Fixing a Crazy Expensive Spectrum Analyser, With Solder
It used to be a spectrum analyzer was an exotic piece of gear. However, these days it is pretty common for a scope to have some ability to do the job — that is, plot amplitude versus frequency. However, a dedicated commercial product will usually have a lot more bandwidth and other features. [Signal Path] picked up an Anrtitsu 7.1 GHz portable spectrum analyzer. An expensive bit of kit — anywhere from around $4,000 to $8,000 on eBay — if it is working, but this one was not. It needed power, but it was also missing the internal flash card that the device uses to boot.
Being portable, there’s a lot of digital and RF electronics crammed into a very small space. The initial tear down didn’t look very interesting because it was mostly an RF shield. However, many tiny screws later, you can finally see the actual electronics.
The first board up was the tracking generator which uses a special-purpose Analog Devices chip to produce four different digital transmission streams to be converted to analog. The filters are very visible on the PC board as well as some additional options that were not installed in this unit.
The input board uses relays for input switching which has plusses and minuses. The instrument is not a current product and [Signal Path] mentions that many of the components seem large compared to what you would see in a newer device.
It looked like it might work, but an LO (local oscillator) unlock error required a bit more effort. It also gave him an excuse to show us some more of the board. A bit of pressure on the board was causing the local oscillator to unlock which suggested a bad solder joint or another kind of wiring problem.
A little detective work narrowed the problem down to a small shielded circuit. Inside the shield was a varactor and some associated components. Reworking the solder joints seemed to cure the problem.
Most of us won’t tear up a $10,000 instrument but thanks to videos like this one, we can watch while someone else does. It is a lot easier to get your nerve up to rip open a $3 multimeter. On the other hand, at least this spectrum analyzer didn’t cost over a million.
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