[SOLVED] Changing the value of the fixed sma 3 db attenuator in a high pass mixer.

[hafrse]

Hi,

Have a question, in my spectrum analyzer I have a high pass mixer coupled with 3db attenuators on each port: RF-in, LO and IF-out.
The analyzer shows a level dip of about 2.5db on the entire range (6-20ghz) of this mixer, it probably needs calibration but wanted to fix it by "hardware".

My question is, Is it safe to replace the 3db attenuator on the RF-in input of the mixer with a 0.5db fixed sma attenuator to compensate the level drop of 2.5db?

Or, can I replace the 3db attenuator on the IF-Out port instead with a 0.5db fixed sma attenuator ?


Many thanks
George

 

[PatrickPribyl]

Hi,

Have a question, in my spectrum analyzer I have a high pass mixer coupled with 3db attenuators on each port: RF-in, LO and IF-out.
...
My question is, Is it safe to replace the 3db attenuator on the RF-in input of the mixer with a 0.5db fixed sma attenuator to compensate the level drop of 2.5db?

Or, can I replace the 3db attenuator on the IF-Out port instead with a 0.5db fixed sma attenuator ?

There are two reasons I can think of for the attenuators - the first is power. The question is whether the various mixer inputs are rated for 2x the power they are being run at (i.e. +3dB relative to the current configuration). Too much power might blow the diode(s) inside the mixer, so obviously this is not OK.

The second has to do with decoupling between stages; it may be that the load presented by the mixer is not a good match to 50 ohms. By introducing some damping in the signal paths any results of this mismatch on other parts of the circuit may be lessened. Introducing this attenuation can flatten the frequency response as well.

In particular, I have had the experience where an output amplifier stage oscillated when driving a particular load, but adding a 3 dB attenuator prevented this. Such oscillation in another part of the circuit may result in damage somewhere if it was much larger amplitude than the relevant signals.

So the short answer is if you know that you are well away from damage thresholds, it MIGHT be ok, but you would have to try it to find out if the results are what you are looking for.

 

[jiripolivka]

Follow the above advice for safety. A good spectrum analyzer has a dynamic range over 60-80 dB, so you can even increase the attenuators to 10 dB each and still get good data.
Attenuators like those you mentioned are often used to improve mixer response with mismatched sources or loads.

 

[hafrse]

Thank you all for the excellent explenation, will not touch these attenuators, will send the analyzer for calibration instead.