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Now what do you do?

You are an elite member of a very small group (0.01%) who actually takes to heart some of the suggestions I make on this blog. There you go, you bought the Heathkit Crystal Filter for less than $15 and the burning question is how to install this filter in a circuit? 


The P3ST IF Module is a great starting place as shown below.


The 500 Ohm resistor smack dab in the middle of the schematic is for simulation purposes representing a 500 Ohm Filter. To use this schematic for the Heathkit Crystal Filter you simply insert the Filter where the resistor is shown and modify two of the filter matching transformers. 

The original primary (L3) is set for a 50 Ohm to 500 Ohm transform. L3 is changed from 6 Turns to 3 Turns so now that 50 Ohms looks like 2000 ohms to the input side of the filter. The secondary remains at 19 Turns thus 361/9 = 40 since the 2000 to 50 Ohms is a 40:1 transformation. On the second transformer, the secondary is likewise changed to 3 Turns and you are there.

The 2N2222A's are biased HOT so you need heatsinks. The P3ST uses relay steering so that this module is used both on transmit and receive. I refer you to the https://www.n6qw.com website for full details of the P3ST build. 

NOW for a big caution about using the Heathkit in a single conversion design as the Cf is 3395 kHz and 2nd harmonics are close to the 40M band (2X3395 = 6.790 MHz) and a loose BPF might see some of that 2nd Harmonic. Whereas the 4th Harmonic (4X3395= 13.58 MHz) is less of problem on 20M. 

The real nut to crack is 40M as the LO for a single conversion at the Low side say for 7.2MHz yields 7.2 - 3.395 = 3.805 MHz LO -- 2X3.805 = 7.61 MHz -- again too close to the Band Pass. A high side LO is better as that would be at 10.595 MHz and less of a risk. Don't hit that send button too fast. W7ZOI has a software program called "spur finder" and it will spit out all of these combinations.

That said this schematic is ideal for a dual conversion rig where the 1st conversion is to 8.3 MHz and a second conversion with a 5MHz LO drops you at 3395kHz.

You now have a way of integrating this filter into a PROVEN simple circuit using a small number of components.

Them that know can make things go!

73's
Pete N6QW

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