After some more experiments with the Pixie kits I am suspending the Pixie for a while.
Versions of the Pixie modified for transmit only, 80m version and 630m version with modified inductors and capacitors in the frequency dependent circuits, and the components specific to the receive mode have been omitted. For example, the diode and switch for RX oscillator offset has been removed. Further, a variable capacitor ("Polyvaricon") has been connected from the crystal to ground, in place of the diode offset "capacitor")
80m:
The oscillator does oscillate in the "RX mode", and the frequency range is about 3500 - 3600kHz. When keying, however the oscillator frequency still changes frequency, sometimes in an unpredictable way (sometimes up, sometimes down).
I suspect 2 reasons:
- the oscillator voltage is not stabilised
- the load of the oscillator changes when switching to "TX" mode.
Further, the output of the transmitter with the ceramic resonator in the circuit is too low to register on the SWR/PWR meter. Not exactly a resounding success. This could be due to a lower Q of the ceramic resonators, resulting in a lower oscillator output.
630m:
I could not register any output from the oscillator. There can be several reasons:
- the oscillator may be more loaded by the buffer stage
- the capacitive load of the oscillator capacitors may be too high
- the frequency determining capacitors may have a low Q
- the Q of the ceramic resonator may be too low for solid oscillator loop gain
- the capacitor ratio in the feedback loop may result in a loop gain below 1.
My conclusion is that the basic design of the Pixie is too simple for a wide range (ceramic resonator controlled) transmitter, though it may be brought to work OK with a crystal, with some VXO functionality. A few switched crystals may provide some coverage, especially in the 40m band. I have crystals resonating on 7000, 7015, 7023, 7030 and 7040kHz, possibly 7035kHz. From box73,de 7005, 7010, 7020, 7035, 7050, 7052, 7055 and 7070 are available This should provide some coverage of the 40m CW band, so I might try that and see how much of the band can be covered with such a setup.
I see that several Xtal frequencies for the 10MHz band are available at box73.de , so a 10MHz version of the switched Xtal TX may be feasible. Frequencies available from there:
10105, 10106, 10115, 10116, 10120, 10135 and 10145kHz. This should provide good coverage of the 10MHz band with a VXO circuit.
Also from Box73.de :
80m : 3530, 3540, 3550, 3555, 3560 and 3570kHz. Generally available is also 3579kHz Partial coverage of 80m looks possible.
160m: 1800 and 1820 available from box73.de. Generally available: 1843kHz. I do have a crystal for 1963kHz.
Experiments with crystal control of the Pixie should therefore be available to me on the following bands:
- 160m
- 80m
- 40m
- 30m
While I do have crystals for higher bands (14,18,21,24 and 28MHz) I consider that the Pixie is probably not suitable for those bands. I could do a test on 28MHz, just to see what the kit does, but I do not expect good results. It may possibly work with better transistors, and with lower power.
I am still looking for crystals in the 5250 - 5450 kHz band.
Receiver experiments, with the offset diode circuit removed, should prove interesting enough. For a receiver a stabilised voltage supply can easily be arranged
Other experiments have been suggested by VK3YE on his page :
The Pixie Hack Challenge
I may try out a few of those at a later stage.
As you can see, there are still many experiments/hacks possible with the Pixie kits. That is why I purchased quite a few before starting the experimentation.
I may have a pause in the Pixie play now. Many other projects I want to do. We shall see.
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