## W

for convenience. A value of 10,000 ohms at the standard frequency is handy. The phases of the voltages taken to the primaries of T-2 and T-3 are 45 degrees ahead and behind the voltage supplied from T-l, or have a phase difference of 90 degrees. If the impedances of 7 -2 and T-3 are high (interstage coupling transformers with a step-up ratio of 6 : 1 are suitable), then connecting them across the elements R and C w ill not materially affect the phase of the voltages. The combined effects of the transformer load -ing and of the impedances across the secondaries generally make it necessary to readjust the elements of the phase shifter somewhat.

If the standard frequency supply is distorted in waveform, a circular sweep cannot be obtained. A low-pass filter is then necessary, as indicated in the diagram.

If a cathode-ray oscillograph having a radial deflector is vised, the unknown frequency can be placed on that electrode. In the more general case, it is convenient to introduce the unknown on the vertical plates as shown at T-4.

The type of patterns obtained with a circular sweep are illustrated in Figure 5. With no unknown frequency introduced, the pattern is a circle, with the spot traveling once around the circle for each cycle of the standard frequency. If a frequency equal to five times the standard frequency is introduced on the vertical plates, a pattern as illustrated in the second part of the figure will result. The frequency ratio can be determined by counting the tops of the waves, as at A, /?, C, D, E. (If radial deflection is used, the pattern is not distorted, and the frequency ralio is found by counting the outer tips of the waves.)

Note that, even when the unknown frequency is not set exactly to five times the standard frequency, the pattern does

0 0