?
?
Reading that, I do get the feeling that some of the improvements made between the 160-A and 260-A, would not be necessary if an instrument was made with a microprocessor doing some number?crunching. For example, they reduced the injection resistor from 40 milli ohm to 20 milli ohm. But still it¡¯s a source of error at high Q which one can correct with the aid of a calculation. If an instrument was made nowadays, one might just as well correct every reading ?Maybe the resistors used in the PW design would be okay if a microprocessor did the corrections, rather than
A digital display would definitely contribute to the accuracy of repeated measurements, but I would still
maintain the analog meter for pealing the tuning. I find it much easier to watch the meter bob up and
down than deal with digits changing. What can I say, I am old school.
I don¡¯t understand why those thermocouples are so easily damaged.?
Nor do i know for sure why.? My guess is the resistor was pushed to its power dissipation
limits as the factory calibration procedure was to use a DC Current source which would
provide 1 AMP across the resistor when the resistor was at room temperature. With that
1 amp current the lab specified a voltage measurement of 19.6 milliVolts across the
resistor at room temp. Once the resistor warmed up and drove the meter to its cal
mark, the voltage drop across the resistor was suppose to be 20.0 milliVolts.
There must have been some variance in the thermocouples as there were two resistors
that were hand selected at the production floor in series with each lead of the thermocouple.
The values were selected so that when the resistor presented with 20 milliVolts across it,
that the Multiplier meter would indicate "1" on the meter scale. The provided information
was that those two resistors in series with the thermocouple were inside the sensor head.
The sensor head was sealed with glass.
Looking back at some notes from the conversation with the retired Boonton engineer,
the resistor was porcelain that was fired with a layer of platinum. Each resistor was
hand trimmed using an abrasive substance to reach the precise 20 milliOhms. The
reason provided for all of this effort was to mitigate the resistor's inductive effects
on Q measurements.
What I find interesting is that the inference is that the resistor will have 1 Amp of
current flowing through it when the Q Meter's Multiplier meter is set to "1".? Looking
at the schematic, the rf power source is the 5763 tube based oscillator. There is
a winding on the coil assembly which I take is to reduce the output impedance of
the tube to a low ohm value to drive the resistor. While the 1 amp current seems
outrageously high to me, I^2R still yields only 20 milliWatts. Talk about some
"outside the box thinking",
You may find this article on the Boonton model 280 Q Meter to be of interest. It is
the Boonton app note for the unit.
It was designed to measure Q between 210 MHz and 610 Mhz up to 25,000.
_._,_._,_
?
If I bought one I would send it to Keysight - assuming that they will still calibrate it, which I suspect that they will. I have a healthy scepticism of calibration labs.?
Based on my experience, if Keysight has continued the policy of former HP 7 Agilent,
they will refuse to service the box. In 1995 one of the five
HP-4342A's in the lab I worked pegged the meter out. We? contacted HP in Maryland
to arrange return for repair and calibration, but were told it was an old and obolete
product which they no longer supported. A few weeks later we encountered the same
issue with? an HP-8505A network analyzer. Very frustrating. Of course they were very
much ready to sell us new network analyzers.
Fortunately there are many metrology service labs willing to repair older equipment
so the more complex devices can still be repaired and calibrated.
?
Chuck
