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The TF1245 comprises a very low impedance element that is associated with a resonant circuit. When a voltage is developed across this defined impedance the corresponding "magnified" voltage across the resonant circuit is measured using a high impedance valve voltmeter. The ratio between the two voltages represents the circuit magnification factor. As the low impedance element is small compared with the overall circuit loss, this magnification factor approximates closely to the "Q" of the test inductor applied to the instrument terminals. Hence the description "circuit magnification meter" is more correct than the frequently used term "Q Meter". The low impedance element is a 20 milliohm low reactance resistor for the 1 kHz to 50 MHz test circuit and is a 0.1 nH inductor for the 20 MHz to 300 MHz circuit.
There are two oscillators available for the TF1245. The TF1246 covers the frequency range 40 kHz to 50 MHz and the TF1247 extends this to 300 MHz. I'm still keeping an eye out for a TF1247!
Dedicated oscillators were necessary for the TF1245, due to the very low input impedance (about 0.5 ohm). A transformer (TM5728A) was available for matching 600 ohm LF oscillators to the TF1245 input, thereby enabling use down to 1 kHz. A range of fixtures for the measurement of dielectric properties of materials was also available.
As with most MI products of this era (1960s), the build quality is superb and the TF1245 is one of the best "Q Meters" ever made.
There is excellent technical detail in the TF1245 manual, available below.
TF1246 oscillator circuit
TF1246 power supply
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