Spectrum Analyzer IF Amplifier


IF Amplifier cooling down

I’ve decided to hold off posting spectrum analyzer modules until they’re complete.  I’ve been collecting tons of pictures along the way, so hopefully the post for each module will be interesting and visually appealing.  With that in mind, I can’t post about any of the modules with the exception of this one.

For whatever reason, my kit didn’t come with any voltage regulators, and this is the only module besides the mixers from my last post that don’t use any.  The IF Amplifier, as the name implies, amplifies the Intermediate Frequency on the analyzer.  I made a simplified version of the block diagram from Scotty’s site.  I’ve also indicated the modules I’ve finished in green.

Block diagram, green modules are completed as of this post.

As you can probably see, there’s a lot to be done.  Luckily both DDSs, the Log Detector, ADC, Control Board, and one PLO are done once I get some parts to replace the missing ones.  I also need some parts for the Master Oscillator.  Once that (actually quite small) order comes in, I’ll be almost finished with the boards.

Anyway, back to the amplifier.  I was one of the first people on an order of boards back in August ’08 because I agreed to look-over a new revision of the design files.  Of course, by having me look them over essentially guaranteed that there would be errors.  As it happens there was an error on this board.  The engineering change order (ECO) is luckily quite simple, the only problem was that a short section of the border was missing.  Here is a photo of the completed fix:

Fix for the slight error,

You should be able to see that along the border there is some solder wick saturated with solder bridging the gap, and connecting to the capacitor.  Also, on (at least) the 2 resistors (R3 and R4) there are tiny balls of solder.  These happen during the reflow stage when a bit of solder squeezes out from underneath the device.  I haven’t gone over the board picking all these out yet in this photo.  I think it’s important to remove all of these because they may cause shorts.  Also, you may be able to see a slight, shiny, residue around everything.  This is the solder flux that’s included in the solder paste.  I remove this later with some 99% rubbing alcohol.

Completed IF Amplifier

This is the completed amplifier board.  There are actually 2 amplifiers here, mirrored.  In practice, the output of one will be the input of the other.  This serves to roughly double the gain.  Finally, notice on the right side, that many of the parts have been omitted.  On the schematic, Scotty simplified this section.  Technically, he has 2 zero-ohm resistors (basically jumpers) in addition to the capacitor.  I chose to just jump the capacitor across and leave the resistors out.

Anyway, I enjoyed building this one.  I hope you’ve enjoyed reading about it.  I can’t wait to finish the other ones!

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