User Manual

SECTION 1: THEORY OF OPERATION
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(870MODE line), it provides a serial interface for the MDT-870. Notice that only the RX data (RXD) and
all handshake lines are buffered. Transmit data (TXD) is derived from a modem interface circuit.
Injection Synthesizer
The dual synthesizer chip (U38) is the major contributor of the injection synthesizer. This device contains
the key components of a phase locked loop (PLL), including a prescaler, programmable divider, and
phase detector. The selected frequencies are loaded into U38 as a clocked serial bit stream via the PLL
DATA and PLL CLOCK inputs. The microcontroller provides the serial data.
Frequency stability is determined by a temperature-compensated crystal oscillator module
(VCTCXO) (Y5) at a frequency stability of 1 PPM from –30C to +60C. This device has an input
(REFMOD) that accepts transmit modulation and voltage from a RX FREQ ADJUST pot. The pot allows
the receiver to be fine-tuned to the exact operating frequency.
Two (2) voltage control oscillators (VCO) are formed by integrated low-noise oscillators with buffered
outputs (U39 and U40) and associated circuitry. The VCO’s generate receiver and transmitter injection
signals. The receiver control voltage is generated by the phase detector output (PDOUT-L) of U38
driving a loop filter consisting of R111, R112, C185, and C186. It integrates the pulses that normally
appear on PDOUT-L into a smooth DC control signal for U40. The output of U40 is buffered by U41 and
split by a resistive splitter consisting of R126, R127, and R128. The outputs of the splitter are RXINJ1
and RXINJ2. A second output of U40 is returned to the synthesizer FIN input via RXFB. This completes
the loop signal path.
The transmitter control voltage is generated by the phase detector output (PDOUT) of U38 driving a loop
filter consisting of transmitter R135, R118, C195, and C196. It integrates the pulses that normally appear
on PDOUT into a smooth DC control signal for U39. Upon transmit, the analog signal from the modem
and transmit processing circuitry is applied to the U39 tuning circuit at CR7. R137 is provided to center
the frequency of the transmit VCO. The output of U39 is the TXINJ signal. A second output of U39
returns to the synthesizer F-IN input via TXFB.
Transmitter/TR Switch
The transmitter section consists of a voltage controlled attenuator, exciter (U36 and U35). The exciter is
built around an RF power amplifier chip (U36). To transmit, 5-volt power is applied to the KEYPWR line.
This causes the U36 to power up and amplify the TXINJ signal input through a voltage variable absorptive
attenuator (U16). PA12V line is also powered up. This causes power amplifier (U35) to boost the RF
power to the desired level. Up to 60 watts are available from the transmitter. Harmonic suppression is
provided by C82, L11, and C83. Additionally, C165, L23, L24, and C164 along with CR3 provide
added suppression of the second harmonic.
Receiver 1 Front-End
This section consists of the components that form Receiver 1 Front-End. These components include
surface acoustic wave (SAW) filters, a low-noise amplifier, and a MMIC mixer.
Incoming signals pass through a low-loss SAW filter (FLT7) that selectively provides a high degree of out-
of-band signal rejection. A low-noise amplifier (U3) amplifies the selected signals and another low-loss
SAW filter (FLT8) provides additional selectivity. The output from FLT8 passes through a mixer (U4). U4
is a MMIC mixer which heterodynes the receive injection (RXINJ1) signal from the synthesizer. The result
is a 45 MHz IF signal and the IF signal goes through crystal filters (FLT3 and FLT4) to the Receiver 1 IF
section for further processing.