[Test-Equipment] Q measurement

[Richard Knoppow]

----- Original Message ----- 
From: "Brian Burns" <brian at lessonsinlutherie.com>
To: "'Discussion of Electronic Test Equipment'" 
<test-equipment at mailman.qth.net>
Sent: Thursday, June 19, 2014 9:01 AM
Subject: [Test-Equipment] Q measurement


> Hello All,
>
>
>
> Thanks for all the good information on Q meters!
>
> Anything that resonates has a Q, and I measure the Q's of 
> my wooden Spanish
> guitar parts to determine their acoustic loss properties. 
> I used to do it by
> exciting them with a sine wave audio source, coil, and 
> rare earth magnet,
> and then tuning the audio source above and below the 
> resonant frequency
> until a microphone registered a 3db drop in volume.
>
> That half-power bandwidth divided into the resonant 
> frequency gave me the Q.
> Now I use an audio analysis program, Spectra Plus, that 
> calculates the Q of
> any resonance it looks at. But the old slow way is 
> certainly repeatably
> accurate.
>
> Though it might be a bit labor intensive, I would think 
> that it would be
> possible to measure the Q of a tuned circuit with an 
> accurate signal
> generator and a VTVM by essentially the same method. Is 
> the convention in RF
> measurements to use the bandwidth at the 6db down points, 
> divided into the
> center frequency, for Q's? In audio we use the bandwidth 
> at the 3db down
> points divided into the center frequency.
>
> Cheers,
>
> Brian

      Essentially, the Boonton Q meter is a combination of 
an oscillator and a VTVM. The combination is convenient and 
makes it possible to have better contol of stray and 
spurious reactances coupled to the measurement.  The 
commercial Q meter had two voltmeters: one is a meter across 
the thermocouple which measures the input to the test 
circuit and allows standardization of level; the second is 
across the standard capacitor and indicates the resonant 
rise of the voltage in the resonant circuit and is 
calibrated in Q.
      Q has several definitions but is essentially the ratio 
of reactance to resistance in an inductor, a capacitor 
(although usually the inverse or dissipation factor is used) 
or a resonant circuit.  The Q can also be measured from the 
resonance curve. It is the ratio of the bandwidth at the 
half power points (voltage down 0.707) from the peak at 
resonance.  I think some confusion may happen when this is 
stated in db since half power is 3 db and half _voltage_ is 
6 db.  The measurement is at half power which is 3 db in 
either system.  Note also that there is a difference in the 
effect of resistance in the circuit between series and 
parallel resonance. The familiar formula for resonance 
applies strictly _only_ to series resonant circuits. In a 
parallel resonant circuit the resonant frequency will be the 
same only if there is no resistance.
     The Boonton handbooks for the 260-A and some articles 
in the Boonton house organ the "BRC Notebook" has 
considerably more on Q and its measurement. All of the 
Notebooks are on line free, I think on the IEC site.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com