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23rd January 2009 16:22

Alan Yates wrote...


For sweeping xtal filters that particular sweeper is a bit primitive. It works, but you'd probably be better off making something with more buffering. Your counter will be handy for accurately measuring the filter's passband, it should have at least 1 Hz resolution and good stability, absolute accuracy is important but not as much as the stability. There is a VXO tool which is designed for filter work. It can be tuned very precisely and is very stable and well buffered. Its buffer with its Norton amplifier would be a good choice to replace the buffering scheme in the LC sweep generator. I'll probably build something similar next time I need to build a narrow xtal filter. With SSB bandwidth filters you can be a bit more leanient than CW or measurement receiver ones. It is possible, but not all that easy to tune them by-hand using just the sweeper. At the very least the xtal frequencies should be matched to within 10% of the desired filter bandwidth.

If you don't already have it, consider buying the Experimental Methods in Radio Frequency Design book. It isn't too expensive any more and goes into great detail about how to build SSB radios (and so much more). It covers xtal filters in some depth and talks about various design methods. It comes with a CD containing software which can design filters from measured xtal parameters. The measuring is simply a matter of building a simple circuit with a switched capacitor and accurately measuring the two frequencies of oscillation. Often you can get a bag of 100 xtals cheap and select several sets of rocks that will make good filters. You can't really just plug together random xtals, especially for a CW filter.



23rd January 2009 05:41

Frank Moskal wrote...

This is just the information I am looking for. I build all my gear and Single Side Band is my new interest. I need circuit diagrams of your exciter, especially the crystal network and any other pertinent information for their use. I shall build your RF sweeper. I have a good frequency counter, which also includes a PLL for VFO stability if required. Thank you for any information.

Frank Moskal