Q Meter cal accuracy

[Domenic M. Mallozzi]

Hi

First, If anyone has a Boonton 250-A RX Meter and does not have the Boonton
"BRC Notebook" issue Summer 1954 let me know and I'll get you a copy. It has
a description of the design of the 250-A and a note about the adjustment of
the bridge trimmer that may be useful to you. All I ask is you reimburse
postage when you get it. Just e-mail your address and let me know you want
the 250-A info.

This is a preliminary note to one I will post at a later date when I get back
to the office and can get to my notes and manuals collection in the plant cal
lab.

Just some quick notes for now: Boonton and HP both also made Q meter
standards. (In fact I think HP (now Agilent) bought Boonton's Q meter
business.). A bunch of people made Q meters including Marconi and Wayne Kerr
in England, and a bunch of small companies in the US that produced them on
special order. At one point HP moved all its Q meter design to a subsidiary
(YHP) in Japan that specialited in LCR measurement instruments. Boonton, HP
and YHP all made fine equipment.

It is important that you use the Q standard at the frequency stamped on the
name plate. Their calibration values are only good at the specified
frequencies. Some cal labs also did produce calibration curves for Q
standards that showed the values over a range of frequencies. (Its hard to
find a set with those curves). A single Q standard used between 500 KC and
1.5 MC has a Q that varies between 180 and 240 (25% if you use the peak of
240 as a reference)

Also, if you add parallel caps to your Q meter to resonate it the caps
themselves MUST be high Q. Air caps are obviously the best but some Micas
types are very good. Also the leads of the resonating caps must be short. In
fact most Q meters have extra banana posts in paralle with the "L" posts so
you can make a good low impedance connection in parallel with the inductor
under test and the internal resonating capacitor.

As for accuracy of the Q meter:

Boonton 260-A
Frequency Accuracy  1%
Q Accuracy (at a reading of 250)
       50Kc to 30 Mc        5%
       30 Mc to 50 Mc       10%
Inductance Accuracy      3%
Internal resonating Capacitor Accuracy (the cap attached to the L dial)
       Main cap   1 % or 1 pf (whichever is greater)
       Vernier       +/- 0.1 pf

The Heath QM-1 manual I have shows no accuracy specs for Q. (Frequency
accuracy is 3%). But, based on a bunch of design issues I don't think an
accuracy for Q of 20% would be surprising.

Obviously, the Q readings are not always on the nose. In fact they do vary
from Q meter model to Q meter model. Boonton published a study of the
differences in readings between a Boonton 160-A and a 260-A using the same
standards and differences as high as the 30 % range were reported (the 30 %
error occurred at 80 KC not high frequency as many would expect). In fact the
only place the two types of instruments correlated well were between 700 Kc
and 3 MC where they stayed within 5 % of each other. The errors get larger as
the Q increases. (The errors above are for a Q of 250).

So don't get overly upset that your meter and a friends of a DIFFERENT MODEL
could be 20 % different at various frequencies and capacitor settings.

Hope that was boring enough

73's

Dom
N1DM

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