Power measurement of broad band amplifier - Page 1
Forum section: electronics / rf-microwave

author: GridWork
author_rank: Contributor
timestamp: September 20, 2023, 12:56:54 pm
content: If one wants to measure the output power of a broad band amplifier should you use a low pass filter on the input to the power meter? A hypothetical example: The RF amplifier has a operating bandwidth of 1 GHz to 10 GHz. When testing from 1 GHz to 5 GHz there will be at least 2nd harmonics that will also be amplified. Is it proper metrology to add in a 6 GHz (or so) low pass filter when doing power measurement in the lower part of the band?In this example the only tool you have is a thermal or diode type power meter, no fancy spectrum or network analyzers.

author: xrunner
author_rank: Super Contributor
timestamp: September 20, 2023, 02:01:31 pm
content: I recently got fooled by the same thing on a cheap RF amp specified as good for 1 to 700 MHz. I wanted to amplify 146MHz input because I needed a stronger signal to test something else, but I wasn't seeing believable results. I was measuring power with an hp 437B power meter. I took a look at the output and it was filled with harmonics. So yea, you either have to filter it or attenuate the output and check with a spectrum analyzer.

author: GridWork
author_rank: Contributor
timestamp: September 20, 2023, 03:56:30 pm
content: Quote from: xrunner on September 20, 2023, 02:01:31 pmIn terms of metrology though what is really the proper method to do this testing? In your case the amplifier was actually making that power, it was just residing in the harmonics. The amplifier was being run in the compression region where it is effectively clipping and thus generating the harmonics. If more fundamental power was required then an amplifier with a higher P3dB point would be required.It kind of feels like cheating with the data to intentionally filter it, in that it makes the output look a little cleaner than it otherwise would be.

author: xrunner
author_rank: Super Contributor
timestamp: September 20, 2023, 04:15:02 pm
content: Quote from: GridWork on September 20, 2023, 03:56:30 pmWell of course that's true but ...Quote... if you want to know what's happening with the output of the amp (it seems that you do) then you need to know if there are harmonics, so, you really need an item of test equipment that you don't currently posses. I don't know what else you can do.

author: LM21
author_rank: Regular Contributor
timestamp: September 22, 2023, 12:03:34 pm
content: Heh. TinySa (Ultra) does not cost too much, IMO.https://groups.io/g/tinysaThere are others a bit more expensive, but they still don't require your limbs or kidneys to buy.

author: coppercone2
author_rank: Super Contributor
timestamp: September 22, 2023, 12:44:52 pm
content: you can measure it just fine if you put a attenuator and have an array of filters to put on it (everything characterized)the question is why, unless you are stuck on mars. Good for episode of startrek if engineering is filled with hostile aliens and poison gas. I don't know the frequency of this force field an we are shut out from engineering. God damnit O'brian get me in there before the Cardasians initiate a reactor overload! The jefferies tubes on DS9 seem to be infested with microwave hardware, so for them its like tearing out a light switch. Primary cause of wave guide explosions over there is related to engineering forgetting about chicken sandwiches they left in the waveguide conduit. Ever wonder why its called a bird watt mater? one of those time travel consequences