two tone measurements
I tried using the attenuators before and after the combiner but still the same problem. The input two tones are used at 0dBm but the output tones have a diffeence of around 6dBm between them...?
It would help if you posted a picture of the SA display, since the terminology can mean many things.
In a two tone measurement, there are many more than two tones at the device output. There are the two frequenceis that you are inputing to the device f1 and f2. Then there are the 3rd order products"2f2-f1 and 2f1-f2. There are higher order products too. Exactly WHICH tones at the output are you talking about being 6 dB different.
My guess would be you have sigificant something going one that is casuing AM and PM modulation in the output, maybe some AM-PM conversion in the device you are testing. Since AM and PM tones combine on one side, but are 180 degrees out of phase on the other side, one side looks higher and the other looks lower.
I am talking about the two input tones. I am using two signals of 0dBm each at 50MHz and 51MHz in a two stage amplifier. The spectrum on the SA doesnot show the two tones to be equal as well as the differnce of levels becomes more in 3rd order products.
That is clearly am and pm signals interfering with each other.
Added after 1 hours 18 minutes:
Looked around for a paper to describe, and there are some on actually characterizing AM to PM conversion by using a two tone or three tone test, and looking at the un-equal heights of the 3rd order products. Unfortunately, could not find papers in the "clear", they all had the stupid IEEE copyright legend from IEEEXplore and I can not share them without getting in trouble.
You can look at page 21 of the attached for some clues about improving the test:
I have access to the IEEE papers through my University so if I could get the name of the papers?
Added after 13 minutes:
Initially, I used the signal from the generators, combined them in the combiner and then to the DUT, the results of which I have posted. Then I tried using 3-6dB attenuators before combining the signals in the combiner. But using the attenuators didn't help. Any idea on what type of isolator should be used ?
Or if you could throw some more light about this am pm interferance..
I assume, that you can exactly calculate, why each line in the output frequency spectrum should have the level you see.
But I wonder if it's of much use. Generally, the result shows a high amount of cross-modulation products. Either the amplifier is
operated in a non-linear B, C whatever mode, or it's completely overdriven. Why don't you start with a measurement of a fairly linear
amplifier respectively a reasonable power level?
Also the analyzer can be overdriven, of course.
As a more general remark, looking at the power level, it looks like a PA measurement. A two-tone measurement
with a non class A PA doesn't make much sense.