The 50Ω resistor is probably there to keep the filter happy when a regular scope probe is used. If you are putting a 50Ω step atten on the input, the resistor should NOT be there (it would reduce the input to the filter to 25Ω).
Replace the 50Ω resistor with a dedicated 1-dB attenuator (in addition to your step atten). That should keep everything happy. The downside is an extra 1-dB of loss that you will need to account for.
As far as I understand, the 1 dB attenuator was intended to achieve at least partial filter matching with unmatched input impedance. But why 0.1 dB? It seems just useless.
A 1dB error on a SA like this is not likely to even be noticed. You're not expecting to be able to measure to 1dB, are you?
You need to put a 0.1dB atten in front of the filter in a simulator with a 1M across it and examine the response of the filter. The problem with improperly terminated filters is that the whole response starts to collapse. After you simulate the 0.1dB atten, do the same with the 1dB. Find an atten where the response is acceptable. A constant attenuation can be calibrated out.
The SLB-1 mixer is pretty well matched to 50 ohms. VSWR 1.4:1 max. So adding another 50 ohm resistor at the input connector...is kind of like wishing for a 25 ohm input impedance! I would leave it off.
I actually would prefer what others have recommended, adding a pad at the input instead. That way even if the mixer is mismatched, the effective return loss at the spectrum analyzer input would be improved, which is important if you are trying to make an accurate power reading. With a bad input impedance, and any cable length, you will get some ripple in the power measurement. I would go for at least a 3 dB pad.
Also, I am thinking of using power resistors ~1-2W for this input attenuator. Is that true that if I use say 2W resistors the power input to the analyzer can be up to 2W, or it does only depend on the attenuation value, or both?
No, it's not true. If you build a 3dB pad with 2W resistors and input 2W, 1 watt of power would emerge from the output. This is too much power to input to your mixer, which has a max input of 50mW. If you need to input large amounts of power, get some power attenuators to drop the power to the input of your SQ.
Thanks. And what is a good value for the isolator, so that it allows for good matching and isolation but without too much lost of the input signal? 3db, 6db, ...
First you have to answer what is your intended use for your SA?
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