The Q Meter and Its Use: Experiment 4-17

25 August 2007
Exp 4-17 The Q Meter
EXPERIMENT 4-17 THE Q METER AND ITS USE

Experiments in Electronics and Communication Engineering E H Schulz and L T Anderson Harper and Brothers 1943 pages 89 to 95
In most radio frequency work it is important to obtain a large ratio of reactance to resistance in the reactive elements of the circuit. This ratio is called the Q of the circuit.
Q= XL L X 1 = 2f = C = R R R 2fCR
A high Q is required to obtain good efficiency, good wave form, good frequency stability, high gain, etc. The Q meter is an instrument that measures the Q of a reactance element directly. It may also be used to measure: the reactance and resistance of a circuit, the distributed capacity of a circuit, the resonant frequency of a tuned circuit, etc.
Fig. 432 shows the fundamental circuit of the Q meter.
Page 1 of 7
25 August 2007
The instrument consists of the variable frequency oscillator, the thermocouple ammeter I, the standard resistor R, the standard condenser C, and the vacuumtube voltmeter EC.
In the Boonton Type l6OA the frequency range is 5075 megacycles; C has a range of 30-450 microfarads; the Q range is 50 to 600. In the Type l70A the frequency range is 30-200 megacycles, C has a range of 1060 microfarads, and the Q range is 80 to l200.
The oscillator supplies a current I to R and the unknown and C. Since R is very small compared with the impedance of the external circuit, the voltage E is equal to IR. Hence the reading I gives an indication of the voltage impressed on the circuit. If the unknown and C are tuned to resonance (i.e., C adjusted until EC = Max.)
I'=
E R
(1)
where R is the effective series resistance of the unknown and C. Also

EC = I ' X C = EX C = QE R
(2)
Hence if E is held constant by holding I constant, EC reads directly proportional to Q and this meter may be calibrated directly in terms of Q. Since EC is proportional to E, I may be calibrated in terms of a multiplying factor for extending the Q range of the instrument.
If the distributed capacity cd is appreciable in comparison with C, the actual Q will be = Q(1 + cd/C). This correction is usually negligible.
The above method is applicable for measuring not for measuring capacity or very low or high inductance. This difficulty may be overcome by L across terminals AB, resonating the circuit,
inductance but values of connecting a coil and reading C1,
Page 2 of 7
25 August 2007
Q1, and the frequency f. The unknown impedance Z is then connected in series with L as shown in Fig. 433(a), or in parallel with C as shown in Fig. 433(b).
C is again adjusted for resonance and the new values C2 and Q2 are read. The constants of the circuit may then be calculated by means of the equations given in Table 41.
For simplicity in calculating, the values are given in terms of the equivalent series circuit of the unknown for the series
Page 3 of 7
25 August 2007
connected test and in terms of the equivalent parallel circuit for the parallelconnected test. Table 42 gives the equations that may be used to change the values from the equivalent series circuit to the values for the equivalent parallel circuit, or vice versa.
The method of arriving at the quantities in Table 42 may be illustrated by deriving the expression for the Q of a circuit as measured by the series connection method in Fig. 433(a). Let X1 and R1 represent the reactance and resistance of coil L, and let X and R represent the reactance and resistance of the unknown impedance. For the measurements with the unknown removed from the circuit
Q1 =
and
X1 1 = R1 C1
1 C1
(3)
X1 =
(4)
Similarly with the unknown inserted in the circuit
Q2 =
and
X1 + X 1 = R1 + R C 2 ( R1 + R)
1 C 2
(5)
X1 + X =
(6)
Page 4 of 7
25 August 2007
where X would be negative if it were capacitive. The Q of the unknown will be
Q=
X R
(7)
One limitation of the Q meter is that the size of the various components must be such that the circuit can be tuned to resonance by means of C. This condition may usually be satisfied by the proper choice of L and of the correct connection. For lowimpedance circuits the series connection is usually preferred, and for high impedance the parallel connection is usually better. The distributed capacity of a coil may be measured by the method shown in Fig. 432. The coil is connected to AB, C is set to about 50 picofarads, and the frequency is adjusted to resonance. The frequency is then set at half the previous value and the circuit is adjusted to resonance by adjusting C for resonance. The distributed capacity Cd is then
Cd = C 2 4C1 3
(12)
where C2 and C1 represent the two settings of C.
Page 5 of 7
25 August 2007
The resonant frequency of a tuned circuit may be determined by loosely coupling this circuit to the inductance L in the Q meter circuit. The Q of the Q meter circuit will drop when the external circuit is tuned to the same frequency as that of the Q meter circuit. Precautions 1. The circuit must always be tuned to resonance by adjusting C for maximum reading of the Q meter. 2. All leads must be short and heavy. 3. The terminals A and B are not grounded; hence the unknown circuit should not be grounded except when the parallel connection is used. 4. The accuracy of the device for a very low or a very high Q is definitely limited. 5. The coils L must be shielded and precautions must be taken to prevent picking up extraneous signals. Procedure 1. Measure the Q, R, and I of a number of coils covering a wide range of inductances. Be sure to use both the series and parallel method. 2. Obtain data for and plot curves of Q and R against frequency over a wide range. Repeat this for several different types of coils. 3. Determine the effect of a coil shield on the Q of the coil. 4. Measure the capacity and Q of a number of condensers. Be sure to use both series and parallel methods. 5. Obtain data for and calculate the distributed capacity of a coil. Calculate the resonant frequency of the coil (i.e., frequency at which Cd and the coil inductance are in resonance). 6. Measure the selfinductance of both windings of a radio frequency transformer. Measure the inductance of the windings connected series aiding and series opposing. Calculate M from
LSereies = L1 + L2 2M
REFERENCES Boonton Radio Corp., Instructions and Manual of Radio Frequency Measurements for Q Meters, Type 100A 160A and 170A.
Page 6 of 7
25 August 2007
Brown, H. A., Radio Frequency Electrical Measurements, pp. 69 71. Glasgow, B. S., Principles of Radio Engineering, p. 30. Hund, A., Radio Frequency Measurements. Rao, V. V. L., The Q Meter and Its Theory, Proceedings, IRE, November, 1942, p. 502.
Page 7 of 7

The Q Meter and Its Use: Experiment 4-17

Uploaded by

Copyright:

Available Formats

The Q Meter and Its Use: Experiment 4-17

Uploaded by

Document Information

Original Description:

Original Title

Copyright

Available Formats

Share this document

Share or Embed Document

Sharing Options

Did you find this document useful?

Is this content inappropriate?

Copyright:

Available Formats

The Q Meter and Its Use: Experiment 4-17

Uploaded by

Copyright:

Available Formats

25 August 2007

Exp 4-17 The Q Meter

EXPERIMENT 4-17 THE Q METER AND ITS USE

Fig. 432 shows the fundamental circuit of the Q meter.

Exp 4-17 The Q Meter

where R is the effective series resistance of the unknown and C. Also

inductance but values of connecting a coil and reading C1,

Exp 4-17 The Q Meter

Exp 4-17 The Q Meter

Similarly with the unknown inserted in the circuit

Exp 4-17 The Q Meter

where X would be negative if it were capacitive. The Q of the unknown will be

where C2 and C1 represent the two settings of C.

Exp 4-17 The Q Meter

Exp 4-17 The Q Meter

You might also like