Re: Frequency response flatness in conventional sampling (say 7S11/S


 

I think using a sampling oscilloscope for flatness calibration is a
great idea. The sampling heads are both very high bandwidth and have
a very predictable frequency response. The weakest links will be the
SWR match and termination but that applies to any system. You can do
away with cable losses by using a sampling head extender.

If you take the 3db bandwidth numbers I posted earlier and divide by
4, that is the point where the sample head output will be down by 2%.
The second number shown is where they will be down by 1%:

S-1 260 MHz 190 MHz
S-2 1.18 GHz 869 MHz
S-4 3.60 GHz 2.61 GHz
S-6 2.99 GHz 2.17 GHz

I think this is the first time I have had a need to do math involving
a sin(x)/x function.

On Fri, 15 Feb 2013 17:45:49 -0000, "Ed Breya" <edbreya@yahoo.com>
wrote:

Albert, I wouldn't use a sampling scope or even an analog one pushing its BW to calibrate RF generators, except maybe for comparing one against another. A power meter like HP848X heads with HP43X meter is a good way to go, but expensive.

You can also roll your own with modern power level detector ICs - they can also be used directly in the generator leveling loops to improve performance. I'm familiar with some from Analog Devices, but I'm sure the usual companies have plenty to choose from too. These are very common ICs used in huge quantities for all the wireless gear out there.

Here's a place to start looking:

http://www.analog.com/en/rfif-components/detectors/products/index.html#RMS_Detectors

It would take quite a bit of investigation and experimenting, but the trick is mostly to get the desired broadband flatness in the coupling of the signal to the IC, and of course, the right dynamic range. Logarithmic and linear responses are available. With linear, you can expand the measurement for better resolution over a small range for calibration purposes, but have less overall dynamic range.

These devices usually measure true RMS voltage over the entire bandwidth, so if your waves have lots of distortion, the harmonic power will be counted too, up to the BW, then diminished. This also could give a quite different result than looking at the p-p envelope even on a perfectly flat scope - another reason to not use a scope, unless p-p voltage is what's wanted.

Ed

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