Page 74 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

Also, the modulating signal is produced by frequency dividing down from the
10.7386-MHz crystal to generate an 839-Hz waveform at pin 5 of the 74HC393
counter. This is done by dividing the 537-kHz signal from pin 3 of the 74HC390 by
5 via its second divide-by-5 circuit at pin 7. The output signal (537 kHz/5 or 107.4
kHz) at pin 7 of the 74HC390 counter is further divided by 128 via binary
counter/divider circuit 74HC393, U4B cascading to U4A to produce an 837-Hz
square wave at pin 5 of the 74HC393 counter. Thus the modulating waveform is a

triangle wave at 839 Hz across C4, which feeds into the load resistors R10 and R5
to provide pulse amplitude modulation at 455 kHz and 537 kHz, respectively.
Chapter 5

Low-Power Tuned Radio-Frequeincy Radios

The very first radios were tuned radio-frequency (TRF) radios back in the early
twentieth century. TRF radios were made commercially from the beginning of
amplitude-modulation (AM) radio to the twenty-first century. As of 2012, there are
still commercially made TRF radios for the AM section of an AM/FM radio, such as

the Kaide Model KK -205 radio that sells for about $5 to $6 on eBay. Also the Kaide
Model KK-9 AM/FM/SW (shortwave) radio that sells for about $7 on e8ay has a TRF
AM radio section while having a superheterodyne circuit for the FM and SW bands.
This chapter will explore TRF radios that drain very little power, and a number of
designs will be presented.

Design Considerations for TRF Radios

In the world of low-power design, we are talking about radios or receivers that
drain less than 1 mA on one or two cells. However, we will set a goal for something
in the range of less than 300 ~A. Why such a low current consumption? Well, the
milliampere-hour capacity of an AA alkaline cell is about 2,500 mAh, whereas an
alkaline C cell has about 7,000 mAh, and a D ceU has about 14,000 mAh. Thus, for
a radio that drains about 300 IJA continuously, one or two D cells will last about 5

years, whereas a C cell will go for about 2.5 years, and an AA cell will run for about
11 months.
As a single-cell battery drains, the voltage drops from about 1.5 volts to about 1.2
volts or 1.1 volts before becoming unusable. Thus the radios must work at the
lower voltages to extend useful battery life. For the most part, low-power radios in
this book shall drive crystal earphones. Conventional magnetic or dynamic

earphones or headphones normally are low resistance and require milliamperes of
current drive, which thus will shorten battery life considerably.
Now, what types of devices shall we use for low-powered radios? 80th bipolar
transistors (BJTs) and field-effect transistors (FETs) are available. For the greatest
voltage gain or power gain per any given operating current, bipolar transistors