Spectrum analyzer imput impedance questions

[neazoi]

How a SA is usually used, is selecting an appropriate attenuator to knock the power down to a level that is safe for the input of the SA. In your case, it is < 50mW... I would stay < 20mW. If I was interested in viewing a 2W transmitter's output and harmonics, I would use a 20dB pad with at least a 5 watt rating.

As for viewing a SSB signal, the stability of the oscillator is adquate for that task. It probably isn't good enough to be able to measure frequency precisely, but for most everything else it should be okay. I see no need to ovenize the oscillator. Anyway, ovenizing the oscillator isn't as stright forward as it seems.

As for your input, I would drop the filter (and the 50Ω resistor) and replace them with a 1-dB pad. The pad will help improve the match between the input and the mixer. It is easily calibrated out, so there will not be much of an impact on measurements. It is the engineer's responsibility to know the limitations of his equipment and how to compensate his measurements to account for those limitations. For instance, if you are looking to measure the output of a 90MHz transmitter and its harmonics, you will know that the 2nd harmonic on will be OUTSIDE of your equipments capability, so you will have to take steps to filter out those harmonics prior to the input to your SA. If you don't, you will see image products IN YOUR PASSBAND that will try to confuse you.

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?

 

[SLK001]

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

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.

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 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.

 

[neazoi]

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 https://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?