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 PIXIE
2 QRPp CW Transceiver
Information and Mods from WE6W
Note: The following emails were sent to
AD6DI from WE6W. Some of these may have been forwarded and the
original recipient is unknown.
In all cases I'd like to thank AD6DI and
WE6W for providing the following information. These discussions are VERY
valuable to anyone wanting to build or modify a Pixie 2 QRPp CW
Transceiver.
--------- Begin Old email discussion --------
> I ran across your web page and was intrigued with the Pixie II QRPp
> transceiver you presented. I purchased the parts kit from Halted
> electronics (my junk box is pretty sparse these days), and have been
> delighted with it. I noticed the circuit is described as a 160-10m
> transceiver, with a change of one inductor and crystal all that's needed
> for switching bands. Although I am coordinated enough to build the
> radio, I am not smart enough to design a circuit. Could you by any
> chance give me the inductor values for the bands other than 80m and 40m
> (I CAN figure out which crystals to use!). Thanks!
Basically 50 Ohms reactance for the components. I just now wrote
a quick program and have it for you:
Exact Values for QRP Frequencies.
Band CW SSB
---- --------------- ----------------
160 1.810 1.910
1.843 (Europe)
80 3.560 3.985
3.710 (Novice) 3.690 (SSB EU)
40 7.040 7.285
7.030 (Europe) 7.090 (SSB EU)
7.060 (Europe)
7.110 (Novice)
30 10.106
20 14.060 14.285
17 18.096
15 21.060 21.385
21.110 (Novice) 21.285 (SSB EU)
12 24.906
10 28.060 28.885
28.110 (Novice) 28.385 (Novice)
28.360 (SSB EU)
6 50.060 50.885
50.285 (SSB EU)
2 144.060 144.285
144.585 (FM)Both filter caps (C6 & C7) are the same.
Values are for CW operation.
160m =4.4 uH, .00176 uF
80M = 2.23 uH, 894 pF
30m = .79 uH, 315 pF
20M = .57 uH, 226 pF
17m = .44 uH, 176 pF
15m = .38 uH, 151 pF
12m = .32 uH, 127 pF
10m = .28 uH, 113 pF
6M = .16 uH, 64 pF
Obviously there might be a little bit of tweaking on the coils to adjust
the match to your transistor, but it is not that difficult. Also, you might want
to add a Bandpass tank circuit on the output to clean up the signal if you get above 20 Meters.
Just link couple to the tank about 2 or three turns from the bottom. That way the 50 Ohm
taps won't desensitize the 'Q'.
Enjoy. And 72 de Ed/WE6W
--------- Begin Old email discussion --------
Good Morning Ron. I thought over your questions and decided I had just better explain the circuit for you.
Ok. The basic pixie2 consists of a crystal controlled oscillator, a second transistor providing: Mixing, Detection, and RF Amplification.
A LM386 Audio Amplifier, And a LOW Pass output Filter (PI configuration)
OSCILLATOR: Colpitts oscillator, single frequency set by installed crystal. Broadband design and components allow any crystal from
at least 3Mhz to about 22Mhz to be used to set the 2n2222 or 2n3904 transistor to the crystal frequency. Any crystal above or below
this range will require a change in the two feedback capacitors attached to this transistor.
The output of the oscillator is capacitively coupled to the second transistor which serves to MIX, Amplify, and DETECT (convert to audio),
signals entering the Base and Collector.
MIXING: The second transistor is biased near cut-off when the key is up. You'll see that a 10 Kohm resistor is in the emitter lead. When
key is down, the resistor is bypassed to direct current and the transistor has high Gain and sufficient DC current to amplify the RF entering the
transistor Base junction. A 100 microhenry choke from Base to Emitter
allows base bleeder bias to keep the transistor stable. Signals entering the Collector mix with the oscillator thereby producing
harmonics. Signals close to the oscillator frequency result in a difference that is
in the audio range. these are fed to the Audio AMP.
RF Amplifying: The second transistor also amplifies. The crystal frequency enters the Base. When you key down, bypassing the 10Kohm
resistor, the transistor now has MUCH current to work with. The 15 to 22 uH inductor
on the collector provides an RF load. The transistor DC current is strobed at the oscillator frequency and develops a voltage across the
load inductor. The output filter isolates and lowers the impedance of
the 15 uH inductor to that of the antenna, also the filter is of low-pass design, where all frequencies above the oscillator frequency
are attenuated.
(NOTE: A band Pass filter can be used too. But the input/output must be carefully coupled so as not to load the filter or broadbanding occurs
and efficiency rolls off quickly.)
DETECTING: When the key is up, the FINAL transistor is nearly biased off and works as a diode, mixing the two signals, Base/Collector, and
the difference of these two signals result in a DC audio wave heterodyne that is fed to the audio amplifier.
OK! That's how it works.
Modifications: The frequency you operate on is set by the crystal. This is a single frequency rig. It is possible to vary the particular
crystal operating frequency by changing the crystal capacitance, either by adding a capacitor load to ground, or a series inductor to ground which
tunes out the crystal capacitance. Some people use both.
Since the oscillator contains non-tuned feedback in the form of two 100 picofarad capacitors, it operates as a simple voltage divider. The
signal at the oscillator transistor enters the base, and is fed back to the base with an amplified in-phase portion of the oscillator output.
Due to reduced gain above 22 MHz, the oscillator begins to fail at those
higher frequencies, and below 3 Mhz, insufficient in-phase signal is coupled and/or fed back to the oscillator input (Base). So for lower
frequency operation, the 100 pF caps should be increased to a reactance similar to what the 100 pF have at say 7 MHz.
The only other limitations are the output transistor load coil, and the PI filter network. The filter has two capacitors to ground, and an
inductor in series with the antenna. A pixie2 built for 3.579 Mhz needs the stock capacitors and a 2.2 uH inductor. For 7.040 or whatever 40
Meter pixie2, you need a 1 uH coil. (Obviously the frequency of the rig
is set by the crystal as explained earlier). So for a 80 meter rig you should have a 80 meter xtal installed, and the output filter should
have the 2.2 uH inductor. A 40 meter rig has a 40 meter xtal, and the output filter now has a 1 uH inductor instead of the 2.2 uH inductor.
Other frequencies: As a frequency spotter for tuning other rigs, you don't need really good output power efficiency. Just change the crystal
and leave a 1 or 2.2 uH inductor in the final. But to operate, say on 10 meters,
Get yourself a 10.116 xtal, modify the PI network to use capacitors of perhaps 100 ohms reactance to ground, series inductor of about 100 Ohms,
and output capacitor of perhaps 100 ohms. The load inductor on output final should be changed to have 500 or so ohms reactance at 30 meter
band.
So you see, it isn't too mysterious. just compute the reactance values of the PI filter, the Output Load inductor at the collector, and install
the xtal for the band these are valid for.
Coil Reactance formula: Xl=2*PI*F*L (Primary units Hertz, Henry's)
Capacitor Reactance formula: Xc=1/(2*PI*F*C) (Again Primary units.)
Now! If you still have any more specific question we can begin anew. I will try to do a line by line explanation.
This is fun! I really do enjoy circuit design and hope
I can help you further if necessary.
Keep in touch. Happy Holidays Ron.
-------- End of Old email discussion. --------
-Ed, we6w
--------- Begin Old email discussion --------
Nick Franco wrote:
>
> Ed,
>
> Enjoyed your post on the mods you made to your 3rd Pixie. I've built
> about 5 or 6 of these little gems and tried in my very limited
> understanding of circuits, to make some mods. Most didn't' work. I do
> use a manual switch for RIT or TX offset. I agree with you; it's nice
> to be able to TX on either switched freq.
>
> I get tons of AM broadcast RFI; I have no sidetone; I'm not happy with
> the offset and trying to hear the other party. I like what you
> mentioned but don't understand most of it. I do not have QRPp, but
> maybe I should break down any finally join NorCal. I figured I live on
> the opposite side of the USA. Now that NEQRP is gone, I guess I should
> stay in touch with these articles.
QRPp and QQ magazines are well worth it! (I have no financial
association.)
>
> Well, I would love to add some of the things you mentioned but don't
> have a schemo or any other info than your printed post from QRP-L. If
> you get a chance, could you explain the 60pF cap in the RIT switch area
> and also the blocking out of the AM junk. I was about to build the W1FB
> version with the audio filter and the auto-switching offset, but maybe I
> should continue to experiment with my old faithful some more. You've
> introduced some fresh ideas for me to try.
I have the UPDATED/CORRECTED W1FB schematic on my web page. (See sig.
below)
OK. You've got a XTAL. For a NO-RIT rig, just ground one end of the
crystal, the other end to transistor Base input as shown on the
original Pixie2 schematic. (also on my web page.)
Instead of directly grounding the crystal, you can ground it THROUGH
a .001 capacitor. .001 uF is a low impedance at the RF frequency, and
you still get a single freq., no-Rit pixie2.
RIT: Permanently ground the crystal through a smaller capacitor,
say 50 to 60 pF. This raises the xtal frequency about 600 hertz.
So a 7040 xtal == 7040.6 KHz. Connect a SPST switch such that allows
you to parallel a .001 uF cap across the 60 pF, thereby totally
providing an excellent ground to the xtal. The crystal now operates
at it's proper (Lowest) possible frequency. There you have it.
A smaller, say 30 pF cap will provide 1Khz offset or so. Just remember,
the well grounded crystal operates on the lowest frequency it was
designed for.
AM BC BAND: Get Ferrite Chokes. They are the size of 1/4 watt
Resistors.
Do NOT use wirewound molded inductors, they are miniature antennas!
The ferrite chokes I use are 5 millihenrys and have a green paint drop
supposedly revealing their value in millihenrys. Green=5.
Looking at the schematic you see the KEYING LINE and Ground; SPEAKER
LINE and Ground, POWER Positive Line and GROUND, a 1 Kohm resistor
feeding reduced current/voltage to the LM386. Also, the Detected
Audio from Q2 line goes to the audio input pin 2 or 3 of LM386.
OK. Place a RF CHOKE: Audio Output; Key Jack; Detected audio to
LM386 Pin 2; Power Supply Positive line. Next; add small Ferrite
bead to Positive Voltage line going to each Transistor.
The Following is probably Optional:
This next part is Tricky! I wanted a stable 5.62 volts to the LM386
and decouple it from the power supply (Car Battery). On the Pin 6 side
of the 1 Kohm resistor (R5), put a 5.62 Volt Zener Diode. Change R5 to
500 ohms, or parallel another 1 Kohm as I did. Put in 250 uf to 470 uF
electrolytic capacitor from pin 6 to ground of the LM386. YOU WILL
GET IMMEDIATE IMPROVEMENT IN AUDIO AND BC INTERFERENCE REDUCTION!!!
Add 100 ohm resistor from D1 switching diode Cathode to Keying line.
This suppresses the now SURGING current from 470 uF powersupply
capacitor.
Without the 100 ohm resistor, the keying device has to pass too much
current to be reliable. My keying relay contacts stuck sometimes...
Also, some current flow is possible thru D1 and the 10KOhm emitter
resistor
and the Q2 amp gets stuck in Self-Transmit mode!
OK! Mine works FB and although I still get a small amount of BC
interference from the AM station about 2 miles away, I find it
disappears when I touch the enclosure. So it is just stray capacitance
and when I put the lid on my PC Board constructed/soldered enclosure,
I expect all BC qrm to be gone.
>
> My Pixie is a 40/80 dual band switchable and I beefed up the 2
> transistors to a 2222 and a 2219 can type for the amp. I get about 1
> watt out on a 12vdc battery. I'm very happy with the output and I do
> make some contacts but copy is very rough. I've checked into the Klites
> net on Sunday eves but that's a tough one and usually rely on my Icom
> for that.
I tried the 2219 and it is FB. Currently using a 2n5109. But the
2n2222A works well too. But I put in a bigger socket that fits
the 2n2219 and 2n5109 so I'll keep those.
>
> Thanks for the post,
>
> Nick - KF2PH
> QRP-L # 13
Thanks for your input.
I hope I have helped!
Vy 73/72 -- Ed Loranger, we6w
--------- Begin Old email discussion --------
Here's some email from another person:
ently threw together a Pixie to play with, and put it
on 20 meters (but that's another story). Haven't had a QSO
with it yet (last weekend was one of *those* weekends..),
but I did get to listen a bit, and geez, I guess I've been
spoiled by good crystal filters. What with all the digital
stuff going on up several kHz, and rag chews down around 14.050,
I had quite an earful!
I got to looking at the datasheet for the LM386, and a little
circuit labeled "Audio Amp with Bass Boost" got me thinking
that it would be easy to add some filtering to the thing.
So it's as simple as this:
- Change the 10uF cap between pins 1 and 8 to 4.7uF.
(This puts the low freq -3dB point at about 200 Hz)
- Put a 2k ohm resistor and a 6800 pF cap in series, and
wire the unconnected ends between pins 1 and 5 (the
output). This rolls off the high frequency gain. The
upper -3dB point is about 1.6 kHz
- I also tried reducing the output coupling cap from 10uF
1 uF. This reduces the gain below 200Hz, but the value
you need will depend upon your load (I had 500 ohm headphones.)
The result is still no 6 pole crystal filter, but it's an
improvement, and doesn't complicate the rig very much. The center
freq of the resulting "filter" is about 600 Hz. If that's too low
for you, try a 2.2 uF cap between pins 1 & 8 and reduce the 6800pF
cap to 4700 pF.
--------- Begin Old email discussion --------
More info...
Ok, the stock pixie2 uses about 20 to 25 milliamps and is capable
of running off of a 9 Volt transistor battery. I'm not sure, but
another problem develops with the 741 in this circuit due to the
additional current demands. Try the thing on a pair of 9 Volt batteries
in parallel to check that.
Explanation of what I did and why I changed the 1 Kohm.
This is a bit complicated, but when I hooked the stock pixie to the
9 volt battery I noticed a few things.
1) The LM386 Motorboated as the battery voltage got to about
7 volts. The spec sheet on the LM386-N1 recommends it run
between 5 and 12 volts.
1A) 7 volts should have been fine, I deduced the LM386 was
being starved for current.
2) The LM386 is a good detector for AM BC band. I decided to
further decouple the LM386 from the power supply by putting
a 5.62 volt zener to ground on the LM386 side of the 1 Kohm.
2A) I lowered the 1 KOhm to 500 ohms by paralleling another
1K. This allowed me to stabilize the LM386, decouple the
Battery and Leads from RF, and supply more current as
needed.
3) The LM386 had problem with strong signals and I observed problems
with audio on CW peaks.
3A) I added a 470 uF electrolytic to pin 6 of the LM386 to serve
as a current reserve for those peaks. This also improved
audio response SIGNIFICANTLY, and eliminated more BC
interference.
4) The mods ran too much current through the diode on key down. Also
the idle current through the 10 Kohm resistor - Diode - to power
supply was too much and this turned on the power transistor as
if you were keying. (BAD PROBLEM!)
4A) I put a 100 Ohm resistor on the Cathode (Key) side of
the diode. This reduced the current through the key
contacts during key down, also kept the output transistor
from turning on.
There you go Bill. I hope this helps. It was a lot of work,
but very worth it! I have a very quiet pixie2 and works FB. The
audio is strong enough to drive a speaker and the key clicks are
virtually non-existent. The 470 uF bypass cap on the LM386 was
totally amazing. Really helped the circuit in three ways: Audio
quality, LM386 stability, Reduced BC interference to to RF detection
of the current starved LM386.
Let me know how it works as you go.
Best of luck.
-Ed
--
72, Ed WE6W (CW only/VP-0); http://www.qsl.net/we6w Santa Rosa, CA
QRP-Z#106 QRP-L#1068 AR#112 NC#2227 ARCI#9397 QAA#006
--
-Ed AR Millennium Q's=>1200/2000 QRP-L#1068 Old NC#2227
72, Ed WE6W, A-1 OP; http://www.qsl.net/we6w Santa Rosa, CA
QRP-Z#106 AR#112 HI-QRP#64 ARCI#9397 ARS#275
--------- Begin Old email discussion --------
One more thing Casey. You will probably find that
a 2n3904 might not oscillate at 10 Meters. A
selected 2n3904 or 2n2222 might work. You may have
to find a 2n5179 to use for the oscillator. And be
sure to change the 100 pF feedback capacitors to something
around 47 pF or so. You may get too much offset if
you use a capacitor switched in/out for XIT, so play
around with that a bit too.
Gotta run. Have fun! 72/Ed we6w
--
-Ed AR Millennium Q's=>1200/2000 QRP-L#1068 Old NC#2227
72, Ed WE6W, A-1 OP; http://www.qsl.net/we6w Santa Rosa, CA
QRP-Z#106 AR#112 HI-QRP#64 ARCI#9397 ARS#275
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