Re' a Q meter.
This is only a thought idea, not tested hi.
Have a small enclosure with sockets on opposite sides. each socket
has a
single turn loop attached with sufficient distance between the two
loops
to allow the coil under test to be positioned between them. one turn
is fed
from your sig' gen' and the other feeds a scope or RF meter.? A
sufficient
level from the sig gen may require an amplifier. With the coil to be
tested
placed between the loops tune the sig gen for maximum throughput, ie
resonance. Then tune the sig gen lower until the throughput is 3dB
down, ie 0.71 reduction in voltage or 1/2 power. Repeat by tuning
the sig gen
higher than resonance for a similar drop in throughput. the resonant
frequency divided by the bandwidth just ascertained is the Q of the
coil.
The coil being tested can, of course, have a capacitor in parallel
if wished.
73 Ken G8BEQ
?
On 04/08/2022 09:19, Dr. David Kirkby,
Kirkby Microwave Ltd wrote:
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I am looking at the possibility of making a Q meter for the
frequency range 1.5 MHz to 150 MHz. One common way to do this is
to inject a voltage from a LOW impedance source, into a series
tuned circuit, then measure the voltage developed across the
tuned circuit with a high input impedance voltmeter. The Q is
the ratio of those two voltages.?
Designs for Q meters usually include an oscillator, but I don¡¯t
think I will bother with that as I can easily use a signal
generator.?
One design appeared in Practical Wireless in the November 1978
issue. I stuck a copy of that in a sub-directory for Q
measurements in the files section of the forum?
/g/Test-Equipment-Design-Construction/files/Q-factor%20%28Q%20is%20the%20inverse%20of%20dissipation%20factor%20DF%29.
There are two problems I see with that design
1) The meter has measurement
ranges of only 0-20 and 0-100, but many inductors have Q¡¯s much
higher than 100. The HP 4342A can measure Qs from 5 to 1000. The
old Boonton 160-A measures Q up to 640 (from memory).?
2) The output impedance of source is too high?- it is
approximately 2 ohm on the Q=0-20 range and 10.2?ohm on the
Q=0-100 range.?
Can anyone think of a way of turning a signal generator to have
an output impedance of 1 milli ohm? I think I have a 5 W
Minicircuits amplifier around, so I can afford to waste a bit in
heat. But it is not practical to put 50 ohm in series with 1
milli ohm to terminate the amplifier in the required 50 ohm. I
will have so little voltage left, it will be difficult to
measure the output voltage, even though it¡¯s multipled by the Q
of the coil. ?
A step-down transformer seems the most obvious way, but that
requires a turns ratio of sqrt(50000)=224. Even with a single
turn on the secondary, there will be too many turns on the
primary for this to work at 150 MHz.
?
I have not yet looked at the HP
4343A meter service manual. That will probably give me some
ideas.?
I suspect the answer
is to not try to get such a low output impedance, accept that
the voltage generated across the LC combination will be less
than the Q, and correct for that in software.?
Any other thoughts??
PS, Does anyone have any documentation, apart from the
user manual, on the HP 42851A Q-adapter, which is used with
the HP/Agilent 4285A Precision LCR meter??
?
?
