Trying it out

I've spent far too much time recently writing software for this thing and not enough time holding a soldering iron. So last night after building the step attenuator I decided to try the thing out in anger.

I started this project to help me make a Spectrum Analyser. I started this ambitious project a few months ago but soon realised that I'd need a decent sig-gen / wobbulator if I was to have a chance of setting up and testing out any of the tuned circuits and filters in this thing.

Here's a few shots of it testing out one of the first components I built for the Analyser. It's a SA614 chip setup as a RSSI meter. Basically this is a linear to logarithmic converter, it should output a nice linear signal over a 90dB range...good enough for a basic analyser! The RSSI circuits I've made use an IF of 10.7Mhz and have a couple of ceramic filters to set this up (robbed from old radios).

RSSI Circuit

The pictures are produced by using the Wobbulator function on the sig-gen. it is set to scan over 2MHz starting at 9.6 MHz. The SYNC output is fed to the scope's external trigger input and the wobbulator, via the step attenuator into the RSSI circuit's RF input. The RSSI output then goes to the scope's normal input.

First Scan - too peaky!

At first glance I was very happy with this trace - well it proves the wobbulator is useful! There is about 0.8div of "passband ripple". That equates to almost 15dB! I could measure this by switching in the the attenuators until the tallest peak ended up where the dip is. There is about 350kHz bandwidth niceley centred on 10.7MHz. Again this was easy to measure by "tuning" the wobbulator until the edges of the trace just touched the left hand side of the screen.

The circuit has two 10.7MHz filters and as can be seen from the scans these are a bit peaky I've tried messing about with the circuit to produce a nicer passband but all it's done so far is make it an odder shape...however it's now a million times easier to experiment than it is using a normal siggen and graph paper. I can tack solder components in while its running and see the changes on screen in an instant!

Here's a shot after a bit of messing about...more theory needed I think!

Trace after messing with the circuit