Mcl Sbl-1
 

I have used this device before a long time ago in projects designed by other people without problems.

I am now trying to build a project myself & have struck a basic problem.

The project is a home made Spectrum analyzer and I am trying to mix L.O of 15Mhz (approx) with the output of a video I.F strip of 39Mhz as RF feeding a xtal filter of 21.4Mhz.

My problem is with the L.O.

The specs for the SBL-1 states that I need +7dbm and the input of the device is 50 ohms.

I can generate enough 15Mhz approx 2v p/p (+10dbm) as measured on a scope.
I then knocked up a 3db 50 ohm pad and terminated it into a 50 ohm load and all worked approx 1.4v p/p across the load.(+7dbm)

Next step is to remove the load and connect to the SBL-1 (pin 8) via the pad and the levels drop considerably to about 0.75v p/p +1-+2dbm with a little distortion on the +ve peak.

This seems to indicate to me that the mixer input is not 50 ohm but a lot lower. I tried terminating all other ports with 50 ohms with no noticeable difference. What am I missing?

Why can't I see the correct level in to the device. I have plenty of drive before the pad but it reduces when connected to the SBL-1.

I am missing something. Can anyone prompt me?

Mcl Sbl-1
 

This seems to indicate to me that the mixer input is not 50 ohm but a
lot lower. I tried terminating all other ports with 50 ohms with no
noticeable difference. What am I missing?

Why can't I see the correct level in to the device. I have plenty of
drive before the pad but it reduces when connected to the SBL-1.

I am missing something. Can anyone prompt me?




--
vk3akc

You are on the right track: The SBL-1 LO port input impedance is not 50 ohms
as you have correctly measured. On top of that, it is rather unlinear, i.e.
drive level dependent.

Reason: LO drive has to turn mixer quad diodes in and out of conduction. The
RF current needed for that depends on the instantaneous voltage/current
point on the diode characteristic.

Conclusion: Important thing is that the three ports of the SBL-1 are
terminated in 50 ohms, i.e. 'see' a low swr/return loss from the mixer's
viewpoint. Your 3 dB pad adds nicely to that. You have done just the right
thing.

Ease back and enjoy the result!

vy 73
OZ7S Sven

Thank you very much Sven for going to the trouble of answering my query.

I will continue on with the next phase of the project with confidence.

Kind Regards
Kevin