howardh@ihu1e.UUCP (Howard Hill) (08/21/84)
(Dick Lincoln's response set off by '>'s) > Not so with relatively recent vintage Escorts from Cincinnatti > Microwave. They have circuitry to be sure they are receiving pulse > modulated signals, as any real range or velocity measuring radar device > must use. Leakage from super-het receiver oscillators is an > unmodulated signal which can block an Escort by saturating its RF front > end, but not trigger a "false alarm". The biggest source of spurious > alarms from Escorts are commercial X-band burglar alarms that also use > (hopefully) very low power radar as motion detectors. Such radar is, > of course, pulse modulated, and thus very hard to distinguish from > police radar. Both police and burglar alarms use CW radar. A unmodulated microwave beam is sent out and the return is redshifted or blueshifted by the velocity of the target. The return, at frequency Wc +- Wv is mixed with the a sample of the outgoing Xmitted beam and the difference frequency, Wv, is filtered out. This frequency (generally in the audio range for the usual range of target velocities and radar frequencies is amplified, limited, and fed to a detector. Police radars usually use a phase lock loop to filter out the strongest signal out of a noisy and multiple return environment. The output of the loop's vco is fed to a frequency counter. This is calibrated to display the output as mph. With a motion detector type of burglar alarm, the output is merely sent through a bandpass filter and detected. In this situation, we are not concerned with measuring the velocity of the target or sifting the strongest return out of multiple signals - merely determining whether or not a target is present. Leakage from the local oscillator in a superhet radar detector will probably be within the bandwidth of another detector, thus, it could set it off. JAMMING police radar (I AM WARNING YOU NOT TO DO THIS), usually involves transmitting an AM modulated signal within the bandwidth of the radar. If memory serves me, an AM modulated X band signal modulated at 2160 Hz corrsponds to a doppler shift of 55 mph. The AM signal is detected by the front end of the radar and, since it is much stronger than the return from the target, overwhelms the legitimate return. The detector in the front end of the radar demodulates the AM signal and feeds the trusty jamming tone to the radar's signal processors; thus fooling the radar. If the police suspect that they are being jammed, there are a few simple tests they can run. (I WILL NOT REVEAL THESE HERE) -- from the depths of plywood city, Howard Hill (ihnp4!ihu1e!howardh)