I recently received a couple of excellent photos of completed Willamette transceivers, and I just can't help but brag on these wonderful creations.
W8BH completed his rig quite a while ago but recently upgraded it by adding a KD1JV digital dial. Looks very sharp, Bruce!
I'm really getting a kick out of this one. K3PG, appropriated an old Sonic drive-in tray to create a chassis for his Willamette. I really enjoy seeing the more mechanically inclined hams build cool enclosures out of interesting scraps. He did a great job with the shielding between the VFO and mainboard. FB work Paul!
I'm very pleased to report the first (to my knowledge) completed Willamette transceiver which was born in Europe. Vincenzo, IZ5GVP sent along to me a couple of photos of his very nicely constructed Willamette. He reports that it works well and that he's considering modifying it to operate on the 30 meter band. Three cheers for Vincenzo!
Thanks to some prompting from a Willamette builder who inquired about some performance issues with his rig, I was able to identify an error in the bill of materials which causes a significant degradation in receiver sensitivity.
Fortunately the fix for this problem is very simple. Audio preamplifier collector resistor R49 was incorrectly called out as 10 k, when it should have been listed as 4.7 k. If you have a 4.7 k resistor in your junkbox, just swap it in at the R49 position. A 5.6 k resistor will also work fine here. Alternately, you could just parallel another 10 k resistor across the existing one. I will be happy to supply the correct resistor to any builders who purchased a kit from me and need one.
I've determined that this wasn't a design flaw, but a transcription error. My original hand-written notes have the correct value and my prototype does work correctly (I'm sure NA5N would have caught this problem in the prototype he evaluated). Somewhere in the process of creating the schematics in my schematic capture program, I entered the wrong value. I did build a beta rig with all of the same schematics/BOM that everyone else did, but I didn't catch the error at the time. I will update the schematics and BOM posted on my website in short order to prevent any further problems.
Please accept a most humble apology from me for not realizing this significant error for a very long time. I do believe that you'll be pleased with the difference in sensitivity once you install the correct resistor. The receiver should sound like you would expect a proper direct conversion receiver to sound. After the modification, you should be able to run the AF gain at ~75% or less during most operating conditions.
I'd like to thank W0EP, N1RX, WB8ICN, and WB9VTB for their assistance in resolving this matter!
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:
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.
I know that the blog updates have been a bit light over the last week or so. Although we have been in our new house for three weeks now, it seems like the chores just keep piling up. However, I have done a little bit of work in the homebrewing department. Inspired by messages from WB9VTB and KB9BVN, I decided it might be nice to create a simple code practice oscillator based on the twin-T sidetone oscillator from the Willamette. The discrete component CPO published by the ARRL is really neat because it is very simple and has a unique build method. However, the circuit is your traditional astable multivibrator, which produces a near-square wave. I guess I'm spoiled, but I like listening to a clean sine wave. It certainly doesn't take any more components to build a twin-T compared to an astable multivibrator.
I experimented with a few different ideas for simple, discrete component audio amplifiers to pair with the oscillator, but settled on perhaps the simplest of all: an emitter follower. The twin-T oscillator puts out a waveform with a fairly large voltage, so all I really needed to do was tack on an emitter follower to provide some high impedance buffering for the oscillator. The entire circuit is extremely simple and produces a pleasant tone at 600 Hz, which you can sample here. The circuit can easily drive low impedance headphones, but if you wanted to listen on a speaker, you would need more amplification. An easy solution would be to plug in a set of amplified speakers, but it wouldn't be hard to add another stage of amplification.
I've created a Manhattan layout for the circuit, and I think that I would like to develop a complete kit with full build instructions at some time in the future. Something that would make it easy for the complete homebrewing novice to successfully build. I know that CPOs are a dime-a-dozen, but I think that the simplicity of this design (2 NPN transistors, handful of resistors and caps, a few 1/8" phone jacks) is a bit unique.
This weekend, two of the first wave of Willamette builders posted some photos of their completed rigs to the qrp-l.org mailing list. I present to you some of these great pictures with the permission of the builders.
First up is the excellent work of Chris Howard, W0EP. Chris placed his finshed rig in a very nice Ten-Tec enclosure (very similarly to how I was planning on building mine, I might add). Here's his description of the construction:
I put the VFO in a little box. The VFO controls are set back from the
face of the VFO box and I have some fiberglass shaft extensions
coming through the VFO box, out to the front panel. I was attempting
to minimize interaction with the VFO (but my mechanical rigidity isn't
all that great so I get a little bit of microphonics)
My faceplate isn't lettered, so I just have some penciled-in control
captions. From the left it's key, phones, volume, frequency and RIT.
The little circuit board on the back, and the pushbutton on the front
are a K1EL keyer. I may put in some more pushbuttons eventually
to do keyer memories, etc. I'd also like to locate a reduction drive
for the frequency control.
We also got some photos from DAve Goodrich, WB9VTB. DAve shows skills in fabrication and construction that blow away anything that I could possibly do. It's a great feeling to see someone take something that you've designed and improve it to an even better state. DAve also incorporated a PA2OHH binary frequency counter into the rig, you can see it right above the tuning knob. Here's a quick description of the photos that follow:
The VFO is below the MB and the freq display is in a separate enclosure tucked in next to the VFO enclosure on the right side of the case. The black RCA jack is for the freq display and the red RCA jack is for the VFO.
Congratulations to both Chris and DAve for jobs well done! I'm really proud to see such fine work done on the Willamette!
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.
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.
Pardon me while I take a moment to direct your attention to the website of KC5WA, who has been simultaneously working on builds of both my Tualatin and Willamette transceivers. RC does an excellent and really thorough job in documenting his work on his projects, and his web pages are a great resource for those who might be building or troubleshooting these rigs. Both pages are still a work in progress (as are the rigs), but RC updates them on a regular basis. Check out his very detailed page on the Tualatin or the one that he is currently working on about his Willamette build.
I just got my Summer copy of the QRP Quarterly and got a nice little surprise in the 2008 FDIM Building Competetion and Show & Tell article on page 32. Right down at the bottom-center of the page is a photo of the W8BH entry, his homebrew Willamette rig! Bruce did an incredible job with his build and deserved to win an award (in my very biased opinion). You can see some better photos of his work here and here. FB job Bruce and congratulations for making it to the pages of QQ!