As far as the correct thermocouple, I have found nothing in Boonton (HP) notes to indicate what
the type was. Given that the wire reaches 1600¡ã F nominal, I suspect if a standard thermocouple
"Type" was selected, it would have been the Type N which measures from 650¡ã C to 1260¡ã C
(1200¡ã F to 2300 ¡ãF).
Boonton accounted for variances in the meter measuring the Nichrome wire temperature with
resistors in series with the wires to the meter that connected the thermocouple output. The meter
as I understand is in the 100 ?A range. The compensating resistor value was split and two equal
value resistors used, one resistor in each meter lead.
Also keep in mind, the temperature of the precision resistor is not being measured. When
calibrated at the factory, 1 Amp DC was injected into the input of the thermocouple sensor
assembly. With no inductor on the terminals, the nichrome heating wire current only flows to
the 20 milliOhm precision resistor. E=IR thus 1 Amp X? 0.02 ¦¸ = 20 milliVolt.? Boonton engi-
neers used a precision lab quality method to determine the voltage across the precision
resistor. It does not matter if you use RF or DC current in the lashup. 1 Amp DC produces the
same voltage across the resistor as does 1 RMS Amp of RF. Both produce the same power
dissipation in a resistor and that 1 Amp of current DC or RF will produce a voltage drop of 20
millVolt DC or RMS.
The 260 was designed before I was born and I am 71 years old. Thus I am not aware of what
choices Boonton had for resistors in the day. What I do know is, their scientists and engineers
went to a lot of trouble to minimize the precision resistor inductance.? Pete Olin who worked for
Boonton in New Jersey, explained that the precision resistance is obtained with a thin layer of
platinum applied to a ceramic disc. He stated the precision resistors were often hand ground
with a very fine abrasive to bring the resistance down to the desired 20 milliOhm. The catch
when grinding the resistor was the conducting platinum layer removal had to be consistent
across the surface as variance in the thickness introduced inductance that caused errors across
the operating frequency.
Given the attention to detail in producing the precision resistor, I do not think the resistor is the
limiting factor in operational frequency range of the 260. Nor do I believe that the inductance
in the nichrome wire is coming into play. If anything, I suspect it is the length of connections where
inductors and capacitors under test are connected. The VTVM in the Q-Meter may also add to
the problem with its rapid rolloff approaching 50 MHz. If I find a 260 with a blown thermocouple,
I plan to connect the precision resistor up to a network analyzer and see just how high in frequency
the resistor can be utilized before the series inductance and stray capacitance become significant
enough to impact measurement accuracy. My SWAG is that the resistor can be used well up
beyond 150 MHz.? But that is just a SWAG.
Regards
Chuck
