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A Poor Man's Q Meter

Simple circuit mates with an RF signal generator and multimeter for quick-and-easy "Q" and inductance measurements.

By Maurie Findlay, MIEAust

Experimenters, and even professionals, setting up a test bench have to think hard before buying test instruments. Depending on the special interest, items such as a multimeter, regulated power supply, counter, oscilloscope, RF and AF signal generators would come high on the list. Money can be saved by building test gear described in Silicon Chip over the years. Sometimes out-of-date equipment from schools and government departments can be overhauled and brought into service.

But purchase of a Q meter would probably be pretty low on the priority list.

Click for larger image
With less than a dozen components, a digital multimeter and practically any RF signal generator you can measure Q and inductance very easily. The old-style point-to-point wiring is housed in a shielded metal box.

There are at least two reasons for this. Inexpensive hand-held bridges can measure inductance reasonably accurately, provided the values are not too small (say below 10 μH). Second, the selective components used in modern equipment usually come in block form such as ceramic, crystal or mechanical filters with the characteristics specified by the manufacturer. No longer does the designer have to specify the inductance and Q of a whole series of coils to make up a filter for, say, the intermediate frequency section of a receiver.

On the other hand, inductances to a fraction of a μH are used in the signal frequency circuits of both transmitters and receivers for filters, tuning, coupling and decoupling circuits. Inductors used for coupling between tuned circuits and to active devices are critical because they are usually quite critical but they are not adjustable.

So - this discussion is about a simple test jig which, used in conjunction with a signal generator and an electronic voltmeter, allows the inductance and Q of small coils to be measured accurately by resonance with a known value capacitor. It comes into its own when dealing with inductors below about 10 μH. It can easily be adapted to measure a range of inductance by altering the value of the capacitor.

Most readers will regard this as an ideas article rather than a constructional project to be copied component for component. The model illustrated is just one of many ways the basic idea can be used.

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