Author: William Meara

Mythbuster Video #2 — 10 Pole Crystal Filter

This is a 5.2 MHz crystal filter. I used the G3UUR method for determining the crystal’s motional parameters. I then used Dishal and AADE software to design a 10 pole Cohn Min-Loss filter. I tested the bandwidth with an Antuino Scalar Network analyzer (thanks Farhan!) and a NanoVNA. I found the passband to be a bit tight for SSB, so I replaced the capacitors with caps of a slightly lower value — this broadened the passband. It is still a bit tight, but the SSB audio — while not enhanced or Hi-Fi — sounds fine.

Passband filter shape as seen in the Antuino and in the NanoVNA. The -20 db line in the Antuino actually corresponds to no loss.


The schematic provided by the AADE software. Dishal software may have come up with better, more correct values for the capacitors.

The passband as predicted by AADE. Skirts so nearly vertical as to strike fear in the hearts of SDR owners!

Filter under construction — waiting for the caps from Mouser.

First scan with the NanoVNA. Insertion loss looks very high but that is only because I am terminating the filter with resistors — I just wanted to see the passband shape.

A Video Series on the Mythbuster 75/20 Rig — Video #1

I am happy to report great progress on the Mythbuster project. I have the receiver working on both 75/80 and 20 meters. And it in fact inverts the 75 meter LSB signals, turning them into 5.2 MHz USB signals for passage through my 5.2 MHz USB filter/BFO combo. No switching or shifting of the BFO is needed.

I am following Farhan’s BITX20 advice — I have paused in the construction and am enjoying the receiver that I have built. I’ll build the transmit circuitry later.

Inspired by Frank Jones (you really should be reading the FMLA articles) I have this rig prototyped “Al Fresco” on a pine board that I found discarded on a neighbors front stoop.

There is no RF amplifier in this rig. Following the advice of multiple receiver gurus, I ran the BP filters right into the ADE-1 diode ring mixer. I have the TIA amps set at about 24 dbm. There is a lot of audio gain from the LM386 and the audio pre-amp. This seems to be enough, even on 20. I hear the band noise when I connect the antenna on both 75 and 20.

Here is the first video in the series. I’m posting them first on Patreon, then, a few days later, here and on the YouTube channel.