So I bought an amp 80W 28V MRF173 transistor. I hooked it up to an analyser and it's putting out a huge spur of -10db at 1/2 frequency and another spur at + 1/2 frequency of about -30db. Is this a common occurrence? What can I try to fix it? I purchased it from a reputable company (broadcastconcepts) so I don't think it's defective. I also checked the exciter and it's good.

Amplifier probably doesn't generate 1/2 frequency spur. What spectrum analyser do you have?


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Normally a 1/2fo oscillation is associated with bipolar amplifiers due to a mismatch or overdrive of the base. It's unusual on a FET amplifier.
Could it be that you're using an exciter which has a doubler? It may be that the input of the amp is not tuned correctly and the exciter has put out these spurious tones.

Could it be that you're using an exciter which has a doubler? It may be that the input of the amp is not tuned correctly and the exciter has put out these spurious tones.

That turned out to be exactly it. It needed a 40pf on the input to fix the problem.
Not sure why the amp didn't come tuned properly.

What spectrum analyser do you have?

DSA815 if I remember correctly.

It is probably because your exciter is producing spur. Broadcastconcept amplifier are very good so it can not com from it unless there is broken componants on it.

What kind of exciter have you?

Also are you sure you are not saturing your spectrum analyser ? How did you samples the RF? With coopler attenuator ?

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