Direct conversion receivers and very low intermediate frequency (VLIF) receivers having chopping mixers are known generally as disclosed, for example, in U.S. Pat. No. 6,125,272 entitled “Method and Apparatus Providing Improved Intermodulation Distortion Protection”, which is assigned commonly with the present application and incorporated herein by reference.
Chopping mixers however are known to cause a spurious frequency response, which may adversely affect receiver performance. The spurious frequency response is produced generally by mixing residual spurs occurring at multiples of the chopping frequency with RF interference. RF interference is otherwise referred to herein as RF blocker signals or merely as blockers.
In some cellular communication applications, for example, GSM, EDGE and DCS band communications, among others, a spurious frequency response may cause exceptions, for example those failing the −43 dBm absolute blocking specification by as much as 25 dB, depending on the choice of the chopper frequency.
The spurious frequency response in direct conversion receivers is especially problematic at higher RF frequencies, for example those typical of DCS band communications, since filtering is not as effective at higher frequencies.
It is also known to reduce the spurious response in mixers by spreading the undesirable signal power spectrum outside the filter bandwidth of the receiver. Khlat, Nadim et al., “Spread Despread Chopping Mixers”, 1999.
The various aspects, features and advantages of the present invention will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description of the Invention with the accompanying drawings described below.