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Old 7th Aug 2008, 11:02 pm   #16
renato_enca's Avatar
Join Date: Feb 2007
Location: Lisbon, Portugal
Posts: 51
Default Re: 80 meter superhet

Please be aware that if the filter is homebrew and has more than 1 crystal (eg. a crystal ladder) then you may have serious problems with frequency mismatching between crystals, due to tolerances in manufacture. One aproach that may be pretty expensive but that's the way to do with crystal ladders is to buy 10 or 20 crystals of the same frequency and from the same factory, preferentially. Then, with an 74ls04 oscillator and a frequency meter you select 3 or 4 crystals that are pretty close to each other (better to be less than 100Hz). Unfortunately, as said, you must have a frequency meter, and the crystals of that particular frequency may be too expensive. For the poor guys like me, dual-conversion (may be cumbersome) or 455KHz (quick and easy) are definitely the ways to go.

You will REALLY (and I mean: ABSOLUTELY) need a good impedance matching between the filter and the rest of the receiver and a REALLY good frequency matching between the crystals, or you will have a "crystal barrier" than a crystal filter.

About the mixers, please note (I don't actually believe that you hadn't noted) that SA602 mixers have 1.5kHz input/output impedances.

Also note that a diode mixer needs quite a bit if drive to overcome the threshhold voltage of a diode of around 0.6V. Too low drive power gives a lot of loss in the mixer.
Don't be afraid: use a diode ring mixer made with BAR28 schottky diodes (0.2V voltage drop). Those marvels can even pick-up DRM stations quite amazingly!!

Last edited by renato_enca; 7th Aug 2008 at 11:08 pm.
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