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 Sometimes out-of-date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.
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.
