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Unveiling a best kept secret.

Who doesn't like knowing the best kept secret which I am about to unveil. So here goes.

I will always suggest for those considering a homebrew SSB or CW transceiver and who for them, is a 1st time outing on such a project to use a commercial crystal filter. Once you have successfully built the complete radio, then is the time to get on with the science project of homebrewing a crystal filter. One problem at a time is always a good approach

So, what is the best kept secret? It is a Heathkit Crystal Filter found in many of their rigs. Many can be had for under $15 like this one.


I guess the low price is based on that these filters are of a low frequency like 3.395 MHz and a set of BFO crystals will cost more than the filter. Most of the Filters have a Zin/out of 2K Ohms.

Actually, the filter is the best kept secret part. With an Arduino and a Si5351 for about $20 you get the whole digital LO and selectable BFO RF sources. The added bonus no drifty analog VFO!

Now the low Center Frequency of the Heathkit filter makes things a bit tricky in single conversion designs. But it is a great choice for a dual conversion rig. There is that 3rd clock on the Si5351 that will yield a heterodyne frequency. Think of a 1st conversion in the range of 8.3 MHz and with a 5 MHz LO the IF is 3395 kHz. This is the HW-101 frequency scheme. 

As an example, if we have our 5 MHz LO at 5.2MHz and the BFO at 3396.5 kHz and the 1st conversion frequency at 15.7965 then 7.2 MHz would be copied. The LO would tune in reverse so that a 7.3 MHz signal would use a 5.1MHz LO frequency to yield 7.3 MHz on the display.

How to match the pesky 2K impedance to 50 Ohms? A solenoid winding of 19 turns tapped at three turns would do it as 3^2 = 9 and 19^2 = 361. We have 361/9 = 40:1 and 2000/50 = 40:1 -- all on a FT37-43 core.

If you check https://www.jessystems.com and look for the Tri-Band Transceiver link you will see the use of the Heathkit Crystal Filter ($15).

Them that know can make it go.

73's
Pete N6QW

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