Re: Spectrum analyzer imput impedance questions

What is the gain from removing the input filter? It is there to filter the image so it is good.

I am curious if it's oscillators would be stable enough to max span (~1-3KHz), after your comments. Others have mentioned they would not be that stable, but thinking logically, the short term stability should be good, because in a room, (lab) temperature is not going to change too much within minutes (a measurement does not take much longer). So it should not be affected too much within this time window so that no measurements could be done.

What do you think?

The main reason for removing the filter is the design is not very good. There is too much reflection caused by the filter (for my tastes). It could probably be improved, but that is a task for you.


The problem with so narrow of a scan is that you will need a RBW filter of around 1kHz BW or less - and that means a crystal filter. The sweep speed will also have to be slow, so a digital scope will be needed (or a storage scope) to capture the sweep. But this is way outside this SA's design capabilities.

But build it anyway. It is not that costly and you will gain a wealth of knowledge implementing and using the design.

I will build it, I have collected all the components. I know the things about the filters. I have two +/-3KHz filters and I may replace them with even more narrow. Slow sweep rates is not much of a problem to me. After all this will be connected to the LPT of the PC not a scope, using this simple circuit http://www.dosprinter.net/lptscope/hw.html so software can make up for the slow sweep rates.

My main concern is the stability of the center signal at max span, which relates to the stability of the oscillators, mainly the first. But as I mentioned the short term stability might be ok for this... possibly?