[Elecraft] OT: Bird Wattmeter "meter failures"

[Don Wilhelm]

Harlan,

That is correct - improper test leads are often a 'culprit'.
   For instance trying to measure an HF signal with a 'scope.  The 
'scope may b rated for 200 MHz, but if you try to us a "hunk of coax" 
for a probe, the frequency rating may diminish to 2 MHz (OK, that is 
just an example).  You need to consider the whole measurement system.  A 
scope's frequency rating must also be coupled with the frequency rating 
of the probe.  The use of 10X probes is necessary to keep capacitance 
loading of the circuit under test to a minimum.  And those probes have a 
frequency rating too.  Take the entire setup of the test equipment into 
consideration when doing measurements.
Make certain that every part of the test equipment being used is 
suitable for the frequency involved.  A simple coax probe may be OK for 
AF frequencies, but it is not adequate when the frequency is increased.

A 'scope can show proper RF voltage amplitude, but the frequency rating 
of the probe and the 'scope should be taken into consideration.  For 
instance, a 35 MHz scope will only show a signal that is 3 dB down at 35 
MHz.   I would suggest that valid RF voltage measurements be made at 
1/10 of the 'scope's frequency rating (and that goes for the probe too).

Again I state, trust no measurement equipment until its accuracy has 
been verified.

73,
Don W3FPR


On 11/21/2015 10:09 PM, hsherriff wrote:
> I'm the supervisor of a metrology lab and that is one of the first 
> things I ensure a new technician understands. And many times it isn't 
> the test equipment that is at fault, nor the unit under test, but the 
> methodology of using that test equipment.  Not using the equipment 
> "guard" correctly,  induced voltages, improper test leads for the 
> job.... and many many more.
>
>