Check Inductors With This Simple Q Meter

While LCR
meters are readily available at reasonable cost, they do not measure
the Q of an inductor. This circuit enables you to measure the Q of
inductors with the aid of an RF signal generator. A capacitor is
connected in parallel with the inductor to form a tuned circuit. By
varying the frequency, you can measure the resonance frequency of the
tuned circuit and its -3dB bandwidth. The Q is then the resonance
frequency divided by the -3dB bandwidth. Transistor Q1 is an emitter
follower acting as input buffer to drive RF transformer T1. The
secondary winding of T1 then drives the parallel tuned circuit formed by
the inductor under test (Lx), T1’s secondary and tuning capacitor VC.

The tuned circuit so formed is buffered by JFET Q2 and transistor Q3 which form a cascode stage with about 3dB of gain. The JFET
provides a high impedance so that the loading of the tuned circuit is
minimal (note: an MPF102 can be substituted if you cannot obtain a
2N5485). The RF output from Q2’s collector can be monitored by an
oscilloscope to easily find the point of resonance and read the
frequency. Alternatively, the RF output can be read by an external
frequency meter. Diodes D1 & D2 and the 5.6nF capacitors form a
voltage doubler rectifier to drive a 100µA DC meter so that the
resonance can be found (in the absence of an oscilloscope).

Trimpot VR1 provides a sensitivity adjustment for the meter.
Transformer T1 is wound on a 12mm diameter ferrite toroid core. The
primary winding consists of 50 turns of 0.2mm diameter enamelled copper
wire, while the secondary is a single turn consisting of a strip of
brass 0.5mm thick and 2.5mm wide bent into a horseshoe shape and
threaded through the centre of the toroid. VC is a small AM tuning
capacitor with both gangs connected in parallel.

To measure Q, the output of the RF signal generator should be around
0.5V peak. Adjust the frequency until the meter’s reading peaks, then
adjust VR1 so that the meter reads full scale (100µA). Read the
resonance frequency F0 from the frequency scale of the signal generator
or better still, the reading on a frequency meter.

Next, increase the signal frequency until the meter reads 70µA and
note this frequency as F2. That done, reduce the frequency on the signal
generator below the resonance frequency until the meter again reads
70µA and note this frequency as F1. The Q can now be calculated as:

Q = F0/(F2 – F1)

While using a variable tuning capacitor will enable a wider range of
inductors to be tested, the main advantage is estimating the
distributed capacitance of the inductor as well. To do this, you have to
calibrate the tuning scale with a capacitance meter, by measuring the
capacitance across the tuning capacitor with no inductor connected. This
is done with the unit switched off. Marking off increments of 20pF
should be sufficient.

Set the tuning capacitor to say ¼ of its maximum value and note this
value as C1. Adjust the RF signal generator frequency so that the
inductor under test is at resonance and note this frequency as F0. Now
set the RF generator frequency to half F0, adjust the tuning capacitor
until resonance and note this capacitance as C2. The distributed
capacitance of the inductor is (C2 – 4C1)/3.

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