how to homebrew your own crystal frequency！
when operating a rockbound transmitter such as my
10w one-valve 80m cw transmitter, you will often get frustrated by qrm on your frequency that drowns out any station further than the end of your street. or you hear someone calling cq nearby but you can't get the frequency low enough to work them. pulling the crystal via a variable capacitor helps a little but doesn't entirely resolve the situation. what you really need is a vfo. but if you don't want that complication, then a few extra crystals will help.
i already had a crystal for 3.560 mhz, the international qrp calling frequency, and wanted one for 3.558 mhz the fists club calling frequency. an advantage of having two nearby frequencies is that if you're listening on 3.560 mhz without the cw filter, you can hear cq's below you on 3.558. but, you can't buy a 3.558 mhz crystal anywhere, so you have to either have one made to order, or make one yourself as follows.
before that though, what about ceramic resonators? they are stable (though not as stable as a quartz crystal) and they can be pulled over a much greater range. a 3.79 mhz ceramic resonator certainly covers the band of interest. so i tried one in my one valve transmitter, but the results weren't good. basically the power dissipated in the crystal or ceramic resonator is quite high because it's a valve circuit, and the crystal gets quite hot. unfortunately when using a resonator the heat results in a degree of frequency drift which is unacceptable, leading to severe "chirp". on key down, you can hear the transmitted note drop by many hundreds of hertz in the first second or so of keydown.
this idea for choosing your own crystal frequency comes from an article 'penning down' crystals by richard wells g0rxh, published in sprat #99. in the article richard recommends selecting a crystal frequency above the frequency of interest, and removing the case of the crystal and drawing on the surface with an indelible ink marker pen to lower the frequency.
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剩下的自己到作者的网站去看吧……
http://www.hanssummers.com/radio/penning/index.htm
