Abstract: A frequency scheme for a police radar detector enables the K band and the K.sub.a band to be scanned during a single sweep of a local oscillator which drives a mixer at its fundamental operating mode to produce intermediate frequency signals around 5 Ghz. The frequency scheme simultaneously activates two or four frequency conversion paths in the police radar detector. Accordingly, a radar warning alarm can be given upon detection of radar signals in any one of the frequency conversion paths. The ambiguity as to which one of the two or four frequency conversion paths is receiving a detected radar signal is resolved by applying first and second modulation signals to first and second local oscillators, respectively, so that the radar band of the detected radar signal can also be included within the radar warning alarm. The first and second modulation signals are in quadrature to one another and quadrature correlation is used to determine which frequency conversion path is receiving a radar signal.

Abstract: An input stage for a police radar detector includes a single mixer together with at least one preamplifier to detect radar signals in the X, K and K.sub.a bands. A preamplifier may be used on the X band alone, the K band alone, the K.sub.a band alone, the X and K bands or the X, K and K.sub.a bands. The use of these preamplifiers provides better noise figure; however, multiple responses cannot be scanned at the same time in bands utilizing a preamplifier with sufficient selectivity to reduce noise in its respective image bands to tolerable levels so that some or all of the receiver responses are swept independently with the preamplifier or preamplifiers being enabled one at a time as appropriate for each band being scanned. Since more sweep time is required when multiple responses are no longer swept simultaneously, a currently preferred form of the input stage couples the K.sub.a band signals to a single mixer through a preamplifier which permits multiple responses to be swept in the K.sub.

Abstract: An input stage of a police radar detector is configured so that a near end of a mixer is coupled to an antenna for receiving high frequency signals and a far end of the mixer is coupled to the antenna for receiving low frequency signals. A local oscillator and an intermediate frequency amplifier are also coupled to the far end of the mixer. The low frequency signals, X band police radar signals, are coupled to the far end of the mixer through a band rejection filter, which serves as a diplexer, a preamplifier and a first bandpass filter. The local oscillator is coupled to the far end of the mixer through a second bandpass filter and the far end of the mixer is coupled to the intermediate frequency amplifier through a low pass filter. The high frequency signals, K band and K.sub.a band signals, are coupled to the near end of the mixer from the antenna through a high pass filter.

Abstract: A radar detector suppresses nuisance alerts due to detection of a third harmonic of a common LO signal of other radar detectors by detecting a second harmonic of the offending LO signal. If a detected radar signal in the K.sub.a band is of a frequency within a suspect range corresponding to possible third harmonic spurious LO signals, a determination is made as to whether a potential alert blocking signal around the second harmonic of a nominal 11.55 Ghz signal is also present. In the illustrated embodiment, the second harmonic alert blocking signal which is checked has a frequency which is 2/3 the frequency of the detected K.sub.a signal .+-. a guard band of, for example .+-.40 Mhz, and is within a range of frequencies from about 22.813 Ghz to about 23.8 Ghz. If no blocking signal has been detected within about 10 seconds before or within three sweeps after detection of the K.sub.a band signal in the suspect range, the detected K.sub.a band signal is reported.