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Wednesday, December 7, 2016

A Charming Detail About the BITX40 Module

In the BITX20 yahoo group, Farhan responded to a question about the BFO and the crystal filter in the BITX 40 Modules.  In his response we learn more about the work of the members of the women's collective.  Uma is the name of the lady who has the job of sorting the crystals. She sorts 1000 a day:

Farhan writes:

There is a longer explanation to this. The crystals we use in the BITX are microprocessor grade. This means that they are not very precise. Uma is the crystal sorter. She takes a bunch of 1000 crystals a day and sorts them by their frequency into different bags. Each board takes its five crystals from any one bag. Hence, the central frequency of each board will be slightly different from another board.

The central frequency of the ladder filter moves down from the oscillating frequency of the same crystal. That is why, in a set of matched crystals, one can be directly used as the carrier/beat frequency oscillator without needing any trimmer : it directly sits on the higher side skirt of the filter.

The BITX40 board's BFO usually ranges from 11.990.0 to 11.997.0, though in almost all cases it is very close to 11.998.0. The best way to determine the BFO frequency is to take it another ham's shack, tune in the BFO to zero beat on USB or LSB and note the frequency. I use the Rigol scope's built-in frequency counter to measure it off the modulator transformer's primary.

So, the IF offset should ideally set to the measured BFO's frequency. These are however, very subjective choices. Given that 2.2 KHz is not an ideal passband, 3 KHz is more like it, setting the BFO will determine how you would like to hear the receiver. Setting it close to the passband will make it bassy, setting it away will make it tinny. Setting it a few hundred hertz away will make it hollow. You get to choose which way you want to make your radio sound bad (harr! harr!)

- f


  1. Hi all,
    A question about the crystal type, I notice the BITX is successfully using the short type of crystal, I have read various reports saying not to use that type as it has "Lower Q" so is it a compromise to keep costs low or no problem using short type crystals ? Anyway congratulations to Farhan for his design.

    Regards.... Peter... GW4ZUA

    1. Hello Peter: I remember Farhan looking into this a while back. Obviously he concluded they would work well, and I think they do. 73 Bill

  2. Time for another rule in Homebrew amateur radio. If it works then it proves it is working. No problems with the short type found. Though, not really tested.

  3. Hello Bill and Anonymous,
    Thanks for the replies, while I appreciate that they are working as filters I would have liked a bit more info on their use, as I have a quantity of 10.7 & 8 mHz short type that I would like to be confident to use.


  4. Peter: I say give 'em a go! Just solder them in an sweep the filter. I bet it works fine. I have done a quick check of my bitX 40 filter and it looks as good as the filters with larger rocks. 73 Bill

  5. Peter Webb, my motto to home brewing is the Nike slogan, JUST DO IT.

    The only issue i have found, and what Farhan has explained in his post, is that you require a bigger bag of xtals to find a matched set. But the cost of x.000 xtals compared to x.00000 makes for buying a bag of 100 a trivial matter. The downside being the extra time it takes to measure the parameters of 100 xtals compared to 10.

  6. Hello Bill & Rob,
    Thanks again for the replies, looks like I just need to do it.
    73 ............Peter..GW4ZUA

  7. How about a matched crystal pool. Everyone puts a fixed amount in to buy an industrial quantity from China and one of the bretheren with the big gun analysers characterises them. Then sets can be made up and distributed to the investors?


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