Etherkit Grab Bag

A little of this and a little of that

As Project Yamhill nears actual physical implementation, it’s time to start building some more real-time, two-way communication. We’re starting to get some beta testing and discussions going! If you’d like to participate in Project Yamhill chat, please click on the link below. Thank you!

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Ascending the mountain to a well-polished, finished product is a long process; relatively easy in the beginning, but more arduous and straining near the end. However, reaching the goal makes the exertion well worth it.

Photo by Susann Schuster on Unsplash

These last few weeks have yielded no great breakthroughs on any of the irons that I have in the fire, just the grind of trying to wring out a lot of the tougher bugs. So here’s a synopsis of what I’ve been working on lately, what I’ve discovered, and what still needs to be done in order to progress. Even though these are all separate projects, both Party Line 80 and EtherKeyer Mini directly relate to Project Yamhill, as both have elements that will be incorporated into Yamhill (firmware in the case of EtherKeyer Mini and some hardware design in the case of Party Line 80).

Project Yamhill

The Front Panel PCB has been the focus of most of my recent work, and I’ve already touched on a lot of the changes and fixes needed for Rev B. The audio chain has mostly functioned as I expected with the exception of one thing: the op-amp low-pass filter block. It introduces far too much hiss into the audio when used in the place where I had originally planned for it to be.

Thinking that I had used a poor op-amp choice in my first prototype, I switched to a TL084 that I found in the junk box, which is very commonly used for this exact type of filter. This component swap yielded no improvement in noise. I then considered that perhaps I was using resistors with too much resistance in the filter elements, introducing thermal noise into the inputs that way. Recalculating the filters and trying resistors roughly an order of magnitude lower didn’t help any either.

At this point I realized that I was most likely using the filter at too early a stage in the audio chain. I think these are filters are inherently noisy when you try to use them at low levels and that they have to just before the final audio PA, at earliest. I also think I’m pushing my op-amp audio preamp stages a bit too hard, which is introducing some additional noise that could be eliminated with a redesign. So I’ll most likely be adding at least one more op-amp stage, with more moderate gain in each of the stages. I’d love to hear from any of you who are experienced in these types of circuits if you have any suggestions.

EtherKeyer Mini

Nearly all of the work to be done on EtherKeyer Mini relate to the firmware. The circuit itself is very simple, and appears to be working as expected (with one small exception, mentioned in a bit). The existing firmware already strains the limit of the 8 kB flash program memory, so a lot of what I’ve been working on here is code optimization to reduce program size without affecting functionality.

After a bit of refactoring, I was able to shave a decent amount of bytes off the firmware image, which allowed me to implement some more important features. EtherKeyer Mini now has a setting to reverse the paddle lever functions, as well as the ability to operate in either Iambic A or Iambic B keying modes. The last major remaining bit of functionality that I think must be implemented is the piezo sidetone. At that point, EtherKeyer Mini should have a complete enough feature set that it will be ready for wider deployment for casual CW operation and activities like POTA activations. Any further features that can be squeezed into the firmware would just be gravy.

One unexpected behavior cropped up that at first I did not realize was caused by EtherKeyer Mini. While working on getting Party Line 80 (see below for more details) fully bug-free, I discovered a subtle problem with the EtherKeyer Mini hardware spec. I’ve been able to use EtherKeyer Mini with both my Icom IC-705 and Icom IC-7300 for daily CW use for about a month now (eating that dog food!) with absolutely zero problems. However, when pairing EtherKeyer Mini with Party Line 80, I was getting a really nasty thump keying artifact, as well as a strange sounding sidetone. I thought the problem was with the Party Line 80 until it occurred to me that I should try a different keyer just to make sure. When I used my CMOS-4 with Party Line 80, the keying acted as I expected. I discovered that on key down on EtherKeyer Mini, there was about 6 Ω of resistance across the key jack, compared to something more like 0.5 Ω on the CMOS-4. My working theory on this is that the low-voltage available from the ATtiny85 running on a CR2032 coin cell (approximately 2.8 V) isn’t quite enough to fully turn on the 2N7000 MOSFET keying transistor. It’s not a problem with the commercial radios keying circuits, but it’s enough resistance to cause a problem with the keying circuit used in Party Line 80. I’m going to order a few different MOSFETs from Digikey with a lower V_gs(th) on my next order to see if that will correct this issue.

Party Line 80

Party Line 80 Prototype

I’m very close to getting all of the bugs in the Party Line 80 Rev A PCB worked out. After figuring out that EtherKeyer Mini was the source of some of my problems, the last thing that’s tripping me up is a slight remaining keying artifact on key down. It’s not super offensive, but I think I should be able to get rid of it. The audio chain thumps a bit on key down, but it doesn’t happen on every keyed element. It does come back after a second or so of no keying, so it seems pretty obvious that it’s related to a capacitor charging and then suddenly discharging on keying.

Actually, I have a pretty good idea at this point what is causing the problem, but I’m not sure how to fix it yet. By methodically circuit tracing and disconnecting various stages of the radio, I can see that the keying transient is caused by the dip on V_CC when keying causing an impulse on the collectors of the transistors of the W7ZOI feedback amplifier which immediately follows the mixer. That amplifier is already pretty well decoupled, and beefing up the decoupling and even adding an active decoupler does not help. I really want to track this down and fix it soon, as I’m eager to get this rig on the air and make some QSOs!

What’s Next

I’m fairly close to getting both Party Line 80 and EtherKeyer Mini in a solid enough state to do a wider release on them. With any luck, I’ll be able to report on successes on that front on the next post. After that, I’ll be back to working on the final changes to the Project Yamhill Front Panel PCB and hopefully a new spin in two to four weeks.