Aligning the Receiver

Author: J.B. Hoag

The alignment up to the limiter is the same as for an a.m. superhet. A sensitive d.c. milliammeter is connected in series with R₁ of Fig. 33 D, at the grounded end, and a test oscillator injects a series of frequencies (into the input end of the i.f. stages) which cover the full band-width (usually 30 kHz). If nearly symmetrical readings of the meter are not obtained on both sides of the center frequency, the i.f. trimmer condensers must be readjusted. At the edges of the band, the output should not be down by more than about 25 per cent.

To check the limiter, disconnect C₃ of Fig. 33 E, also the upper ends of R₁ and C₁, and insert a sensitive rectifier milliammeter in series with R₂ at the grounded end. The meter should show no change as the frequency of the test oscillator is changed over the band. Now restore all broken connections.

With the meter in series with R₂ as before, and with the test oscillator at center-frequency, adjust the trimmer condensers of the discriminator until the meter reads zero. Then note the meter readings when the test oscillator is set on the two frequencies (one above, the other below the center-frequency) which correspond to the desired frequency deviation. (The meter terminals will have to be reversed.) If the meter readings are not the same, adjust the primary trimmer, then readjust the secondary trimmer for zero reading as above and repeat.