Datron TW7000MS, Technical Manual

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3: Theory of Operation
3-8 TW7000MS
3.3 Frequency Generation
For a diagram of the boards used in frequency generation, refer to the
"Transceiver Frequency Mixing Schematic" figure on page 3-7. All
frequencies are tied to the reference oscillator located in the Reference/
Control board. This oscillator has a frequency of 16.777216 MHz, and
connects between the Reference/Control and Synthesizer boards via an
internal coaxial cable (J61 on both boards). The synthesizer takes the
reference frequency input and generates the three local oscillator (LO) output
frequencies in the TW7000 mixing scheme. These are as follows:
These LO frequencies result in intermediate frequencies (IF) of 75 MHz and
5 MHz. The beat frequency oscillator (BFO) has different values depending
on the mode selected. This allows the use of only one sideband filter instead
of the usual two (USB and LSB), giving the transceiver both modes as
standard equipment. The third value of BFO (5.0036 MHz) is for USB
operation when the optional wideband data filter is installed. The first LO is
offset by 3 kHz in USB mode with the standard voice-grade filter, and by
3.6 kHz in USB mode with the optional data-grade filter. A few examples:
Frequency Value Level
First LO 75.1 to 105 MHz
(10 Hz steps) 0 dBm ± 2 dB (50 ohm)
Second LO
≈
70 MHz 0 dBm ± 2 dB (50 ohm)
Third LO 5.000 MHz (LSB, voice)
5.003 MHz (USB, voice)
5.0036 MHz (USB, data) 0 dBm ± 2 dB (50 ohm)
Channel frequency = 1.6 MHz, USB, voice, 1 kHz modulation
First LO = 75 MHz + channel fre 3 kHz
= 75 MHz + 1.6 MHz + 3kHz
= 76.603 MHz
First IF = First LO - input frequency
= 76.603 MHz - (1.6 MHz + 1 kHz)
= 75.002 MHz
Second IF = First IF - second LO
= 75.002 MHz - 70 MHz
= 5.002 MHz
Output
Audio = BFO - second IF
= 5.003 MHz - 5.002 MHz
= 1 kHz