Technical Manual Technical Description
© SPECTRA ENGINEERING 2006 Revision 4.2.3
67
Provision is made for the optional injection of an external reference
frequency. If this option is selected CN3 is fitted and X1 is not fitted.
Components R30 and TR7 are also omitted.
A second RF output from the VCO on SKT-6 also at +3dBm is used as the
main transmit RF amplifier signal source. This main signal is first buffered
by a very high isolation circuit consisting of a 10dB pad and a MMIC
amplifier (IC1). The signal is further amplified by IC2 and a variable gain
wide band amplifier with 40dB control range and power output of 300mW.
The drive power of this stage is used to set the output power to the main
power amplifier under the control of the DC voltage on SKD-4 from the
Micro Controller board.
The VCO boards and synthesiser circuits are the same for the exciter and
receiver modules. The VCO consists of a 10mm ceramic coaxial resonator
with common base oscillator for low phase noise for bands 805-960 MHz.
Frequencies below 520 MHz use a LC tank circuit. The power supply to the
VCO consists of an 8-volt regulator and active filter for maximum noise
rejection. It is controlled by the Micro Controller through SKD-7, which
connects to switch TR6. For standard modulation, transmit audio is fed on
SKD-8 to the conventional point of the VCO varactor. For 2 point
modulation, audio is also fed via SKD-19 to the voltage control pin of the
VC-TXCO, this in effect cancels out the PLL error that would otherwise have
occurred for low audio frequencies, hence resulting in a flat VF response.
4.2 Receiver Module
Refer:
Receiver Block Diagram: BD001
Receiver Front End Circuit Diagram: CS003-2
Receiver IF Circuit Diagram: CS003-1 (Rev F)
CS017-1 & CS017-2 (Rev G-I)
RX VCO Circuit Diagram: CS005-1 (Band A to Q)
CS012-1 (Band R to X)
The receive signal from the antenna enters on CN1 a 3 section Bandpass filter
which provides the initial filtering for the front-end amplifier. The front-end
amplifier IC12 is a broadband high performance MMIC with a gain of 18dB,
noise figure of 3dB and 3rd order intermodulation intercept of +36dB. A 4dB
pad and a second 3 section BPF follow this, and a high level double balanced
mixer M1.
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