Electrical – Understanding the Michigan Mighty Mite

crystaloscillatorresonancetransmitter

I'm quite interested in this rather simple transmitter design called the Michigan Mighty Mite, described here:

https://makerf.com/posts/mighty_simple_shortwave_transmitter

I'm familiar with resonant circuits and crystal oscillators such as these:

enter image description here

I know that a crystal resonator can operate in either "series" or "parallel" modes as shown in the picture above. I understand the principle behind these basic circuits: the crystal acts as a resonant filtering circuit and when a voltage is applied to it, it starts to oscillate at its resonant frequency. The oscillation is prevented from dying out because it's continuously being fed energy by the transistor amplifier.

But still I can't quite figure out how the MMM works. I'm quite confused as to whether is belong to the series or parallel class of oscillators. Here is a picture from the website:

enter image description here

Here it looks like the L1 and the variable capacitor form one resonant circuit,
but then there is also the crystal. I assume the crystal here is
the element that determines the frequency, so what does the
L1 and variable capacitor resonant circuit do? Also the L1 is tapped, and the tap is going into the collector of the transistor, which is also a difference to the oscillators in the above picture.

So these were my initial observations to the differences of this transmitter to simple oscillators. I tried googling but I couldn't really find sources that would explain this circuit beyond very brief overviews. I would like someone knowledgeable to explain this transmitter in greater detail. Thank you!

Best Answer

Redraw the Michigan Mighty Mite so that it conforms to the Pierce oscillator circuit. The similarity should be obvious. R2 has been ommited in the MMM circuit...Q2 is operating at much higher current than the small-signal Pierce.

The antenna-coupled link winding on MMM's L2 has been omitted, and the key switch in series with R4 (27 ohm) is replaced with a short circuit:

schematic

simulate this circuit – Schematic created using CircuitLab
In both circuits, the crystal feeds back radio frequency signal from transistor output (collector), to transistor input (base).
The high-impedance collector load of the RFC choke in the Pierce oscillator has been replaced with a parallel resonant LC circuit in the MMM (L2 in parallel with 365 pf).

The MMM is a power oscillator that puts out more power when L2 is connected to the collector through a tap, rather than going to L2's high-impedance end (where it meets the crystal).

Thinking black-and-white about a series-resonant crystal versus parallel-resonant crystal gets you into a bit of trouble. Most crystal oscillators will give you an output frequency that lies somewhere between the lower-frequency series resonance, and slightly higher-frequency parallel resonance.

A 2N3904 is perfectly fine for Q1 in the low-power Pierce oscillator, but would likely overheat in the MMM circuit (Q2). A bigger transistor should be used, likely with a top-hat heat sink.