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Project Notes

#401 pixie

Research the history and variants of the Pixie QRP transceiver, and build a basic Pixie 4.3 kit on 40m.

Build

Notes

Pixie is the name given to a class of QRP transceiver designs that evolved from the Foxx design described by George Burt GM3OXX in summer 1983 edition of SPRAT magazine. The distinguishing feature is the use of the same bipolar device as the transmit PA and the receiver mixer.

There are many versions of the Pixie available. I picked up a kit based on the Pixie 4.3 PCB from a seller on aliexpress. The kit had a product ID of E0991, but that may just be the merchant’s stock code.

The Pixie may be operated on various bands, but most kits (like the one I’m using) target 40m.

Kit Details

Specs as listed by the vendor:

  • This transceiver kit is a perfect DIY kit for radio amateur.
  • The local frequency is fixed at 7.023MHz when transmitting, the local frequency is 7.023-7.026MHz when receiving.
  • Increase a DC buzzer on the PCB, it will beep when pressing the button. But you can choose whether to install the buzzer as you want.
  • Can be powered by 9V battery(not included) or 9-12V DC power supply.
  • PCB Size: 5 * 5cm / 2 * 2in
  • Output Power: 0.8W(9V power supply) / 1.2W(12V power supply)
  • Power Supply: 9V Battery(not included) / 9-12V DC Power Supply
  • Antenna: 50ohms,unbalanced(antenna is not included)
  • Package Size: 7 * 9.5 * 1.5cm / 2.8 * 3.7 * 0.6in
  • Package Weight: 24g / 0.9oz
  • Power supply: 9V-12V (Recommended 9V laminated battery)
  • Antenna: 50 ohm, unbalanced
  • Frequency range: transmitter local oscillator frequency: 7023kHz; receive local oscillator frequency: about 7023-7026KHz
  • Headphones: low-impedance headphones
  • Transmit power: 0.8W (using a 9V power supply), 1.2W (12V power supply)
  • Suppress spurious (harmonic):-20dB

kit_parts

kit_pcb_front

kit_pcb_rear

How the Pixie Works

The South Canadian Amateur Radio Society has published one of the best references for the Pixie at w5nor.org/pixie/. It includes links to some of the best resources available.

The main trick that I had to get my mind around was how this circuit can operate as both transmitter and receiver.

The “key” is to note the effect of the key on R5, C9, W1 and D3.

With key-down:

  • R5, C9 and W1 bypassed:
    • Q2 acts as a class B power amp for the Colpitts local oscillator
    • output to attenna via pi-network filter that results in a roughly sine wave
    • efffect of W1/D2 padder capacitor is removed from the Colpitts oscillator, resulting in a transmit frequency that should be spot on 7.023 MHz
  • D3 conducts/routes to ground, which disables the LM386 audio amp completely

With key-up:

  • D3 is reverse-biased, enabling the LM386 audio amp
  • R5, C9 and W1 all back in circuit:
    • The W1/D2 padder capacitor allows receive frequency adjustment (7023-7026KHz)
    • high resistance of R5 effectively disables Q2 RF amplifier and turns it into a simple mixer as part of a direct conversion receiver
    • C9 makes a dead short at RF frequencies. This eliminates f1, f2, and f1+f2, leaving f1-f2 (audio - hopefully!) to pass to the LM386 amplifier

Components and Schematic

The kit I bought did not come with a schematic. The PCB is marked as version “4.3” but I haven’t found a definitive 4.3 schematic, so I traced the PCB and with some reference to other sources I redrew the schematic in EasyEDA.

While it turns out that most components are consistent across versions, the following are the components that most commonly vary. Note:

  • the component labels refer to my version of the schematic
  • the “As Used” column is the value I am using in this build
  • “Other v3/4” refers to various schematics that appear to be v3 or v4 variants
  • “Pixie 2” refers to components I’ve seen in Pixie 2 schematics
Component As Used Other v3/4 Pixie 2 Notes
L2 1µH 1µH 1.2µH Some recommend 1µH for 40m and 2.2µH for 80m operation
L3 100µH 100µH 150µH  
C2 10nF 100nF 10nF  
C4 10nF 100nF 82pF  
C5 470pF 470pF 820pF  
C6 470pF 470pF 820pF  
C10 47nF 47nF 100nF  

Schematic

Modifications

Frequency Agility

A common Pixie modification appears to be one to provide frequency agility so as not to be locked on the crystal frequency. The kit already includes the “padder capacitor” modification (comprising D2, R6, C8, W1) that provides some adjustment of diode capacitance.

I haven’t taken this further for now, but have seen some interesting ideas such as vk3ye’s video that covers some crystal and ceramic resonator VXO circuits to add frequency agility.

Side-tone Indicator

The kit included a piezo buzzer side-tone indicator. I did install it, but left the label on the buzzer as everyone knows how annoying they can be!

I did add an LED side-tone indicator (LED_K and R9), which was not in the kit - the LED in the base design is a simple power-on indicator.

High-frequency Bypass

The kit already included a Zobel network on the LM386 output (R7, C11). This is intended to filter high-frequency spikes.

Enclosure

The PCB is a very nice size to fit in an Altoids can;-)

Hooked up for operation:

pixie_build_operating

Operating Results

Not much to report yet - other than it appears to be operating correctly.

Here’s a scope trace of the key-down output into a dummy load:

output_wave

Credits and References

About LEAP#401 Radio
Project Source on GitHub Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is my personal collection of electronics projects, usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting (IMHO!).

The projects are usually inspired by things found wild on the net, or ideas from the sources such as:

Feel free to borrow liberally, and if you spot any issues do let me know. See the individual projects for credits where due. There are even now a few projects contributed by others - send your own over in a pull request if you would also like to add to this collection.