[time-nuts] OT: Power level reference

[Joseph Gray]

Mike S,

What you say about my needs is essentially correct. For ham radio
purposes, my Bird dummy load and Tek scope will give me what I need.
However, being interested in measuring things to greater accuracy than
I really need (aren't we all?), the other responses were also
appreciated. I was curious how others were making such measurements.

BTW, the links to the PDF files look to be very interesting reading. I
just did a quick glance for now and will add these to my reading list.

I also found lots of Google hits for the AD8307. Within its
limitations, this chip seems to be very popular.

If anyone else has different ideas or suggestions, please continue the
discussion. This is all very educational.

Joe Gray
KA5ZEC

On Tue, Dec 1, 2009 at 8:55 PM, Mike S <mikes at flatsurface.com> wrote:
> 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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