Page 191 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

resistor to reduce the I and Q signals by 10. Note that C13 should be replaced with

a 47-pF capacitor if R22 is 22 k

Alignment procedure.' With a reference generator of a known frequency, such as
the 537-kHz reference RF oscillator shown in Chapter 4, couple the generator's
si9nal with a wire near the I,oop antenna. With an oscilloscope set to

alternating-current (AC) coupling, probe either end of C3 or C4, and tune VCl for
maximum si9nal. Then connect a frequency counter to pin 4 of U2 and adjust T2
(or an adjustable inductor) to a frequency about 15 kHz above the RF oscillator's
frequency. For example, if the RF oscillator is measured at 537 kHz, then adjust T2
such that pin 4 of U2 reads 552 kHz.

Figure 12-9A shows a screen capture of the Winrad program with the AM radio
front-end circuit connected to a computer with a received radio station. In the
fi9ure, the tuned desired signal is highlighted by a wide vertical strip, whereas the
image signal is highlighted by a vertical line. The image signal is about 40 dB lower
than the desired signal (tuned signal). Or stated another way, the amplitude of the
image si9nal is about 1 percent of the amplitude of the desired signal (tuned
signa:I).