[Boatanchors] Shade-Tree Scope Calibration

[Bob Groh]

Dave, measuring RF power is a huge question with no easy answer. A scope is
simply not going to cut it at all. I have chased this elephant for more
than 50 years (I graduated in 1964 BSEE and 'retired' in 2004) - the most
accurate answer is using a calormetric approach, i.e. measuring the actual
heat dissipated in a load. HP has such an instrument - the HP 434A
Calorimetric Power Meter. The rf power was dissipated in a 50 ohm load (of
course, it has to be a darned good non-reactive load) and then the device
measured the  rise in temperature. I believe HP specified better than +/-
1% accuracy from DC to 12 GHZ.  It was rather large and rather slow.

Analog Devices does have a family of what they refer to a 'TruPower' RF
power detectors - I have one to play with (in my almost non-existent spare
time) but you still need a very well defined RF load.

HP did have a VTVM (and probably does have today) which was pretty darned
good - specified at +/- 5% of full scale. And you still need to have an
accurate load.

Lots more involved - and that is just to measure RF power!  Amplifiers of
course have many other characteristics that need to be watched - including,
of course, IMD.  It just isn't easy.

73
Bob Groh, WA2CKY

On Sat, Oct 10, 2020 at 6:53 AM David Stinson <arc5 at ix.netcom.com> wrote:

> Shade-Tree Scope Calibration at RF.
>
> IMHO, a properly-functioning scope is
> the First member of the "Holy Trinity"
> of test equipment-
> The Scope, The Meter
> and The ZM-11 Bridge, Amen.
>
> But a scope is only as good as the
> calibration and linearity of it's
> vertical amplifiers.  One can use a
> variable voltage supply and see if the
> amplifier agrees with what it says, but
> who's to say the supply is properly cal'ed?
> Improperly calibrated test equipment
> can have you chasing your tail for days.
>
> For checking the scope at DC input,
> perhaps a 300V supply feeding the top
> of a Zener stack, then stepping down the
> stack, checking agreement at each step?
> Can someone recommend a Zener family
> with closer tolerances than most?
>
> Well and good for DC, but how about
> "where the rubber meets the road"
> at RF?  An old vertical amp in a Tek scope
> might be spot-on at DC or 60 Hz, but that's
> no promise of how it behaves at 4 or 7 MC.
>
> So here's the question:  You don't have
> the NIST-certified standards available
> and you aren't going to take a 2nd mortgage
> to get them.  You need to scope a transmitter's
> output and read the PTP voltage so you can
> calculate the true power output in Watts,
> then use that to calibrate a Wattmeter.
> I have three or four Wattmeters around here
> and none of them agree with each other.
> One Drake is connected to a Johnson Viking II
> with AM 270 V PTP output and the meter says
> the carrier is only 45 Watts.... Uhhh, no...
>
> So, if you calibrate your scopes at RF,
> how do you do it?
>
> TNX OM ES 73 DE Dave AB5S
>
>
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