Design, Homebrewing, Willamette Transceiver

Willamette AF Filter Mod

As I mentioned in a previous post, I did the calculations to change the AF filter in the Willamette from a low-pass filter with a 3.3 kHz cutoff to a peaked low-pass filter with a cutoff frequency nearer to 1 kHz. I finally got around to implementing the mod last night and got a chance to listen to it on the air today and take some measurements of the filter response.

First off, let’s take a look at the filter response:

New Willamette AF Filter Response

If you compare this to the old response, it probably won’t look drastically different, but it does cut off a bit eariler than the original filter. There is a bit of a peak as predicted, although it’s a bit wider and shallower than expected.

However, the real proof is in the listening. I found (purely qualitatively) that the response of this filter was much tighter sounding than the original. Much of the high frequency interference is gone, and you can tell by tuning through a signal that it drops off much more quickly at the higher AF frequencies. You do lose some of the “crisp” direct conversion sound, but I feel like this is made up for in the utility of having greater filtering.

Here are the steps that you need to take in order to modify your own rig:

  • Replace R50, R51, R54, and R55 with 24 kΩ
  • Replace C55, C59 with 100 nF
  • Remove C56, C57 then place a 1 nF capacitor from Q12 base to ground (in the place where C56 was located)
  • Remove C60, C61 then place a 1 nF capacitor from Q13 base to ground (in the place where C60 was located)

One other small thing that you might want to do is replace C65 with a 1 uF capacitor. I noticed that when the AF gain control was set to maximum, that there would be an annoying popping during keying. This change helps to eliminate this problem.

I hope that you enjoy this modification to the rig. In hindsight, I’m not really sure why I designed such a wide open AF filter, although I suspect it was because I wanted to preserve the “DC” sound of the rig. However, I think that utility trumps a nicer sound in this case and will make the rig more usable overall.

Homebrewing, Willamette Transceiver

Another Look at the Willamette AF Filter

A few different things have conspired to bring me back around to finish up the documentation for the Willamette, as well as forcing me to re-examine a few elements of the rig design. One of those things was an e-mail from AA0ZZ (really cool to hear from the designer of the PIC-EL and the IQPro) asking me about the very broad response of the audio filter. This is one of those design choices that made sense at the time, but doesn’t seem so wise now. My theory was that it might be nice to leave as much of the “direct conversion” sound intact, while trying to knock down the most annoying higher frequency sounds. It also would have allowed someone to move the receiver to the SSB band without changing the filter (but honestly how often would that happen?). However, I’ve had a few people question the choice of such a wide open filter, and rightly so.

A comparison of the old LP filter with the newer peaked LP response

Fortunately, it’s easy to change some components to alter the filter to create a much better response for CW. The design for the filter was taken from the active AF filter discussion on page 80 of Solid State Design for the Radio Amateur. The filter topology is not so much low-pass, but peaked low-pass. The peak is based on the Q of the network, which in this case is determined by the ratio of the two capacitances in the filter section. As it is currently designed, the response is nearly flat across the passband, which corresponds to a network Q of 0.5√2 (the ever-popular number 0.707). By changing feedback cap to 100 nF and the shunt cap to 1 nF the network Q is increased to 5, which does pretty much what you would expect it to do. The value of the series resistors had to be changed in order to keep the peak at 600 Hz. Those are the only changes needed to drastically change the filter reponse. In the attached screenshot, you can see a LTSpice comparison of the two responses. The new filter constants give a much steeper high frequency roll-off and also gives better attenuation of the nasty low frequencies of 120 Hz and below.

I haven’t actually made the changes to the filter as of this morning, but I expect to do it very soon. Assuming that it works as designed, I plan to roll out the change to the design in version 2.0 of the rig. I’ll also put the changes in the current 1.x build documentation section about rig modifications.