Thank you for such fast response to my crystal business. Good results!
icmfg.com is really on top of it and responded immediately. The contact person looked at the "Pixie2" circuit and specified a matching crystal which I ordered in unit quantity and it's all taken care of.
For other "Pixie2" builders here is the detail:
Catalog No. 434368
13.56 MHz HC49U package
68 pF
Calibration Tolerance: +/- 30ppm from -10 to +60C
Carl,
That's great news and the specs. being here for others to use is helpful.
Here I go again with another caution: When you solder the crystal into the circuit, use a heatsink on the leads (needle nose pliers, etc.) since heat going up the leads can cause damage. A little heat for a short time won't hurt but don't park the iron on the leads.
Looking forward to reading your "on the air" report.
Neil
This morning I completed the Pixie2 transmitter section and powered it up. Wow! A carrier at 13.56MHz! Now comes finishing up the modulation section, for which I purchased a 7W audio amplifier to provide the input to the modulation transformer.
The modulation transformer will come from a vast assortment of junk transformers collected over the years, until I pick out the best one.
The scary part is going to be the antenna, which needs to be outdoors.
If you will be applying 7 W to the modulation transformer, your transmitter circuit must be designed to produce several watts of RF power. Remember that your output power will be on the order of only about a milliwatt, or so, if you plan to meet the Section 15.225 limits. This would require a lot of attenuation of your transmitter output.
If you will be using a horizontal half-wave dipole to start, the total length of the wire is about 35 feet, cut in the middle. There will be an insulator to join the two wire sections at the middle, and an insulator at each of the other ends of the wire to connect the antenna to the supports holding the wire in the air. Since the antenna will probably not be very high above the earth, 50-ohm coax would be the right transmission line to use. Connect the coax across the insulator in the middle of the antenna. The attenuator for the transmitter output should be designed for 50 ohm output impedance. The other end of the coax would be connected to the transmitter attenuator output.
Just a note to remember that an AM signal in this narrow band will occupy the whole band assignment and the only possible legal frequency choice for AM is 13.56000. Use an accurate source to set your transmitter on frequency,the tolerance here is tight
(1% I think??). Also,please use the harmonic filter in an earlier post to keep it clean. The second harmonic is 27.120 in the cb band.
I am now using a dipole and can be heard some 5 miles away by fellow hams with ease,but no DX , yet...
Hi Guys! I have also ordered a 13.560 MHz crystal and
3 Pixie kits. At roughly $10.00 each, I figured it would
allow me room for any errors in construction, because of
my poor eyesight.
Actually I have received everything that I ordered.
I do things very slowly, so it might be a month or two
until the transmitter is built. In order to get roughly
0.5 milliwatts into a dipole, and comply with Part 15
an attenuator will be needed.
There are 50 ohm RF attenuator schematics on the
web so this part won't be hard.
According to all the descriptions of modulating the
Pixie transmitter on the net, full 100% modulation
is not achievable with the Pixie circuit. But I'm sure
somebody will figure out how to solve that problem.
There was some mention about putting a 2N7000
in the circuit to make the modulation characteristics
linear. One thing that is good is that if the transmitter
is looking into, say, a 20 dB RF attenuator, I think it
will be seeing a load that is mostly resistive and therefore,
may be easier to modulate.
What is cool about this thing, is that not too many radio
enthusiasts have done Part 15, 13.560 MHz AM, as far
as I can tell. So this territory is still largely unexplored.
The HIFER cw guys have done a lot of great work, though.
I think it will be fun to try this whether it works well or not,
because to me, it is just another experiment.
If a few hams or shortwave listeners in my town could hear
my station, I think they would appreciate the "DX" and I
would certainly appreciate their interest.
So I'll let you guys know when my set-up is going.
It may be a little while.
Best wishes to everybody here!
Bruce, MICRORADIO1690/1700
P.S. This probably isn't necessary, but I would even
be open to running the 13 MHz AM thing when the
band is NOT open, so the HIFERS aren't interfered
with, but that is just me and the way I think. That idea
is probably not needed at all. Good luck everybody!
I didn't choose a 7 watt audio amp for purposes of transmitter power increase, but because it was the lowest power small kit I could find at the time. I'm assuming I can run it at a whisper to supply whatever modulation matches the actual transmitter power, which isn't very much.
When "attenuation" is discussed, does it refer to attenuating the audio amp output by a reduction pad at its output, a a variable T-Pad would do, or wouldn't it simply be good enough to lower the input level to the audio amp?
Hi Carl and everybody!
Carl, I am going to be trying almost the
same thing that you are doing, but you
will probably be ahead of me.
I plan to get the transmitter running with
just a carrier into a dummy load, and then
put the RF attenuator on (To satisfy Part
15), and then experiment with modulation.
I plan to use any audio amp lying around
with a low volume and any junk transformer
I can find to modulate the transmitter.
If that modulation approach doesn't work well,
the 13 MHz links have other ideas that can
be tried.
Aside from the RF attenuator, I don't know
about the separate issue of using an attenuator
on the audio amp. I'm just going to try a bunch
of different parts and amps to see what happens.
If something self destructs, well, we all will have
learned something. As you know, the Pixie circuit
by itself runs a fair amount of power. Nobody has
mentioned anything about the duty cycle of this
transmitter circuit. But maybe that's a moot point.
Anyway, after all of the above, I will try the antenna.
But I think you will get there before I do.
Best wishes to everybody
Bruce MICRO1690/1700
Tonight I linked everything together on the test bench. At first I modulated a tone, which came on strong but very sensitive, and in need of further audio attenuation. Soon, though, I began hearing garbled hash and I think the power supply isn't strong enough for both the Pixie2 and the 7watt audio. It's a Vellemen 1 AMP supply, and I suspect we need closer to 2.5 AMPS. I just looked and the audio amp shows a 1.5 AMP rectifier in the illustration.
I don't think anything blew up yet.
MICRO1700 takes the lead!
Hi Carl!
Actually you are still way ahead of me, but at
some point you may see me in your rear view
mirror.
I'm not a big expert at this stuff, but I guess
that audio amp would have stages that are
running class A, so it would take a bunch of
current to keep the amp happy.
I studied electronics theory 40 years ago.
I still remember some of it. What was your
name again? (Just kidding!)
If the Pixie circuit turns out not to be a good
candidate for AM, I'll probably run a whole
lot of tests with it anyway for quite a while.
Also eight "D" batteries in some kind of holder
will serve temporarily in an experimental set-up.
I had to do this recently for a small watt meter
because I lost the 12 volt supply that was
supposed to run the meter. Turns out the
supply was hooked to something else which
I had forgotten about.
Have fun and let us know how it goes!
Bruce
Previously I have said the audio amp I'm using with Pixie2 is "1.5 AMPS" with "7watt output." Now I wish to add better details, having read all the little notes that came with the kit.
Velleman Kit K4001
Output Power: 3.5Wrms/ 4-ohm - 7W max.
Power Supply: 8 to 18VDC / 0.5A
High Input Sensetivity (40 mV)
That looks a little better than the previous guesses I was making.
Today I'll be testing the audio amp on its own not connected to the transmitter.
The audio amplifier checked out very nicely tested by itself, and I ended up using the suggested 50k potentiometer at the input, with an additional 500k resistor at the input for further padding.
But as a modulation driver I was either getting very distorted sound or no sound at all, until discovering that C4, .01 uF from B+ to ground of Pixie2, was missing. Adding C4 brought good sounding modulation, but the results around the rooms of the house varied wildly between audio reception and hum reception. The test antenna was 36" straight wire, estimated 1/4 wavelength, but the range seems very small. Just out the door there is almost nothing outside.
Pretty cool stuff!
About that antenna...
"test antenna was 36" straight wire, estimated 1/4 wavelength
Hmmm - Calculating 1/4 wavelength, how about using this formula for calculating that?
[((11811/F{MHz})/12)/4]*0.95 or:
F = Frequency = 13.56 MHz
11811/13.56 = 871.0177 for wavelength in inches
871.0177/12 = 72.5848 for wavelength in feet
72.5848/4 = 18.1462 for 1/4 wavelength
18.1462*0.95 = 17.2389 to allow for velocity factor
So, I get a 1/4 wavelength antenna @ about 17 feet 3 inches.
But, 36 feet would be a half wave antenna...
Wow, scwis, I must have been dizzy on solder smoke when I calculated the wavelength. Thank you for the significant correction. That'll make a difference.