Sunday, July 31, 2016

Power supply woes?

I built another little preamp today (this time just a CE followed by a CC rather than the shunt thing I used before), just to see if additional drive was the issue.  Did not seem to help.

However, it occurred to me that it might be the power source I'm using.  I've been using alkaline AA batteries.  But calculating things out, for 12v to produce 5 watts, the batteries would need to be delivering around half an amp (not counting what is dissipated in the transistors).  I always assumed that the batteries could deliver more amperage for a shorter time (the amp-hour rating).  However, I forgot that that batteries have voltage curves.  I measured the voltage output when the amp was running, and it was closer to 9V.  So, I think I need to go find a better battery to use :)

(I actually tried a wall wart, but I think I was either getting feedback into it or it was just noisy, was getting a weird output when I tried to bring the power above what I had with the batteries.)

Tuesday, July 26, 2016

Transmitter progress

I moved the circuit to copper, but no love.  Still when I turn the bias up, it all goes haywire.  Actually worse than before.  Thinking it might have to do with the drive level, I tried adding an amplifier stage ahead of the 2n3053, but again no love.

Then I thought "hey, I sure have a lot of wires running around here..."  I had been using hook-up wire to join my modules, as I didn't have any extra coax.  But I did get some recently, so I added some BNC connectors to the output of the PA and the input/output of the LPF.  Bam!  Pretty output!

I'm getting 20v P-P on about 11.3V power, which is about 1 watt.  I can get it up to about 1.5 watts by increasing the voltage to around 13V.  But I'm hoping for 5W.  Here at least some of the issue I think is the drive level.  I tried the little amp I had, but I think that was overdriving it.  Was getting about 3W out, but it was well ugly.  Will need to have a think about what the drive level ought to be.

I might at some point see about using a different IRF510.  I've read that "lower quality" parts may not work as well, and I'm not entirely sure about this one.

Edit: actually, after playing around in LTSpice, I think I may need to change out some of the coupling caps.  The ones he uses in the Mighty Mike look to be attenuating things too much.  In the simulation, if I change those to larger caps (closer to .1uf) and run it at the same power he does, I get >5W out, even with my current drive level.  With his values and my current voltage, I get closer to what I'm getting on the bench.

Edit 2 (next day): I tried swapping out the .001uf for .1uf coupling caps as I mentioned in the last post.  Raised the power some, but not nearly as much as in the simulation.  I also tried swapping out the IRF510 for a new one from Mouser, didn't see much change.

Friday, July 22, 2016

Another try at a transmitter

Trying a different approach, from the circuit here (pdf file).  This uses a 2n3053 to amplify an oscillator and drive an IRF510.

I built it out on a breadboard and after a lot of tinkering am getting about 0.7W out.  Which is nice and all, but for the voltage I'm using I should be getting twice that at the voltage I'm using.  Also, if I change out some of the coupling capacitors the plans use for some larger ones, it works even better in the simulator.  However, when I try to do it on the breadboard, it causes the DDS to stop working.  Not sure if there is some weird oscillations occurring there.

I think the big issues is that when I turn it on, the waveform going into the 2n3053 goes from a nice sine wave to something that has a lot of higher-frequency content.  Not sure what the cause of that is.  It could be just that the parasitics of the breadboard are causing it and that I should just go ahead and move it to copper, but I kind of wanted to get it working better before I did that.  Ho hum.

Tuesday, July 12, 2016

First steps towards a transmitter

I would like to add a CW transmitter to my little DC receiver.  I'm thinking of using an IRF510 running as a class C amplifier.

I got a 74AC240 to use as a driver for an IRF510.  However, the output of the DDS is fairly low.  I was going to use a comparator to convert to a 5v square wave, but was having trouble getting it to work.  I think the issue is that the LM399 I pulled out of the junk box isn't fast enough :/

I then tried making a Schmitt trigger out of an op amp I had laying around, but it wasn't fast enough either. One I tried was a bit faster, and if I got the source down to 1 Mhz it sort of worked.

Then I tried the DDS buffer to directly drive the '240 - success!  The buffer distorts a bit, so the signal out of the '240 isn't that great, either.  But I can clean that up with my low pass filter.  I'm measuring about 400 mW into a 50 ohm load when I use 5 volts on the '240.  I'm using a 12V source for the buffer and the Arduino's 5V output for the '240.

I seem to be running out of breadboard space.  Hopefully the IRF510 stage won't take up too much room.  Once I have something together that works, will see about moving the PA stage out to something more permanent.

Tuesday, July 5, 2016

antenna here is dipole

I finally got a chance to build and put up an antenna.  I followed on from the the ideas that WP4AOH had for building the center insulator, but added a 1:1 current balun as laid out in the W1FB QRP Notebook.  That calls for 12 bifilar turns of #26 wire on a FT50-43 for up to 25 watts.  I used #22 wire, as that is all I could find in my junk box.

Here is a closeup of the center insulator

And here it is living in a tree

It isn't very high - probably only 15 or so feet - but that is all I could manage right now with the arrangement of trees in my tiny back yard.  It is cut for 40M, and is sort of a bent inverted V.  It is about 100 degrees in the horizontal and slopes down to about 6 feet at the ends.  It is fed with 100 feet of RG-58.  No idea how well this arrangement will work, but will hopefully be better than the unmatched random wire I was testing the receiver with before.  I cut the legs too long and will shorten them once I have a method to measure it.

Also, many thanks to my wife and 5 year old for helping me measure, build, and raise the antenna.  I really lucked out in the family department :)