[time-nuts] OT: Power level reference

[WB6BNQ]

Well John,

Neither do you with regard to the hp-3400A RMS voltneter.  It is spec'd to only
10 MHz !  From 50Hz to 1 MHz it is +/-1% and at 10 MHz it is rated at +/-5%.  So
at 10 MHz it is no better then the spec of Bird 43 slug type power meter (if you
believe them).

The old hp-434 Colorimetric power meter (i.e., measures heat content of applied
power) is only good for 5%, but than can be improved somewhat with applied
techniques.  However, this unit has the frequency range of DC to 12 GHz.

The best you can do with an analog scope is 3% measurement of the screen
display.  If you were to assume the load resistor whose voltage drop is being
measured was perfect, then 3% is the best you could do.  Obviously, such is not
the case so some allowance would have to considered because the resistor is not
perfect, so, again, about the best you can do is in the area of 5%.  The newer
digital displayed readout scopes are really no better.

As someone pointed out, the best you could trust an Amateur type power meter
would be 20% into a perfect "load" without some serious calibration efforts and
recording the offsets at various frequencies.  As most all Amateurs would be hard
pressed to understand and quantify their "load," they are really kidding
themselves to use the meter for anything more then a relative forward power
indicator.

"J. Forster" 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,
>
> Really? The most successful 'scope in history.
>
> > 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.
>
> Apparently, you don't know what an HP 3400A is.
>
> Stick to political blogs.
>
> -John
>
> ===============
>
> > 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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