These days, I have been looking around some schematics for building a minimal CW transceiver. However, I do not fully understand their principle of operation.
As an example, here is the Pititico transceiver by PY2OHH:
There are many similar designs around (see the one by HA4OO or this list), but I prefer to focus on this one as of now in order to try to fully understand it.
As far as I know, in the previous schematic the crystal must act as the signal generator and, the BJT, as an amplifier (maybe a class-A amplifier, but I do not fully understand the feedback connecting the emitter with the base). C4 seems to me to be related with matching the output impedance of the antenna, but it is just a guess. U1 is a high impedance headphone and C3 is just a decoupling capacitor, and I cannot name the use of the rest of the components of this circuit which I am trying to decypher.
The thing I find the most disturbing on that schematic is how the separation between TX and RX is done. I read that the network C1-L1-C2 creates an offset of less than 1KHz to diplex both signals. How is this possible? Moreover, how is downconversion perfomed? Isn't there any mixer?
Thank you in advance.
Edit 1: Thank you to the people that answered; I know understand that the TX is a Pierce oscillator directly connected to the antenna, and that the RX acts as a regenerative receiver.
It would be great if someone could explain how TX and RX are combined, namely, how those capacitors (C1, C2 and C4) produce (demodulate in a direct downconversion) an offset signal at the headphone output.
Best Answer
Oh man, there's so many things going on with that circuit...
The circuit is a Colpitts oscillator, always.
With the key up, the strength of the oscillation is severely limited by R2. This does two things: it keeps you from getting nice QSL cards from the FCC for spurious emissions (maybe), and it lets signals coming in the antenna affect the strength of oscillation. Note, however, that you still have an active oscillator connected to an antenna, so it will be radiating. It's kind of a regenerative receiver with a crystal in it. Received signals are rectified by Q1 (cringe) and appear in the headphones.
With the key down, oscillation amplitude goes up, and C2 is partially shorted by L1, and -- presto! -- you're transmitting a fairly strong signal. Because L1 is now active in the circuit, the frequency of operation shifts slightly -- this means that someone talking to you on your frequency will be received as a tone in the receiver.
(Edited because I was originally looking at the wrong schematic)
At any rate - have fun, and stand in front of he mirror practicing saying "yes sir and/or madam, I will stop using that transmitting equipment immediately!".