Installation Instructions

Hardware Installation
RVP8 User’s Manual
September 2005
2–30
2.4 Digital AFC Module (DAFC)
The DAFC is a small self-contained circuit board which can passively “eavesdrop” on the
RVP8’s serial uplink transmissions. Its purpose is to generate a set of digital AFC control lines
that could be applied, for example, to a custom STALO frequency synthesizer. A full size
(3”x3.75”) assembly diagram of the board is shown in Figure 2–3. It can be installed in the
radar system either as a bare board, or packaged into a small metal enclosure.
Figure 2–3: Assembly Diagram of the DAFC
SIGMET recommends that the DAFC board be used in new system designs whenever AFC is
required, as it offers these advantages over other methods of frequency control:
1) The use of a digital frequency synthesizer is superior to using analog AFC because
the stability of a synthesized STALO can be made much greater than that of a tunable
cavity oscillator. Also, noise on the AFC control voltage directly contributes to phase
noise in the received weather targets in analog AFC systems, so cabling of the control
signal can become tricky.
2) The RVP8 Connector Panel can also be made to output 8–bit AFC (TTL or
RS422). However, this is not in general recommended because of the potential for
noise on the cable which is typically run >2m into a radar cabinet. Using the DAFC
module is preferable because the board can be physically located very close to the
STALO. The length of the control cable and its susceptibility to noise and ground
loops are therefore reduced. Also, the DAFC board can supply up to 24 output
control lines, rather than just eight.
The digital output lines are made available as TTL levels on a 25-pin female “D” connector
(P1). There are 130W resistors (R1–R25) in series with each output line to help protect the
board against momentary application of non-TTL voltages on its external pins. However, these
resistors do impose a restriction on the input line configuration of the receiving device. To