Simple Spectrum Analyser

Low(no) budget

How did this project started life:

After reading some articles on the internet about creating cheap spectrum analysers using simple electronics I got interested in this subject.

Since I find it exciting to know what type of different signals are being transmitted and on which frequency.
This was a subject that has always been of interest but the profesional equipment is way beyond my reach.
This low end solution was good enough for me. But the problem was could i get all the parts needed to create such a device.
Then one day when cleaning up some stacked electronics I fond some interresting and essential parts.

At first I was planing on making it with a digital TV tuner wich is controled via a I2C bus but I did not have one of those I could always use it as practice on makeing my own car radio (near future plan).
I had found a project I made from a well know electronics magazine Elektuur. It was a radio tunable from 48Mhz until 860Mhz using a standard TV Tuner.
This design used a analog voltage to control the tuning freqency and after a couple of days thinking on how ik could hook this thing up to my Scoop. I came up with a simple solution to get a first attempt.

How does it work

I Would use my functiongenerator to create a sawtooth signal 20Hz which with a DC offset would be fed into the tuning pin of the tuner. The width of the sawtooth would represent the frequency band scaned. This same signal but now only the AC component would be fed into the Scoop to act als the X-deflection.
The signal used to reflect the signal strength (Y-deflection) is the AM signal that came out of the mixer.
In the project there was a AGC circuit present to get better tuning. This was set to a fixed value to avoid signal changes due to counteractions taken by this circuit.

The Result:

And this picture is to give u some idea of how it then looks like on a Scoop. Some nice spikes on a noise floor. Now that I know the concept works, special circuits will be designed to create the propper signals to control the Tuner. Then the full range of the tuner can be selected. Most of the things will be made very fexible because I have no idea how or what the precice values have to be to get the whole thing working.

Finaly a picture from the circuit board of the tuner that produced the pictures above.

To the right you see the function genrator. The output used are the DC for the tuner and the AC goes to the Scoop. The transformer is used to feed the tuner circuitboard. More details about the tuner board will be posted when the schemetics have been drawn. But it is a very standard circuit using a NE612 to create the audio signals then there are the supplies and an audio amplifier. To the left you can just see the board of a stereo decoder that has been hookup afterwards, it is obsolete when used for this purpose.

More to come.......