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the Talking Pixie

The "Pixie2" is a simple QRP CW transmitter that dozens of ham radio operators have successfullybuilt. (QRP is ham jargon for low-power operations, and CW is the simplest method of sending Morsecode merely by turning a carrier-wave on and off.) The Pixie2 is usually built for the 40 meter band butit will work on frequencies from 1000 kHz up to at least 15 MHz. It is said to output a couple hundred

milliwatts of RF.

The circuit can be amplitude modulated quite easily. A small audio amplifier feeds audio current into the8-ohm side of a transformer. The 1 kilohm side of the transformer is inserted in the V+ supply going tothe Pixie's output transistor.

This modified Pixie2 is called the Talking Pixie. It has 18 components (not counting circuit board, jacks,power supply and external audio amp). Building it on a prototyping board only takes a few minutes if allthe parts are available.

The level of the audio fed to the transformer is adjusted until the best sound quality is achieved. TheTalking Pixie will not sound as loud as commercial stations but the user must avoid the temptation toover-modulate; nobody will listen to an over-modulated signal.

parts list

C1: 100 pFC2: 220 pFC3: 82 pFC4: .01C5: .01L1: 150 uHL2: 22 uHQ1: 2N2222 or 2N3904Q2: 2N2222A (metal can type) or 2N3866

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R1: 47KR2: 1200R3: 33KR4: 10 or 15 ohms (experiment!)T1: 1000 ohm to 8 ohm audio transformer

The frequency is crystal-controlled. A crystal for the frequency you're interested in will have to be

ordered if you don't have one handy.

The transformer must be rated to handle at least half a watt of audio; a very tiny transformer will notsound good and will have too much resistance on the 1K winding.

L3, C6 and C7 form a low-pass filter to attenuate the harmonics generated by the circuit. Specific valuesfor various frequencies can be found on ourfilters page.

L1 and L2 are factory-made axial molded chokes.

variations

The impedance and bandwidth of the antenna will affect the sound quality of AM transmitters like theTalking Pixie. What sounds good on the test bench with a 50-ohm dummy load attached to the outputmight not sound as good with real-world antennas like short end-fed wires. Some kind of antenna tunermight be helpful. (By the way, two 100-ohm resistors in parallel make an adequate dummy load for thisrig.) Needless to say the size and efficiency of the antenna will have a major impact on the range.

If you build the circuit on a prototyping board (as shown above), you can experiment with manyvariations on the circuit design.

Here are some modifications that have been suggested...

Martin Spencer suggested using a FET (such as 2N7000) instead of an NPN transistor for Q2. Thiscould give more linear modulation. Replace L1 with a 5K variable resistor; remove the crystal for amoment and adjust the resistance for about 2 mA drain current.

Mark Weiss wrote: "You can put another transistor in series with the PA and use it as a series voltagesource. By varying this voltage control element with the audio signal, highly linear modulation isachieved. Transformers tend to present variable impedances, causing the PA to be less stable undervarying load conditions. A direct-coupled modulator can offer the potential for great tolerance of loadsthat aren't precisely +50 j0."

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If you'd like to offer any suggestions, please e-mail them to mediumwave@hotmail.com

Other QRP CW transmitters can also be modified for amplitude modulation. You will find schematicsfor such transmitters in ham radio books and magazines, and on websites operated by QRP clubs.

This page is explicitly placed in the public domain.It may be freely reproduced by anyone for any purpose.

Its contents may not be copyrighted by anyone under any circumstances.

return to Engineering Dept. orwebsite entrance

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