Interfacing to Open Collector Radar Signals

Engineering Insights Article

When connecting radar signals to radar interface hardware, such as the HPx series of cards from Cambridge Pixel, consideration has to be given to the type of signal being connected. There are single-ended signals, which have a single signal wire referenced to ground, there are differential signals, which use a pair of signals to provide improved noise immunity, and there are open collector signals, which are the subject of this article. It is essential to understand the type of signal and ensure that the radar interface hardware is configured appropriately.

There are several reasons why a radar manufacturer may use an open collector signal type. Probably the most important reason is that the open collector approach can support different signal voltages, thereby ensuring the broadest compatibility with connected equipment.

To begin with, consider the situation with a single-ended signal, which is assumed to be a discrete (two-level) signal, perhaps communicating azimuth, bearing pulses or a radar trigger (range zero pulse). The signal will be characterised by a voltage and an output impedance, which can be modelled as a series resistor in the signal path - see Figure 1. When the signal is in the ON state the output will measure a well-defined voltage, Vout, defined by the drive circuitry - for example it might be 10V with nothing connected. If a load is connected with an input impedance of Rin then the measured signal Vr will be Vout * Rin / (Rout + Rin). What's important here is that the magnitude of the signal seen by the receiving equipment is defined by the transmitter. If the on-load voltage is 5V (for example, for Vout = 10V and Rout = Rin) and the receiver needs a minimum of 8V, say, then the receiver will not see the signal. Cambridge Pixel's HPx series of radar input cards support signal-ended signals by having different voltage options selectable by links.

The situation with the open collector signal is that the sender does not define the voltage of the signal. The sender simply drives the signal to ground (the output transistor is turned on) or leaves it floating (the output transistor is turned off). The output circuit of an open collector signal (for example from a radar) is shown in Figure 2...