Spectrogram is also limited by the dynamic range of many sound cards. I've used it on some computers that had an internal noise floor barely 30 db down! Also the accuracy of the crystal clock in the sound card affects the accuracy of frequency measurements.
Even so Spectrogram is an extremely worthwhile tool for the purposes it has been recommended: align the BFO frequency to center the bandpass on the receive frequency and to see, clearly and graphically, how changes affect the bandwidth and position relative to the center frequency. Considering the price (free) when downloaded from Tom, N0SS's site (http://www.n0ss.net/index_k2.html) it's hard to beat the "investment" for anyone who has a PC running Windows handy. Ron AC7AC -----Original Message----- Hi Brett, My personal preference when looking at the response of any filter is to use a RF Spectrum Analyzer and a tracking signal generator. When it comes to looking at the response of IF filters already built into a receiver the tracking generator can work at the antenna frequency rather than at IF, which makes the generator - pad - receiver connection easy to make, provided that the front end selective circuits have no effect on the measurement of the IF filter i.e.their working bandwidth is >>> IF filter bandwidth. The penalty is that the input of the Spectrum Analyzer needs to be connected to the IF strip at some point before the product detector, and everything after this point has to be disconnected unless the working bandwidth of this part which follows is >>> IF filter bandwidth, *or* has no effect on the measurement. One also has to ensure that the Spectrum Analyzer and its connecting cable are transparent to the cascade under measurement, a high impedance probe helps. AGC has to be OFF of course if the AGC system could be active. Spectrogram does not require this fiddling, AGC must be OFF, but being in effect a Spectrum Analyzer working at audio it gives a limited view - about 20kHz IIRC. What I like about using a RF Spectrum Analyzer and tracking generator is that a cleaner picture is produced without fuzzy noise at the top, and it is possible to sweep over a wide bandwidth to view the ears and spurious responses of the filter. It is also usually possible to see those other unwanted responses caused by radiated feedback or coupling via pcb traces etc. It could be argued that Spectrogram is easier to use for routine alignment, but has limitations for use as a diagnostic tool. 73, Geoff GM4ESD _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
