Tuesday, December 9, 2014

xtal filter making 1

Little History:::

This is the one of the building block which was never mastered to brag about. Years ago I hade made one xtal filter, without any matching and see the way I put the pcb, aluminum shield. After a decade I look and smile at it. It was never used, just soldered it.

After the free gift of BITX40 PCB & cheap/low cost kit supply at the HFI2014 many peoples are making the BITX. from the facebook group it seems that VU3JOJ and VU2SWX are making and uploading the process/fotos on facebook. Few others were also making the bitx.  (As I did not attend HFI2014 at hydrabad, so I did not get the kit or free pcb) I requested for the a gift of pcb if any other ham have spare or not building it. But this gift  did not arrive as of  9dec2014(!) , Then asked a friendly person to make one pcb, not coming. then asked another friend to make one ( which he will at his leisure), meanwhile someone suggested to make Dual bander for coming onto the air.  Asked the kit supplier of HFI2014 but peoples were not giving good opinions. What I can say about it. But if you are reading and want a kit then buy from the VU2SUA. Only he gives the matched xtals.

Whether I make the bitx is not sure but mastering the filter is a must to make my many dc receiver into superhet and DSB TX into the fullfledged SSB transmitters. Lets see.

Designing the Xtal filter :: ( little theory first)

source:- www.phonestack.com/farhan/xcvr1.html.
bitx smd version crystal filter testing.html
bitx.htm by py2ohh
TT's Jotting Blog of VU2SWX ... Building Bitx Part2-The crystal filter. html


First stag is choosing the IF of the receiver. The BFO should not be harmonically related to input RF freq,  for 14mhz band 10 MHz IF is okey. so VFO becomes 4.000mhz and IF is far from one and half of the vfo.

Second stage is you need many xtals so it must be easily available. Higher the frequency greater the VXO shift. Generally Ladder filter does its work (filtering) little lower than xtal marked freuqency.

Third stage is matching its frequencies and make the ladder filter. This musing ends here.

Fourth stage is measuring the bandwidth and other parameters of the xtals. For this I don't have any access to instruments i.e. CRO, RF mili voltmeter , DDS VFO, Compute programs
links are on the scanned page please read it .....

Please see part two for practical selection procedure for crystal and other details


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