5 Jun 2012

Very simple Spectrum Analysers

On the GQRP Yahoo group recently there was some further mention of very simple spectrum analysers with some links to very impressive, yet simple circuits. I was recently offered an old HP analyser but it was very large, very old and quite expensive: I did not want something that might soon go wrong and take up a lot of space, but I still need something simple to allow basic spectrum measurements. Nothing too accurate is needed, just an indication of harmonic levels and the like.

In essence, a spectrum analyser boils down to a receiver with a swept oscillator with its output connected to a display such as an oscilloscope. In its most simple form it could be just a crystal set in which the tuned circuit is replaced by a varicap tuned circuit with the rectified output going to a display.  The voltage applied to the varicap is a sawtooth waveform (for example derived from a 555 timer IC) which also drives the x axis of the scope: this makes the tuned circuit sweep a band of frequencies over a second or so with the receiver's rectified output connecting to the Y axis of a scope. The resulting scope trace is then a picture of the band being scanned.

Shortwave scan on the VK2ZAY mint tin spectrum analyser
An example of a simple spectrum analyser in a small mint tin is at Alan VK2ZAY's site http://www.vk2zay.net/article/256. Although Alan was doing this design for fun, the resulting circuit based around a VHF super-regen receiver in the IF is quite remarkable.  It is a seriously useful piece of test equipment. This is the YouTube video of Alan describing the circuit:

An earlier simple design is available at http://www.qsl.net/n9zia/spec/Homebrew_Spectrum_Analyzer.pdf which uses the sweep voltage to control the local oscillator of an existing receiver.

More complex designs have wide dynamic range, more linear displays and narrow IF filters to improve the resolution. "You get what you pay for", but simplicity is still capable of providing something very useful.

1 comment:

Anonymous said...

These circuits are fun but they are of very limited use.
I think I have designed something better, yet very simple.
Check it out http://www.qrp.gr/sa/
I am finishing the mixer design these days which is a transformerless discrete gilbert cell mixer.
The SA is intended for HF and it does not use a higher LO as one would expect, for simplicity.