The
"Talking" Pixie2 QRPp AM Transmitter
The Pixie2 is a simple QRP CW transmitter that dozens of ham
radio operators have successfully built. (QRP is ham jargon for low-power
operations, and CW is the simplest method of sending Morse code merely by
turning a carrier-wave on and off.) The Pixie2 is usually built for the 40 meter
band but it 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 the 8-ohm side of a transformer. The 1k ohm
side of the transformer is inserted in the V+ supply going to the 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 all the
parts are available.
The level of the audio fed to the transformer is adjusted until
the best sound quality is achieved. The Talking Pixie will not sound as loud as
commercial stations but the user must avoid the temptation to over-modulate;
nobody will listen to an over-modulated signal.
parts list
C1: 100 pF
C2: 220 pF
C3: 82 pF
C4: .01
C5: .01
L1: 150 uH
L2: 22 uH
Q1: 2N2222 or 2N3904
Q2: 2N2222A (metal can
type) or 2N3866
R1: 47K
R2: 1200
R3: 33K
R4: 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 not sound 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 values for various frequencies can be found
on the Medium Wave Alliance's filters
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 the Talking Pixie. What sounds good on the test bench with a 50-ohm dummy
load attached to the output might not sound as good with real-world antennas
like short end-fed wires. Some kind of antenna tuner might be helpful. (By the
way, two 100-ohm resistors in parallel make an adequate dummy load for this
rig.) 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 many variations 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. This could give more linear modulation. Replace L1 with a
5K variable resistor; remove the crystal for a moment 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 voltage source. By varying this voltage control
element with the audio signal, highly linear modulation is achieved.
Transformers tend to present variable impedances, causing the PA to be less
stable under varying load conditions. A direct-coupled modulator can offer the
potential for great tolerance of loads that aren't precisely +50 j0."
Other QRP CW transmitters can also be modified for amplitude modulation. You will find schematics for 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.
Thanks to wodido
for allowing us to reproduce this page.
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