Ripple on downconverter output

Arved M0KDS

Hi all,

I built quite a few downconverters for multiple microwave frequencies in the past. 
One problem I seem to be having with it is that I appear to have a output passband with a lot of ripple on it, it's not a constant straight line.
The ripple consists of ups and downs that are about 2 or 3dB different. It's really noticeable over a wide range of bandwidth. 
I suspect that it may be caused by an impedance mismatch. The cable I'm using on the downconverter is RG6, so 75Ohm coax on the output of a 50Ohm device. 
Do you think that the cable may be causing the ripple? If not, what do you think it might be?

Thanks in advance.


Arved M0KDS


Arved ,
1) If ripple is result of a mismatch on the cable it will be regular in frequency, based on the frequency at which the cable is half wavelength long.
2) Mismatch on the cable is a result of the terminating impedance being different from the cable impedance; nothing to do with the source impedance.
3) Try putting a 75 ohm attenuator at the end of the cable, that may reduce the effect.
4) unless you are trying to make accurate measurements the ripple is probably on no consequence.