marc (02/26/83)
The following is condensed from a note I received last night from Cincinatti Microwave (makers of the Escort Radar Receivers). Two competitive radar detectors TRANSMIT enough energy that they can trigger other detectors in the general area (the Escort apparently can be triggered a mile away under perfect conditions). These 'receivers' transmit signals on both the X-band (10.5 GHz) and K-band due to their design. Apparently these 'superheterodyne' receivers are really superhomodyne or direct conversion type receivers. The manufacturers are in Japan and probably don't care too much about what the customer really needs, only what is cheap to manufacture. The two offending units are: ROAD PATROL XK PHANTOM-1 Radio Shack Speed-o-Matic My Escort detects these guys very well but fortunately (since they transmit on both X- and K-band) have a distinctive (yet annoying) signature. My reason for sending this article is to encourage net.people to seek out a quality product when contemplating a purchase of a radar detector, even if it may cost a bit more. Remember you seldom pay extra, in the long run, for better quality. Flames about use of radar detectors should be directed to me, not the net! marc (smith) ...decvax!genradbolton!marc [Super-XXX-dyne receivers generate a signal internally to mix with the received signal in order to shift the received signal in frequency, usually a lower frequency, that is easier to detect. Often in higher quality receivers, several conversion stages are used (TV's mix the RF to produce IF then mix again to produce video), many super-XXX-dyne radar receivers use only a single conversion stage. A big problem comes in because the mixer allows some of this internally generated signal to escape back towards the antenna. If the internal frequency is different (in HETERO type) a filter can be added to significantly reduce this signal before it can reach the antenna and get transmitted (also other superHETEROdyne detectors, if sufficiently close to the 'transmitting' unit, will reject this signal as out of band). If the internal frequency is the same (in HOMO type) a filter is out of the question, as it will attenuate the input signal. Therefore a large amount of energy is allowed to be transmitted and since it is on the same frequency as the radar, other units detect it as a true 'in band' signal.]
ltn (02/27/83)
Just a few comments regarding superhet detectors, etc.: I don't know what sort of i.f. frequency (difference between the incoming radar signal and the internally-generated local oscillator signal) is used in radar detectors, but it can't be too high (~< 200 MHz, maybe), so if the unit is operating at 10.5 GHz, it would take a *very* good filter to stop the local oscillator from leaking out. Such a filter would cost much more than the Escort. The real way to prevent local oscillator leakage is to use a properly designed mixer, which simply doesn't send much energy back into the antenna. It's quite possible that those cheap, leaky radar detectors are in fact superhet units, but they simply have scrimped on the mixer in order to save money, and decided to let the buyer worry about the radiated signal. (By the way marc, TV's don't really use a second mixer to get down to the video frequency, but simply a.m. detect the i.f. to pull out the 4.5 MHz bandwidth video.) Les Niles (aluxz!ltn)