Section III Model 4342A
Paragraphs 3-62 to 3-66
Figure 3-11 (A) exhibit the stray capacities 3-64. PARALLEL MEASUREMENTS.
added by connecting a sample with a shielded
case. This capacitance increase adds to the Note
stray capacitances (Cl, C2 and Cs) around the
measurement terminals. In a series measure- In the following parallel connection
ment, the shorting strap, for initially measurement procedures, set 4342A Q
short-circuiting the unknown connection ter- RANGE as appropriate unless specially
minals, has its own residual impedance. instructed otherwise.
Additionally, its contact resistances differ
from those of samples. Small changes in the 3-65. High Inductance Measurement.
loss and the distributed inductance of the
tuning capacitor affect measurement accura- 3-66. When the measuring circuit is resonat-
cies. Figure 3-11(B) graphically shows an ed using a reference inductor and then the
electrical model of a variable capacitor. sample (unknown) inductor placed in parallel
The distributed inductance and the loss with the tuning capacitor, the tuning fre-
varies depending on the position of the quency will increase. To restore resonance
capacitor rotor. In the 4342A, these re- at the measurement frequency, the tuning capa-
sidual factors are minimum because specially citance must be increased. The inductance of
designed, high quality variable capacitors the unknown inductor can be determined from
are employed in the tuning circuit. relationship of the tuning capacitances at
the same measurement frequency. After the
Actually, the residual impedances present in sample is connected, quality factor and
the measuring circuit do not cause signifi- equivalent parallel resistance can also be
cant errors except when measurements of ex- calculated from a reduction of the panel Q
tremely high or extremely low impedance meter indication,
samples are taken at high frequency. A full
consideration of the factors of additional To measure an inductance sample by the paral-
errors is not practical except in cases where lel method, proceed as follows:
the experiment requires improved accuracies.
However, it is difficult to make an accurate a. Depress appropriate FREQUENCY RANGE
Q measurement above 1000 (effective Q) at a pushbutton and set FREQUENCY dial
frequency higher than about 1MHz. control for desired measurement
frequency.
3-62. In parallel and series measurements, Q
meter indications are read twice as often as b. Select a reference inductor which
those in direct method measurements; thus, allows the measuring circuit to re-
the accumulation of reading errors and in- sonate with a tuning capacitance of
strumental errors should be taken into con- 30pF to 70pF at this frequency. Con-
sideration. In addition, a more accurate nect it to measurement COIL (HI and
tuning operation is required to minimize LO) terminals.
these additional errors. To improve fre-
quency accuracy, the oscillator frequency may c. Adjust L/C dial and AC dial controls
be monitored with a frequency counter (using for a maximum Q meter deflection.
FREQUENCY MONITOR output at rear panel). Note sum of the C dial and AC dial
readings as C1 and panel meter reading
3-63. When a low Q sample is measured, the Q as Ql.
meter deflection increases and decreases
broadly during the tuning operation. Because d. Depress AQ button and adjust AQ ZERO
of this low resonance sharpness, it is usual- (COARSE and FINE) controls so that
ly difficult to do exact tuning (to get a re- meter pointer indicates zero (full
sonant peak) and to obtain correct indica- scale) on AQ scale.
tions. This limits the resistance value
measurable with parallel and series methods, Note
respectively, as shown in Figure 3-5. As
high series resistance and low parallel re- Press AQ button to release AQ func-
sistance make for very low Q resonance cir- tion and recheck for correct reso-
cuits (below lo), the measurement accuracies nance. Again depress the AQ button
for such samples are thus much lower. and recheck for AQ zero indication.
3-18 |
ACEC
