开云体育

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??

?

?