Thursday, March 3, 2011

Maybe I Should Have Paid More for a SpecAn?

So in my last post, I showed my new IM-ME based spectrum analyzer in front of my Davis VP2 wireless receiver.  I speculated that what I was seeing was the ISS frequency transmissions. But it didn't look right: the frequencies were popping up below the 902.5 - 927.5 MHz range that Davis uses. How to know for sure?

As I think I've alluded to before, I do RF stuff at work and have access to a few nice toys that let me check stuff like this out.
A Good Old-Fashioned RF Throwdown
In the background, we have our champion: a Hewlett Packard 83620A synthesized signal generator good from 10 MHz to 20 GHz.  It was worth around $60,000 when bought new some years ago.   Locked to GPS, it would be frequency accurate to < 1 Hz.  I didn't bother to lock it up because its internal ovenized oscillator is also very accurate.  I'd be surprised if it was more than 100 Hz out.

In the foreground, we have our challenger: a Pretty Pink Pager flashed to work as a spectrum analyzer up to about 1 GHz.  I paid £17.00 GBP on EBay.  Plus £5.00 GBP for shipping.  Pricey, I know, but it isn't easy to get these things North of the 49th parallel.

Let's zoom in on the displays:
Not Looking Good for the Challenger
The IM-ME should show a nice sharp spike at the generator frequency of 950 MHz.  Instead, we have that area of the spectrum blank, and the spike shows up around 915 MHz.  So I think I can pretty conclusively argue that my IM-ME spectrum analyzer is reading 35 MHz too low.  I haven't looked at the code yet to try and figure out WHY this is.  I only know that it is.  35 MHz is a kind of interesting number in the RF world though: it is half of the 70 MHz that is the IF (Intermediate Frequency) in many RF designs.

Anyway, I plan to ping the software's author and see if we can't sort this out.

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