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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!

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  • \$\begingroup\$ I read that you are familiar with the top two circuits but I don't think you understand them. For instance, in the left circuit it is described as operating with a series resonant crystal but, if that were so, where does the extra phase shift come from to make this truly oscillate and not behave like a transistor with a feedback capacitor. I'm not trying to rubbish you; I'm trying to point out there there are subtleties in oscillators that you are not aware of..... \$\endgroup\$ – Andy aka Jan 29 '18 at 12:16
  • \$\begingroup\$ .....For instance, in the Colpitts oscilator (right hand side picture) are you aware that it won't work without the emitter having an internal series resistor. Without this resistor you will NOT get the required feedback phase angle for sustained oscillation. In other words you think you know how A and B might operate but until you more fully understand what you currently think you understand, you will only scratch at the surface. Like I said I'm not trying to rubbish you but to urge you to ask a simpler question where an answer can be provided that delivers understanding. \$\endgroup\$ – Andy aka Jan 29 '18 at 12:19
  • \$\begingroup\$ @Andyaka Yes Andy I understand and I think you're right. \$\endgroup\$ – S. Rotos Jan 29 '18 at 19:30
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    \$\begingroup\$ Hope you do, oscillators can deceive and appear simple so it’s easy to think you know how something behaves then, when confronted with a new oscillator topology there is nothing in the tank for you to use that might help. I think google generally does a poor job here but, good luck, and if you raise another question on oscillators please let me know. \$\endgroup\$ – Andy aka Jan 29 '18 at 20:05
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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.

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  • \$\begingroup\$ The way you've drawn it makes a lot more sense, I can see the similarity to the Pierce oscillator. Few things still puzzle me: Why is the LC circuit replacing the RFC? It kind of seems there are two frequency filtering elements here, the LC circuit and the crystal. Does the resonant frequency of the LC match the frequency of the crystal? I can see that when we turn on power, the LC oscillates at its resonant frequency. But then there is the crystal between the output of the LC and the input of the amp. What does it do there? Which element determines the frequency of the circuit? \$\endgroup\$ – S. Rotos Jan 28 '18 at 22:59
  • \$\begingroup\$ Yes, LC resonant frequency should match the crystal resonant frequency. Since the crystal is much higher Q, it determines the exact oscillating frequency. The LC allows higher power output: the link coupling to the antenna matches the high collector impedance to the low antenna impedance. The LC is also a band-pass filter that reject harmonics somewhat...this oscillator has collector current that is likely harmonic-rich. So 3 things the LC does...high Z at oscillating frequency, impedance match, bandpass filter. \$\endgroup\$ – glen_geek Jan 29 '18 at 2:07
  • \$\begingroup\$ Okay, the picture is getting much clearer now. One more thing: Why is the feedback being fed to the tap in the coil? This seems quite weird. You mentioned that it increases power output, but I don't see how. I don't really understand what you mean by "L2 high impedance end". I apologize for what can be a very simple question but it seems my understanding of oscillators is not as good as I thought! But thank you for answers, I think I almost understand this. \$\endgroup\$ – S. Rotos Jan 29 '18 at 9:38
  • \$\begingroup\$ @S.Rotos "L2 high-impedance end" is where the crystal attaches. Its low-impedance end is where power supply attaches. The tap is mid-impedance. You *could move * the tap to the high-impedance end, it still oscillates. But now the collector sees a much higher impedance. Collector voltage now swings very easily down to the base voltage each cycle...can't go more negative. At the lower-impedance tap point, this voltage saturation happens less: transistor collector current is larger to get to this same saturation point...more power gets out. \$\endgroup\$ – glen_geek Jan 29 '18 at 14:17
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The crystal must pass the feedback to the base of the transistor with right phase shift at the right frequency. If this oscillates, it happens. Obviously the oscillation frequency is near the serial mode resonant frequency of the crystal.

The tapping and transformer structure of the resonant circuit is developed to keep the oscillation waveform sinusoidal enough (=low enough transmitting power at the harmonic frequencies). Your own oscillator examples do not pay any attention to harmonic content.

Tuning the 365pF capacitor needs something. One can add a miniature incandescent bulb (=a lamp with filament for a small flashlight) in series with the antenna wire.

ADD due the comment:

Sinewave oscillators are amplifiers with feedback. They oscillate just like a sound system will scream when one brings the microphone too near the loudspeaker. The oscillation happens at the frequency where the signal returns from the output back to the input through the feedback path just in the same phase angle (=delayed full cycles) as it was inputted to the amp and at least as strong or amplified.

The crystal causes wildly varying phase shifts in different frequencies. Proper crystal oscillator designs have the crystal in the place where the oscillation conditions come true only in one frequency. In the mighty transmitter the feedback from amp output to the amp input is strong enough and causes the right phase shift near the series resonance mode frequency of the crystal.

Harmonics: A pure sinewave is a dream. Practical oscillators distort the waveform. The well known math of Fourier series or a practical test with a transmitter and radio receiver proves that practical oscillators send at a series of frequencies. If the crystal is for 7MHz, there's also some output at 14, 21, 35, 42, 56...Mhz, all at the same time. The resonant circuit at the output can be designed to be as well a part of the feedback circuit and a bandpass filter which attenuates the unwanted frequencies weak enough to stop collecting complaints.

Tapping and transformer make possible the resonant circuit to have at the same time low enough attenuation for the transmitting frequency and high enough attenuation for other frequencies. Proper design needs complex variable mathematics and it was seriously researched already when electron tubes were taken into the radios.

Tuning: This circuit works only if the output filter is tuned to not attenuate dead the frequency of the crystal. You need some indicator of existing output. Adjust the capacitor for maximal current in the antenna wire (=brightest light in the lamp in series). The right adjustment is critical.

Your own example oscillators: The leftmost has the feedback through the crystal. The rightmost is a little tricky. The transistor is used as common base amplifier, the input is at emitter. Feedback is through C1. The crystal in this circuit acts as an parallel resonant output filter and the harmonic content is obviously lower than in the leftmost circuit.

An link to general oscillator theory (no crystal oscillators, all applications are for low frequencies with TI's parts)

https://www.ti.com/lit/an/sloa060/sloa060.pdf

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  • \$\begingroup\$ Thanks but I don't understand at all. In the first part: What feedback and what phase shift? And how is it obvious the frequency is at the serial mode resonant frequency? In the second part: How does the tapping make the oscillation sinusoidal enough? And lastly: What do you mean by "tuning the 365pf capacitor"? \$\endgroup\$ – S. Rotos Jan 27 '18 at 20:48
  • \$\begingroup\$ @S.Rotos You advertised you understand the oscillators. It's not true. Seemingly you do not consider sinewave oscillators as amplifiers with feedback, which is the practical way to think if one wants to understand the designs. The crystal behaves in terms of current and voltage like a ultra low-loss LC circuit, so low-loss that it's impossible to replicate with coils and capacitors. I added more to the answer. \$\endgroup\$ – user287001 Jan 27 '18 at 21:32
  • \$\begingroup\$ But my confusion with this circuit was not really about the theory of oscillators but rather with the confusing layout of the MMM circuit as given in the picture. I have hard time seeing things like where is the output of the resonant circuit, where is the input to the amplifier, exactly what are the signal phases at each point etc. My confusion is hard to describe in words, I will see if I can improve the question.. \$\endgroup\$ – S. Rotos Jan 27 '18 at 21:58

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