Installation Instructions

Hardware Installation
RVP8 User’s Manual
September 2005
2–35
2.5 RVP8 Custom Interfaces
This section describes some additional points of interface to the RVP8. These hookups are less
conventional than the “standard” interfaces described earlier in this chapter, but they sometimes
can supply exactly what is needed in exactly the right place. For the most part, these custom
interfaces are merely taps into existing internal signals that would normally not be seen by the
user.
2.5.1 Using the Legacy IFD Coax “Uplink”
In the previous RVP7 processor the Coax Uplink was the IFD’s single line of communication
from the main processor board. All of the information that was needed by the IFD would arrive
through this uplink; and as such, it contained information that might also be useful for other
parts of the radar system. In particular, it is a convenient source of digital AFC.
The RVP8 uses a single CAT-5E Uplink/Downlink cable between the IFD and RVP8/Rx PCI
card. The legacy coax uplink protocol is no longer used directly; but to help with backward
compatibility, the waveform is now synthesized as an output from the IFD. Any hardware that
used to be attached to the RVP7 coax uplink can still be driven from this new IFD port.
The uplink is a single digital transmission line that carries a hybrid serial protocol. The two
logic states, “zero” and “one” are represented by 0-Volt and +12-Volt (open circuit) electrical
levels. The output impedance of the uplink driver is approximately 55Ω. When the cable is
terminated in 75Ω the overall positive voltage swing will be approximately 8.6-Volts.
The electrical characteristics of the uplink have been optimized for balanced “groundless”
reception. The recommended eavesdropping circuit is shown in Figure 2–4, and consists of a
high speed comparator (Maxim MAX913, or equivalent) and input conditioning resistors. Both
the shield and the center conductor of the coax uplink feed the comparator through 33KΩ
isolation resistors; no direct ground attachment is made to the shield itself. The 500Ω resistors
provide the local ground reference, and the 47KΩ resistor supplies a bias to shift the unipolar
uplink signal into a bipolar range for the comparator.
Figure 2–4: Recommended Receiving Circuit for the Coax Uplink
Max913
or equiv.
33KW
33KW
500W
500W
47KW
Received
TTL
Signal
+5V
GND
GND
Coax
Uplink
Input