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
My review of the CR Kits Pixie with two frequencies supplied
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)
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.
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.
