on SKU-1 at a nominal level of +3dBm is applied to the
fractional-N synthesiser (IC10) main divider input. This signal is compared
with the reference oscillator frequency and the correction voltage from the
synthesiser’s charge pump output is filtered then amplified by the non
inverting low noise op amp (IC11A). This correction voltage is fed back to
the VCO to maintain loop lock as well as being fed to the Micro Controller
via SKD-14. A lock detect signal from IC10 is also fed to the Micro
Controller via SKD-16. The op amp uses a 25 volt power supply (generated
on the Micro Controller) so as to provide a wide tuning range voltage to the
frequency control varicaps located on the VCO board. Frequency
programming data for the receiver is sent to the synthesiser chip from the
Micro Controller via the a serial data line on SKD-18 under the control of the
Clock (SKD-15) and Strobe (SKD-17) lines
The Micro Controller through an enable signal on SKD-8 controls the local
oscillator signal to the mixer. This signal switches the supply to the local
oscillator amplifier and is used to enable or disable the receiver.
Provision is made for the optional injection of an external reference
frequency. If this option is selected CN3 is fitted.
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