Sunday, 29 September 2013

Muppet Show

Got the Larkspur mast guying kit out for inspection today. The valise needs a new sheet of plywood gluing in over the original, which is split, but other than that its not in bag nick really. Now have eight guy lines, but only one base plate pin. The ground spike and base plate that came with it are not much use, so have been replaced with those I already had. I might think about putting the mast up for a test tomorrow.

Building the RF diode probe proved rather simple this morning, and the pen selected was spot on for it. Cutting off the tip a little earlier than the main body left a gap the same diameter as the body of the cut down test probe, which fit in snug, held in place with a dab of superglue.

A small hole was bored through the side wall for the ground clip, and the end cap was slit and a small hole made for the coax. It was then slipped over the coax and pushed back into place, sealing the end and securing the cable. A pair of 4mm plugs and a couple of quick continuity tests later, and it was ready for testing with RF.

RF testing was accomplished by connecting it to my DVM, and probing the BNC output connector of my Marconi 2955B communications test set. With the BNC selected, it will give an RF output up to +5dBm. A quick run through at 28MHz allows comparison with the signals on the 10m WSPR unit -

0dBm = 0.38v                 3dBm = 0.56v
1dBm = 0.44v                 4dBm = 0.63v
2dBm = 0.49v                 5dBm = 0.72v

Of course, these are at 50ohms, so the readings in-circuit wont be exactly the same.

After much testing with parallel capacitors on the doubler stage of the wispy, I finally managed today to get a very high 28MHz output with virtually no 14MHz or 42MHz component. Very good I thought, er, no, actually! When I applied modulation, all I got was a very good AM signal! Whats going on? It turns out i'd forgotten to move the probe from the spectrum analyser from the output of the mixer back to the collector of T2, the doubler transistor!

So, all the work to get the parallel capacitance right is in fact wrong. That all needs to be redone now. I also noticed that the driver stage isnt, it appears to have unity gain, what goes in comes out no bigger. It also gets rather hot for a 2N3904! Something then is definately not right there.

So ive decided to go back to the start and work forward, solving any issues as I come to them. Using my nice new diode probe, I find the output of the oscillator is 0.38v, about 0dBm, or 1mW. The output of the doubler is a touch higher at 0.52v, about 2.5dBm or 1.8mW. This seems low to me, im not sure what the insertion loss of the mixer is, but I bet its more than 2.5dB! Measuring at the crystal itself showed 0.79v, about 5.8dBm or 3.8mW.

So, the questions are - what level output should I be expecting from the oscillator and the doubler? What loss must be overcome in the mixer? What level drive does the mixer need? And, why am I not getting those figures?

Im also concerned about the balance of the mixer. After a lot of searching on the tinterweb, ive finally found a similar design that shows a value for a balance resistor, at 200ohm. Im pretty sure I dont have any 200ohm presets though. Theres also a hint that the insertion loss might be about 8dB.

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