Thursday, July 25, 2013

The QRP2000 Frequency Generator

I'm back to working on ham radio projects and so it's time to get back to blogging about it too.

I'm still seriously into weak signal VHF-UHF-SHF work, an aspect of ham radio that I never thought would hold any interest for me but which I now find fascinating.  My goal is to build the whole line-up of W1GHZ transverters, from 902 to 3456 MHz (see W1GHZ's documentation and also the Yahoo! group I started for other builders).

To that end I have had serious need of a signal source.  Basically, if you've just applied power to a homebrewed transverter, you need to know whether it can receive something.  A simple crystal oscillator will work, allowing you to hear some weak harmonics.  But I craved something more substantial.  Now I've got it.

I settled on the QRP2000 kit from SDR-kits.  Utilizing the Si570 from Silicon Labs (mine has the A grade, LVDS chip) this unit provides frequency selection via USB control from 3.5 - 1500 MHz.  I just finished mine and it is as cool as can be.

The kit took about 2 hours to complete, including mounting in the custom enclosure highlighted at left.

The only hitch in the building process was that I received a tape of 47 ohm resistors that were supposed to be 4.7K.  I should have checked the values before stuffing them.  But I didn't and of course the board didn't work.  After going over and over my soldering and checking the polarity of diodes and such, I finally on a whim checked the resistance on a couple of the microprocessor control lines and found this mistake.  Fortunately nothing was damaged and stuffing the correct values immediately resulted in a working board.

There are a couple of software applications available to set the output frequency.  The frequency ranges for the "A" chip are spec'ed at 10 - 945, 970 - 1134, and 1212.5 - 1417.5 MHz.  Mine doesn't seem to have any of those gaps and the actual range extends beyond each end--I'm getting continuous coverage from 3.5 - 1500 MHz.

Power-wise I'm getting between -10 and -3.5 dBm out from the Si570 chip, depending on frequency.  My plan is to bolster that by 15 or 20 dB in order to be able to use this box to drive a mixer, so that I can use this dude as an agile LO for any given project.  I'll drop another little amplifier PCB in between the main board and the connector and post an update with the results.

But even without the additional gain this puppy is perfect for a signal source to check out the receive on a transverter. It puts out a nice square wave, so it's rich in harmonics.  I found that running at 100 MHz, the 15th harmonic was only 27 dB down from the main signal.  I currently have a "cheater" 902 MHz transverter at home, to get KC9JTL and me on that band for the upcoming ARRL UHF contest and the 82nd harmonic of an 11 MHz source is a welcome sign that all is well on receive.

This unit has a ton of applications, including some for my day job.  If you need a super-flexible signal source, this is hard to beat.

Next up is to build W1GHZ's RF Power Detector board.