TEKCOM Industries Limited
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THEORY OF OPERATION
VHF7100/VHF7000 (For NAVMAN)
November 28, 2003
This PLL controlled VHF marine mobile transceiver provides an accurate and stable multi-channel
operation.
The transceiver consists of 12 main sections
Transmitting stage
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Receiving stage
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Local oscillator PLL (Phase Lock Loop) Circuit
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Memory backup
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Low voltage detection
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Large LCD
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Local/distance circuit
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2nd receiver.
(only VHF7100EC)
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wx alert circuit
[only VHF7100USA]
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PC program
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DSC Feature
[only VHF7000ROW without]
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GPS Message
[only VHF7000ROW without]
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ATIS Feature.
(only VHF7100EC)
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Transmitter Stage
The audio is picked up from the internal Mic. The audio signal is then amplified by Audio Amplifier,
IC205C [NJM3403AM], IC205 B [NJM3403AM] and filtered by a low pass filter IC10C [NJM3403AM]),
IC10D [NJM3403AM]). The audio that is adjusted by VR3 to obtain a suitable RF frequency-deviation,
modulates the carrier of VCO, through Varicap (VD302).
The modulated signal output from the VCO is pre-amplified by Q2, and IC1 (RA35H1516M-01). When the
supply voltage is 13.8V, this signal will be amplified up to 1W or 25W. The signal is filtered by low-pass
filter circuit of which consists of L1, L2, L4, C1, C2, C3, C5, C6. C14, C47 and C62.These low pass filters
are necessary to suppress the second and the third harmonics as higher. The signal is then fed into the
antenna input and radiated out. The signal is also fed into another path consisting of C9, C10, D3, D4 for
sampling, and is converted into a direct current voltage for the Automatic Power Control (APC) circuit
IC12 (NJM2904M) Q5, Q14, Q15,Q24 to control the voltage of IC1 Pin 2 to maintain the output power
stability. Q15 is used control Hi/Lo power
When the unit is transmitting, the channel control voltage is added to the TX VCO varicap VD306. The
capacitance of VD302 is varied following the audio signal, therefore the carrier is modulated to form the
modulated signal.
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Receiver
The receiver uses a double frequency super-heterodyne circuit. The first Immediate Frequency (IF) is
21.4 MHz and the second is 455 KHz.
The RF signal is received by the antenna, and passes through a low-pass filter network L1, L2, L3, L9, C1,
C2, C3, C5, C6. C14, C47 and C62 to filter out unwanted signal. the received RF signal then passes
through a high RF transformer L10 and is amplified by RF amplifier Q9. L11, L12, L13, C15, C18, C20
form the band pass filter. The RF signal then is mixed with the local oscillation frequency by the mixer
