Page 162 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

Inductor-Iess (aka Coil-less) Superheterodyne Radio

Here is a problem to solve: How do you design a superheterodyne radio without
any coils? One way is to substitute every inductor with a gyrator. Or if a band-pass

filter is needed, one can use a ceramic filter or a gyrator.
But what if you want to design an inductor-Iess superheterodyne radio with just a
one-section variable capacitor or, better yet, tune the radio without a variable
capacitor at all? So this was what I was thinking during the last four months before
I was going to write this book. The initial design was to use ceramic filters for the

IF stages and no tuned RF stage. This design would work, except that using a
455-kHz ceramic filter with a single conversion RF mixer circuit would result in
receiving radio signals at image frequencies.
For example, without a variable band-pass filter tuned to the desired frequency and
fed to the converter or mixer circuit, an image signal will convert (or mix down) to
the same IF frequency at twice the IF frequency (e.g., twice the IF frequency = 2

x 455 kHz = 910 kHz) plus the frequency of the desired station. See the following
table for some examples.

Desired Station Frequency IF Frequency Image Frequency
540 kHz 455 kHz 1,450 kHz
610 kHz 455 kHz 1,520 kHz

680 kHz 455 kHz 1,590 kHz
However, there is another technique called double conversion that eliminates
receiving stations at the image frequencies. But a double-conversion radio requires
a second mixer or converter circuit and other IF filters. The double-conversion radio

is workable, but it was decided that the inductor-Iess radio design will stick to just
one converter circuit instead.
Therefore, to get around the problem of receiving stations at image frequencies,
the IF frequency was raised to about 600 kHz (or higher), and a broadband
low-pass filter was used to remove signals higher than 1,600 kHz. Any signal above
1,600 kHz would be filtered out and not able to mix or convert down to the new

600-kHz IF frequency. See the following table for some examples.

Desired Station Frequency IF Frequency Image Frequency
540 kHz 600 kHz 1,740 kHz
610 kHz 600 kHz 1,810 kHz

680 kHz 600 kHz 1,880 kHz
As stated previously, any signal above 1,600 kHz will be attenuated or removed by
a low -pass filter. Figure 11-5 is a block diagram for an inductor-Iess
superheterodyne radio. For the antenna, a short wire of less than 2 or 3 feet or a

telescoping antenna will be sufficient to pick up radio stations in the AM broadcast