[time-nuts] OT: Power level reference

Mike S mikes at flatsurface.com
Wed Dec 2 03:55:02 UTC 2009


At 08:30 PM 12/1/2009, J. Forster wrote...
>In fact, on the Tek 7704 or 7704A (I forget which) there were "No 
>Cost"
>options of maximally flat frequency response OR best pulse response. 
>OR!

Sure, if you're using a 150 MHz to try and measure accurately at 100 
MHz, you're not going to get absolute accuracy. I'm not familiar with 
that particular scope, but would bet the difference appears as peaking 
at the extreme. In the absence of circuitry which deliberately changes 
it, the response of an analog scope is (very nearly) Gaussian.

Can you quantify the difference between those options when measuring a 
50 MHz (1/3 BW) signal?

Since the OP is a ham, I assume his quest is relative to commonly 
available power meters for that market, most of which can't be expected 
to do better than 20% accuracy, maybe 5% at the high end ( 
http://www.telepostinc.com/lp100.html ).

Sure, you can buy an uncalibrated 3400A (good to ~150W), and get some 
unknown amount worse than 5% accuracy. Or a 432a (good to a whole 10 
mW!) for <$100, plus another $100+ for the mount/cable. Now you've got 
something which is good to a couple of %, with an _extremely_ limited 
range (and practically useless for many ham applications).

A Gaussian scope is predictably off ~3% @ BW*0.3. A decent scope will 
have a vertical amp accurate to a couple of percent. With a 1% load, 
you can measure at the 5% level of a $400 dedicated wattmeter (from < 1 
uW to 400W, and to the 2000W ham limit if you invest in a 100:1 probe), 
for little cost.

"Accurate...simple and inexpensive." Many non-appliance-operator hams 
will already have a scope, so the cost is a $10 resistor (for a 100W 
rated Caddock, which should be good for a few seconds of full power, 
enough to make the measurement).

Finally, as I've already mentioned, one can make a simple peak detector 
using a rectifier, and measure the DC voltage off that.





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