Sunday, June 18, 2017

Transmitter experiments

I've never managed to get more than 1W out of my Mighty Mike amplifier.  While I've tinkered a bit on things on it, thought I would try some other circuits.

A few weeks ago I tried was using a 74AC240 octal inverting buffer as the driver for an IRF510.  There are a lot of these circuits out there (here, here, here, here are a few, but there are lots more).  What I ended up building was pretty standard - one part of the inverter with the crystal to make the oscillator, then feeding that into four other inverters in parallel to increase the driving power, and putting that into an IRF510 (with a DC bias just below turn on).  It worked... ok.  Again was having trouble getting more than a watt, and the transistor got pretty hot.  

Some of it may have been the construction technique (I don't usually build RF on breadboard), but I didn't have a lot of time that night, and wanted to see something on my scope :)

Today I thought I would try my hand at building the circuit for the ns-40.  It is a Class E transmitter using a 2n7000 to drive the IRF510.  The actual kit is retired.  Its neat feature was that its inductors were part of the PCB (thus the 'None Simpler 40').  I've seen somewhere else where someone made this circuit using toroids, and I thought it would be a good morning project.

Here are all the parts

Not much there.  I used T50-2 toroids, since that is what I have most of.  Here is what I had two hours later (with my dummy load hanging off to the right):

And here is some output:

Pretty excited it worked the first try.  However, it isn't being a proper class E transmitter.  I was measuring 28.8Vpp while consuming 370mA from the 12v battery.  I'm pretty excited about the 2w result (double what I was getting from an IRF510 before), but that looks to be <50% efficiency, and the IRF510 was definitely getting warm after a while.  I'm not quite sure how to tune a Class E without being able to view the current waveform on the drain.  Also, the gate voltage on the IRF510 (yellow below) isn't pretty, not sure what is up there (though I did just notice that in that screen shot it was putting out 3w!)

Now what I'd like to do is not drive this with a crystal but rather an Si5351 or the Raspi.  I wanted to build it as is first (a) to get it working and (b) I was unsure what the drive requirements were.  I tried simulating the circuit in LTSpice first, but getting crystals to behave in that program is difficult.  Also, I tried driving the 74ac240 circuit with the Si5351 the other week and managed to smoke the chip... 

One other experiment I did over the past few weeks was buying a cheap "Forty-9er" cw transmitter on EBay.  It was pre-assembled and was suppose to put out 3W.

My plan was to convert it to be digital (per the QST article the other month).  Was disappointed to find it only put out 1W, and even then if you held the key down too long the power output went down.  Probably some dodgy parts or something in it.  Oh well.  First day I got it there was some CW contest that weekend - I could hear people, but didn't want to join in.  Tried calling CQ a few days later, but no takers (couldn't even hear myself on a local WebSDR).  Will probably shelve that for a bit.  

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