1022410 – 0001 Rev. 2
UMOD hardware theory of operation 3–13
Figure 3-5
Transmit synthesizer circuit block diagram
SYNTHESIZER
FIR/NCO
ASIC
CONTROL
PROCESSOR
LPF
NCO
40 MHz
D/A
PLL
104-176
MHz
1
OR
2
TX LO
52-88 MHz
OR
104-176 MHz
SYNTHESIZER
FIR/NCO
ASIC
The NCO output is converted to an analog signal by a D/A
converter, filtered to remove alias components, and sent to the
input of a conventional phase-locked-loop (PLL) frequency
multiplier. The NCO output frequency range is in the 5.2- to
8.8-MHz range, and is multiplied by 10 or 18.5 for 52- to 88-MHz
output frequencies, or 20 or 21 for 104- to 176-MHz output
frequencies. A divide-by-two circuit provides the LO signal for
operation in the 70-MHz band, and is bypassed for operation in
the 140-MHz band. The PLL divider modulus of 20/21 is selected
to avoid operating the NCO at output frequencies where there are
large spurious signals close to the prime output. The choice of
modulus is coded into the control software and is selected based
on transmit frequency for optimum spurious performance.
Modulator output must be amplified to the desired carrier level,
and the level must remain stable through changes in frequency,
temperature, time, and unit-to-unit component variation. Bandpass
filtering is employed to limit out-of-band emissions (see figure
3-6).
CONTROL
PROCESSOR
IF
LOOPBACK
ALC
50
Ω
TX IF
OUTPUT
IF ON/OFF
SWITCH
52- T0 88-MHz
BPF
104- TO 176-MHz
BPF
MODULATED IF
Figure 3-6
Transmit IF stage circuit block diagram
An automatic level control (ALC) loop maintains the transmit
output power constant within tight tolerances.
Transmit IF stage