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Review of and improvements to the Pixie QRP CW transceiver

The biggest selling (but not necessarily most commonly heard!) QRP transceiver kit would be the Pixie and its variants. Costing less than a takeaway burger meal, it's the cheapest transceiver kit available. The transmitter puts out a few hundred milliwatts CW on a single crystal-controlled frequency (often 7.023 MHz). The receiver is direct conversion with little selectivity. Performance could charitably be described as 'basic' but the design is ingenious, with many parts shared between the transmitter and receiver.

The first question to ask is whether you can make contacts with such a simple affair. The answer is 'yes', with hard work. I class the Pixie as a novelty project, purchased more for the fun of construction than its operating capability. However if you're persistent, have a resilient sending wrist (for all the calling you'll do) and connect it to a full-sized outdoor antenna then you will work stations hundreds of kilometres away.

So how does the Pixie work? One transistor is connected as a 7 MHz crystal oscillator. It is continuously on, to generate the carrier on transmit and to provide the local oscillator for the direct conversion receiver. The second transistor is the RF final amplifier on transmit. On receiver it operates as a crude detector for the direct conversion receiver. The audio signal at the output of the detector is very low which is why the LM386 audio stage is there to provide some amplification. This is the only stage that operates on receive only.

Other parts of the circuit include the pi-network low pass filter and the transmit/receive frequency offset. The latter is required in all direct conversion CW transceivers and ensures that the transmitter carrier and receiver local oscillator frequency are slighly different so that incoming signals produce an audible beat note in the receiver. The video below describes transmitter/frequency offsets in more detail.

Transmitting on the right frequency with direct conversion CW transceivers

I've built and seen several Pixies. There are small variations between them. Instructions are often rudimentary (if supplied at all) and circuits may have different values to that supplied. Hopefully the differences will be in non-critical parts of the circuit. For instance there should be little difference in performance if 100nF is used instead of 10nF for RF coupling and bypass capacitors. It's worth reading up on others' experiences before you insert and solder the first part. This video on troubleshooting may also be helpful.

Troubleshooting a Pixie QRP CW transceiver kit

You see pictures of Pixies with small 9 volt batteries. I'm not a fan. For a start 9 volt batteries are expensive for the small power delivered. The poor quality ones won't last very long given all the CQ calls you'll make to get even one contact. And compared to 12 volts the transmitter RF output power will be low, and you need all of that you can get. So do yourself a favour and use a rechargeable 12 volt battery source instead.

Testing requires a dummy load, signal generator (eg another transceiver) and preferably an RF power meter. Even when it's in receive you should be able to hear the crystal oscillator on a nearby SSB or CW receiver or transceiver tuned around 7.023 MHz. If you can't bring a wire from the receiver's antenna socket to near the Pixie board. Pressing the key should result in a big increase in signal and a small frequency shift.

If you're lucky you'll just hear band noise and maybe CW signals. If you're unlucky you'll hear breakthrough from AM broadcast stations. This is one of the liabilities of the Pixie – its weak and unselective front end makes it only suitable for use well away from high powered transmitters.

Because it is fixed frequency contacts will come harder with a Pixie than with other QRP rigs. The low transmit power and the broad receiver are other hindrances. But, if you're using a full sized antenna such as a dipole, persistence should allow at least some contacts. Some information on improvements you can make is in the following video.

Making the Pixie QRP transceiver kit slightly less appalling

To summarise the Pixie is unbeatably priced. It's almost worth buying for the crystal and audio IC alone, even if you don't assemble the rest of the kit. If it doesn't work or you're not successful with it the loss can be written off. The kit is also great for soldering practice before you try dearer and more capable kits.

Making contacts is hard work. There would be numerous cases where stations would be workable but are not because the Pixie is on the wrong frequency or there is interference or breakthrough that the receiver can't cope with. Nevertheless its few hundred milliwatts can go a long way and the contacts you do complete will be rewarding.

Items you might need

A Pixie kit if you haven't got one already (why not get several to experiment with?)

Crystals to allow a choice of frequencies (buy several to experiment with parallel crystals)

Variable capacitor (use to give some frequency agility)

Toggle switch (use to switch between crystals to change frequency)

 

Disclosure: I receive a small commission from items purchased through links on this site.
Items were chosen for likely usefulness and a satisfaction rating of 4/5 or better.

 

Next steps

The above improvements can make the Pixie easier to use and allow more contacts. But past a certain point, due to the small circuit board and the interaction between different stages, it's better to put the Pixie to one side and start afresh if you want top performance. This can either be a better kit or a scratch-built homebrew transceiver. Ideas for both these options are elsewhere on this site.

Alternatively you could use the Pixie kit as the heart of another project. Buy a few Pixie kits, study the circuit, let your imagination run free and see what else you can make from one. For ideas and inspiration see The Pixie Hack Challenge.

 

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