Chirp and My Michigan Mighty Mite

I’ve been doing lots with my recently brewed Michigan Mighty Mite.  By lots I mean listening to my Tecsun PL-600 with SSB turned on and twiddling the BFO knob to find the tone being emitted by this new transmitter.  What I noticed early on is that the tone emitted started as what sounded like a pretty pure tone would quickly degrade into a warble before settling out at a slightly lower tone which became mostly stable.  After I little research I confirmed that this is what I had heard referred to as “chirp.”  That all makes sense — a variability to the tone emitted might sound like a like a bird’s chirp.

But what was causing it?  It sounds like a loss of power, so I looked into the stability of the power supply, a 9v heavy duty battery.  Watching the voltage across the battery, I saw an approximately 1v drop.  That looks like a culprit!  (Right?)  So, I proceeded to try using a more substantial 7.6v battery pack I made out of 1.2v AA NiMH batteries.  Did that resolve the issue?  Well, if by the dropping voltage, yes.  The voltage dropped less with the NiMH battery pack.  However, I was sad to find, the chirp was still there.  Next idea!

Maybe the transistor is overheating?  After all, I’m using a plastic case 2N2222a TO-90 case transistor.  Everyone says to use a metal can packaged transistor.  So, I figured it could be the transistor for many reasons.  I tried a different 2n2222a.  No dice.  The chirp was still there.  I tried a 2N3904.  Still there.  A MPS A14… Wait a minute, the sound changed.  It still has a chirp, but it is delayed.  So, I learned that the transistor has SOME effect on the chirp.  Next.  2N4401.  Now the chirp is shorter in duration and softer (it doesn’t get as quiet before returning to a tone).  More effects while only changing the transistor.  9287 — Wait, virtually no chirp.  That’s different.  Still there, but greatly diminished.  Am I on to something?  Maybe.  Next, I got some BD139s from Mouser and tried those since they can handle a lot more heat.  What!?  Still a chirp.  One time I held the BD139 body while keying to see if it was getting warm and something surprising happened — no chirp.

Here’s what my bench looks like right now:

Five transistors laid out on postit notes labeled with their part numbers and their effects on chirp.
These are the transistors I tested in my Michigan Mighty Mite to see if the transistor affected the chrip. The BD139 is in-circuit.

From: http://arcarc.xmission.com/PDF_Electronics/Transmitter%20Faults.pdf

CHIRPING
Chirping only applies to CW (Morse) transmitters. Keying chirps are quick changes in the frequency of a transmitter and occur each time the telegraphy key is closed. […]

In CW mode, if the stage being keyed is too close to the oscillator, then the oscillator may shift slightly off frequency each time the key is closed. These quick changes in frequency sound very much like a canary chirping, hence the term ’chirping‘. Chirping can also occur if the voltage regulation to the oscillator stage is inadequate.

So what’s going on when I hold the transistor?  Well, lots of little things.  I’m not sure which one is causing the change. One thing I realized right after that was that my key is conductive and connected into my circuit.  And, remember, in the Michigan Mighty Mite (MMM, from now on), the circuit is little more than an oscillator…  Hmmm, “if the stage being keyed is too close to the oscillator, then the oscillator may shift slightly off frequency each time the key is closed”  So, by keying and touching the transistor and key I’m altering the electrical (capacitive and possibly feedback) properties of the oscillator.  How can it not chirp?  It’s like licking a tuning fork or something!?  Maybe that isn’t a fair analogy.  I wouldn’t know.  But, it made me smile for a minute 🙂

So, how do I remove the chirp with my hands off the circuit?  I did some reading about Hartley oscillators, which the MMM is styled after, and found that the position of the tap in the inductor in the tank circuit decides how much feedback is delivered to the oscillator.  Then I remembered that the air core inductor I wound on a 1.25″ form for the MMM wasn’t center tapped — it was tapped after like 1/3 of its turns.  So, the orientation of the inductor affects the feedback and therefor stability of the oscillator.  I tried the inductor both ways.  No change.

Then, I remembered something I never should have forgotten: The tuning capacitor.  Doh!!!  I know, it’s a totally noob thing to do.  I did it.  I confess.  So, I tuned the tuning capacitor with the BD139 in place and BAM!  I got a stable pitch (to my ear at least.  I’ll check it on the oscilloscope soon).  That was such sweet music!  Now, I went back and tested all my previous transistors with this current arrangement and used the tuning capacitor.  I found I could make all of the transistors sound a little better than before (with all this experience), and, while there IS variation between different transistors, the BD139 IS still the best.  This leads me to the hypothesis that temperature is a factor.  Am I sure?  Nope.  It is just a likely factor.  If I figure it out 100% for sure later, I’ll share my findings.  For now, it is more stable in my circuit.

My next step is to build this transmitter in a compact and robust package that I can take on the road.  I need to test the limits of my ability to transmit!

Disclaimer: I’m likely to be amazed if I can receive my transmissions just 20 miles away from the town I work in, Orono, ME.  Why?  I’ve never transmitted on anything other than VHF/UHF so far.  My plan is to leave a device recording the output of my Tecsun PL-600 tuned to 7.030MHz and drive to work.  Then, while in Orono, transmit a sequence of code.  Cross my fingers, and when I get home that night, seek to that time of the day and listen!