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Friday, February 20, 2015

Italy, Spain, Gibraltar, a Flight to Prague, and How the Mighty Mite Really Works

Gab IZ1KSW is a true Knack-afflicted member of the International Brotherhood of Electronic Wizards:  He is an Italian homebrewer who lives in Spain and works in Gibraltar.  At the end of this e-mail exchange he has a great story about reading "SolderSmoke -- The Book"  on a flight to Prague with his Greek girlfriend. It reminded me a bit of the problems I've had with fellow passengers while reading "Hot Iron" on the Washington DC train system. 

A blog post about Gab's version of the Mighty Mite is here:
His desire to REALLY understand the circuit is, I think, admirable.  I know that my quick explanation of how the Mighty Mite circuit works isn't complete, and I'm sure that others will jump in with more details.   


Hi guys,
I write to you because I'm a bit lost.
Ok, the MMM is oscillating, brilliant! 
Now I'd like to understand why it's working and how it's working.
I've been sitting on the workbench with the schematics in front of me and I found some resources on the internet, I've understood the concept of feedback loop but what really make me scratching my head is that I cannot match the MMM schematic with anyone of the typical oscillator design I found (Pierce, Colpitts, Hartley).
I've read online that it can be considered as a Pierce oscillator but from what I've found online I cannot find the purpose of the tapped coil. Maybe you can point me in the right direction before my GF starts complaining about the pile of schematics I'm accumulating in the living room!
Also, if you have any books to recommend, I'll be happy to go "back to the books"

Thank you and 73
Gab - IZ1KSW


OK Gab.   I've been meaning to do this.  This little circuit needs some explanation.   I'll take a shot: 

Start by thinking of this circuit as an amplifier.  The 27 ohm resistor from the emitter to ground (negative terminal) puts a limit on how much current will flow.

The 10K resistor from the base to the positive terminal puts a positive voltage on the base and biases it so that current will flow through the transistor.

Now the fun begins!   It is an amplifier, but it has no input signal!  The input signal is the output signal -- it is like a dog chasing his tail!

The crystal is very important.  It is the main frequency determining element, and it is the conduit for the feedback that gets this thing oscillating.  It is a piece of quartz.  If you put a voltage across it, it will begin vibrating (physically) at a specific frequency.  As it physically vibrates, it also creates electrical vibrations.

So, when you turn this thing on, noise in the circuit will put a bit of charge on the crystal.  It will begin to ring, much like a musical tuning fork.  The electrical vibrations from the crystal will go to the base.  They will be amplified by the transistor and will emerge (stronger) from the collector.   From the collector, they go to the 3.579 MHz  tuned circuit formed by the big coil and the variable capacitor.

The coil wound on the film box serves several purposes. The portion of the coil between the positive terminal and the collector carries the 12V DC to the collector of the transistor.  It also carries the amplified 3.579 MHz signal coming from the collector.  This signal goes through the lower portion of the coil and causes the coil and the capacitor to resonate.  The signal at the top of the tuned circuit peaks when the tuned circuit is tuned to.... 3.579 MHz.

The capacitor/coil tuned circuit (with the tapped coil) are set up so that the right amount of energy is fed back from the output to the input, and that this energy is fed back in the proper phase relationship to the signal at the input.  Think of a child's swing at a park:  To keep the swing oscillating, you have to push at the right moment (frequency and phase) and with the right amount of energy. 

The little capacitor across the battery is to prevent "key clicks."   The output coil on the main coil takes some of the energy and sends it to the antenna while converting the impedance of the antenna to a suitable "load" for the transistor.

Whew,  how did I do?  Lots of electronics and physics in those 7 parts!

73  Bill    


Hi Guys,
 Bills explanation is absolutely perfect –but there is some additional Math in the woodworks known as the Barkhausen criteria where kB = 1
Pete N6QW


Well, what can I say Bill? Grazie mille!!
I keep thinking that you would have been a great teacher, you have the rare ability to explain complicated concepts using simple words. 
Yesterday I finished reading your book SolderSmoke GAWE (yes, you deserve an acronym too) and there have been several "eureka" moments while I was reading it. It gave me a lot of motivation to go in depth and understand what's going on in a circuit down to the physics of the components. I got the Kindle version but I'll order the paper version too, I love the hand make schematic and they're not very readable in the electronic version, plus I believe that a book about radio home brewing must be in the old fashioned paper version don't you think?
There's a funny story about the book. Few days ago I was on a flight to Prague with my YL, I was reading the book and zooming on the schematics to see them better, I was really into it and I didn't notice that the guy sitting on the seat next to me started to look at the kindle nervously, he probably though I was an bomb home brewer HI! So I decided to pass the Kindle to Angeliki so that she could read her books. She's Greek and she started reading a Greek book, written with the Greek alphabet which looks quite weird if you don't know it. At that point probably the guy thought to be sitting in the middle of some exotic terrorist... it was funny.

Wow... as most of the Italians do, I talked too much! Thanks again both for the big effort you make spreading the tribal knowledge with the podcast, the ARCI LBS articles and the books. 
Siete fantastici!

IZ1KSW - Gab

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