Peter wrote: > Could you elaborate on this? Why is an IQ mixer needed? The way I see > it the most needed thing is low (very low) phase noise of the carrier > so the LO does not swamp the mixer and the audio after it (although I > am unsure what happens with the phase noise in a direct conversion > receiver when an actual signal is not present). The scheme is the > same as used in DSB/CW direct conversion receivers, whose performance > is rather good usually. Also using a noise source as input makes it > possible to go to very high frequencies without the usual problems. > No? Also it is a very simple and inexpensive scheme. With a $10 mixer > it could cover dc to 4GHz. If a stabilised Gunnplexer is used then > microwave filter (and Gunn and varactor bias, by voltage, to match > an existing filter or structure) tuning becomes possible. What am > I missing? Ok, dynamic range is probably 55dB or less (sound card > dynamic range). With a good sound card and a preamplifier this could > be 90-100dB. Many people already have such a good sound card. If the > LO is replaced with a stepwise DSS or PLL then a spectrum can be > taken with say 40kHz 'windows' from DC (almost) to 1GHz. The DSS > could be controlled by a USB to parallel control for example. Your enthusiasm is causing you to extrapolate way beyond the concept of the original project, which was to evaluate the details of a crystal filter's amplitude response, which was already known to be less than the bandwidth of the sound card. This isn't a "spectrum analyzer" so much as a magnitude-only "network analyzer" (no phase information). In order to turn this into a direct-conversion spectrum analyzer, you'd need to build a filter with a fixed 20 kHz bandwidth whose center frequency could be varied from "DC to 1 GHz". Good luck! If you want a rather good design for a low-cost spectrum analyzer, as well as a discussion of some of the design issues, look for the article by Neal Martini in the next issue (#192) of Circuit Cellar. It's based on a MAX3550 and a PIC18F4520. -- Dave Tweed -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist