Pixie QRP CW Transceiver Kit Project -
Here is the schematic and photos of the completed Pixie. These are very inexpensive - about $7.00 on Amazon, and as little as $3.56 on eBay including shipping direct from China! These crystal controlled QRP (and QSK) transceivers use a minimal number of parts and the transmitter produces a respectable, stable signal with an output of about one Watt depending on supply voltage. The receiver is, let's say, "Hi-Fi". High fidelity is a great thing for music, but for CW maybe not so much. I will try to explain:
The Pixie is a direct conversion CW transceiver. Referring to the schematic diagram, the Y1 crystal oscillator Q1 transistor is always powered. On Transmit, closing the telegraph key powers the single stage power amplifier Q2 transistor by grounding the emitter and amplifies the crystal oscillator output. It has simple QSK, since closing the key also shorts the audio amp LM386 input to mute audio.
The amplified transmitter signal is fed thru the Pi output filter to a 50 ohm antenna and about 800mW is produced with a 12V power supply. Transmitter works fine, and it is rock bound without adjustment. Carl, WB0CFF recorded my Pixie, and I recorded John's, K9KEU Pixie: since the oscillator is always on, there is no chirping common to some other one or two transistor transmitters. (Link to the recordings below.)
On receive, whenever the key is open, the transmitter power amp circuit is used as the receiver detector. Received signals come from the antenna in thru the Pi filter. The emitter current to the PA transistor Q2 is limited by emitter resistor R5 and the power amp base-emitter junction is used as the detector. Input RF signals are mixed with the crystal oscillator signals then amplified by LM386 audio amp IC into a pair of earbuds or headphones.
There is no audio filtering, so the product of ALL received signals in the 40m band are fed into the audio amp and one is treated to full Hi-Fidelity reception! As an added bonus, DC receivers hear both sidebands, so one will hear at least 20kHz of band simultaneously, 10 or more kHz above AND below the transceiver's frequency! The little trimmer pot W1 provides about 1kHz of adjustment for the receiver's offset.
Transmitter and receiver tests are here, along with more detailed infornmation including the recordings by Carl and of John's Pixie: https://youtu.be/oNu0kqH5Jnw?list=PLFiIJJI5T2wfSfddvYLS84k7x97pj-we8
This kit has only 7.023 MHz crystals. So a ham in the USA must have an Extra Class radio license to operate one on the air. That is probably a good thing, because if a novice built one, they would be likely be totally confused by plethora of signals on other frequencies - none of which they could ever work using the Pixie. At least, in the Extra Class portion of the band (below 7.025 MHz in USA), the guy building and using it should theoretically understand the concept and not be disappointed in the results! But in any case, I suppose a cheap little kit like this is a motivational project that might stimulate thinking or even lead to other more practical QRP transceiver kits like an MFJ Cub or Pacific PFR3.
73,
Lynn
