<Homer Simpson voice>
"Mmmmm, sol-der smoke..."
</Homer Simpson voice>
Perhaps someone will have a clue, which I don't
The Pixie2 is generating a signal at 13.56MHz and I am modulating it with a clear undistorted sound, but REGARDLESS OF ANTENNA LENGTH my portable Grundig ONLY picks up a clear distinct signal when located DIRECTLY at either the transmitter or the antenna at any point along the length. When I walk away from the transmitter/antenna I get hums and buzzes and a diminishing signal strength.
Yes, I have the specified filter on the output.
Take the filter off. Why worry about harmonics at flea power?
It's premature to connect an antenna to the transmitter. First, it is necessary to check out and troubleshoot the circuit. Initial testing should be done with a 50 ohm (or thereabouts) resistor used as a dummy antenna. The actual antenna should be connected only after the circuit is determined to be working properly.
It is essential to use an oscilloscope for testing. Without an oscilloscope, you will not have any idea about what is going on. It's like driving a car without being able to see. An audio signal generator is also very helpful. A sinusoidal tone is much better for testing than voice or music. If you don't have these pieces of test equipment, try to borrow them from someone.
Poor me. It's been so many years since I built transmitter devices I actually think you just hook it up and it works. I have an oscilloscope and a 50-ohm resistor, but I needed the reminder. Now tomorrow is planned.
Hi Carl and everybody!
Hey Carl I think it's great you've
gotten this far. Because of family matters that are
going on, I am way behind you.
The Pixie 2 put's out more power than what you need to
feed into the dipole, or whatever you are using for an
antenna. So I plan to make an attenuation network
with 50 ohms impedance on the "front" end and the
"back" end. When the output of the Pixie looks into
this network, it will see something close to 50 ohms
(I think), especially if the antenna (on the other end
of the network is close to or at resonance for 50 ohms.
That should be when things start cookin, if you know
what I mean.
I have a proto board, and will be wiring it up as soon as
I can. I might start at a lower frequency, because the
proto board might not like operating at 13 MHz.
Since the prototype won't be transmitting anyway
except into a dummy load, something like 3 MHz is
OK. The signal won't be going anywhere. After that
is running OK, then I will take all the parts off the proto
board and solder them together and run it at 13.560 MHz.
Although at lot of my electronic ideas never get built,
I have some interesting ideas that are fun to think
about. One is putting the 13.560 MHz transmitter in
a box with an FM radio, hanging the quarter wave wires
(for the dipole) off of each end of the box, and hoisting the whole
thing up into a tall tree. (Not necessarily a permanent solution.)
The 13 MHz transmitter would get my Part 15
station's FM signal (hopefully) from the FM radio. A 49 MHz link
would work, too. (Maybe in the future.) A battery pack can
be installed with an on/off timer circuit of some kind. It could
all be rigged to come on for a certain amount of time during the
day. I think the higher it is, the better it would work. (True?)
It would be sort-of like a square communications satellite
that is stuck in the tree in my back yard. (Weak analogy?)
People that are hams with towers in their back yards can
try other approaches.
Now here's the big question. All the information I have seen
has indicated that 100 % modulation is not achievable with
this set-up. Maybe it's 70 or 80 %, I don't know. But your
modulation transformer may be a better match than other
transformers that other people have tried to use. So you
will have to let us know how loud the audio is when you are
ready.
I don't expect miracles from this 13 MHz set-up, but I am
sure that my shortwave listener friend a mile away will
hear it in his house without any trouble. I am right about
that? We'll see!
Best wishes to everyone!
Bruce, MICRO1690/1700
The Pixie2 has been assembled on the work bench for a couple of weeks, generates a robust carrier at 13.56mHz, but it doesn't seem to want to accept modulation from the exact same transformer as shown in the linked illustration. Different transformers bring about hints that it wants to modulate, but it just doesn't kick in. True, I have a load of sundry transformers that I hope will save having to buy something, but for the moment it's fascinating how "delicate" the threshhold apparently is for getting a "match."
I've built tube transmitters in days gone by and never had trouble getting plate modulation even one time with a doorbell transformer.
Ideas are welcome.
While the Pixie2 is powered up with 12vdc and the audio is being applied to the modulation transformer, 16-ohm/1k-ohm, all I hear on the radio is a hum-carrier.
HOWEVER, when I unplug the power to the Pixie2 (not the power to the modulator) it takes a few seconds for the power capacitors to drain, and AS THE CARRIER DIES THE MODULATION STARTS TO APPEAR AND RISES TO A STRONG LEVEL UNTIL THE CARRIER IS COMPLETELY GONE.
What a clue. But what does it mean?
Hold everything. I got modulation.
I was under-driving the modulation transformer on the belief that transistors only need a pinch of voltage to mix the carrier and audio, but based on that last observation in the previous posting I risked cranking it way up, and like the sun coming out of the clouds I had a distinct 440Hz tone coming out of the radio!
I think we've just set sail.
Thanks and thanks.
I'll report again on antennas and ranges.
Carl,
This is good news and thanks for giving us the blow by blow reports since it helps others.
As I recall from my tube transmitter days the modulation transformer secondary peak voltage needs to match the quiescent plate DC voltage and this is probably true here also. The peak transformer secondary voltage will need to be approximately the quiescent DC collector voltage. An oscilloscope is almost a necessity to gauge this properly.
When I taught Electronic Communications we had a lab where we constructed an AM transmitter where the final amplifier collector was supplied by an emitter follower which provided both the quiescent current and the audio (similar to what is done in the Ramsey AM25). The modulation was not good above about 50% but I expect with a transformer it will be much better.
Nonetheless, it appears that you are on the right track and making progress.
Congratulations.
Neil
Hi Everybody!
Hey Carl, that's great!
I have all the stuff, I just haven't put it together
yet. In a couple of weeks, we can compare notes.
Best wishes to everyone!
Bruce, MICRO1690/1700
Thank you for sharing your good wishes.
Now here's another interesting chapter. Oh, and by the way, I have an oscilloscope, but for the fun of it I am groping in the dark right now because I don't have the desk space to put the scope anyplace.
All right. The good modulation results have come from an audio tone generator with a 600 ohm output into my little 3 watt amp, which has a 10k pot at the input, outputs into a 4 ohm power resistor and the 8 ohm/1k transformer. Perfect.
But when the input comes from my FM radio, which has hi-impedance outputs, even though there's enough head-room to boost the 3 watt amplifier, there is garbled audio. So I'm thinking the input impedance to the audio amplifier needs to be on the low side. Out comes another matching transformer. More tomorrow.
You did a great job so far, especially considering that you were working blind. Now is the time to stop everything else and somehow get your scope into position for troubleshooting.
A sine wave signal generator is able to produce a beautiful (sounding) modulation at 13.56mHz using the little 8 ohm to 1 k ohm transformer shown in the Pixie2 documentation. But I've now used every combination of transformer trying to get voice & music modulation and all I ever get is either nothing or hash which spreads across the spectrum.
The author seems to have anticipated this by his discussion on trying other methods of achieving modulation.
Not sure the oscilloscope could help with fixing this, it will only show when it starts working.
One change I tried which the author mentions is going from 10 ohms to 15 ohms on the transistor on emitter of Q2, there was an ever so slight improvement but not enough.
The transformer in the design is in step up configuration, so I've used every "step up" transformer in the junk box, but they all fail the same way.
Hi Carl and everybody!
Well, Carl, you may not have figured it out yet,
but we will get something going one way or
another.
I have mentioned before that I am almost legally
blind, so it takes me a while to build stuff.
I have all the parts for the Pixie 2 sorted out and
placed in bins now so I can get to them. I am
going to proto board the oscillator section as fast
as I can. Once that is going, I will move on to
the power amplifier. Then after that we will see
what happens.
Keep trying different things if you can. My goal
is to make a reliable 13.560 AM transmitter even
if large modifications have to be made. For me,
this is fun! (It takes me back to about 1968
when I built my first Part 15 transmitter, and
I didn't have a clue what was going on then!)
Best wishes to all!
Bruce, MICRO1690/1700
P.S. There are other 13 MHz Part 15 circuits
on the net, but I will probably experiment with
this one for quite a while until I go on to
something else.