Page 23 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

other radios is taken from the output of the detector, is taken from the output of

the radio-frequency/audio-frequency (RF/AF) amplifier instead.
Circuit IDescription of a Reflex Radio
The RF filter section is formed by variable capacitor VC and cOil/inductor L1, which

also has a (stepped-down) secondary winding connected to the base of transistor
Q1. Note that the base of Q1 is an input for amplifier Ql. RF signals are amplified
via Q1, and the RF signals are detected or demodulated by coupling through an RF
transformer T2 to diode CR1 for envelope detection. At resistor R2 is a low-level
audio signal that is connected to the input of Q1 via AF coupling capacitor Cl and

the secondary winding of Ll. RF coupling capacitor C2 is small in capacitance to
direct RF signals to the emitter of transistor Ql without attenuating the low-level
audio signal. Audio transformer T1 is connected to the output of the amplifier at
the collector of Ql. Tl thus extracts amplified audio signal for Q1.
Superheterodyne Radio

The superheterodyne radio overcomes shortfalls of the TRF, regenerative, and
reflex radios in terms of sensitivity and selectivity. For example, the TRF and reflex

radios generally have poor to fair selectivity and sensitivity. The regenerative radio
can have high selectivity and sensitivity but requires the user to carefully tune each
station and adjust the regeneration control so as to avoid oscillation or squealing.
A well-designed superheterodyne radio will provide very high sensitivity and
selectivity without going into oscillation. However, this type of radio design requires
quite a few extra components. These extra components are a multiple-section

variable capacitor, a local oscillator, a mixer, and an intermediate-frequency (IF)
filter/amplifier. In many designs, the local oscillator and mixer can be combined to
form a converter circuit. Selectivity is defined mostly in the intermediate frequency
filter (e.g., a 455-kHz IF) circuit. And it should be noted that an RF mixer usually
denotes a circuit or system that translates or maps the frequency of an incoming

RF signal to a new frequency. The mixer uses a local oscillator and the incoming RF
signal to provide generally a difference frequency signal. Thus, for example, an
incoming RF signal of 1,000 kHz is connected to an input of a mixer or converter
circuit, and if the local oscillator is at 1,455 kHz, one of the output signals from the
mixer will be 1,455 kHz m,inus 1,000 kHz, which equals 455 kHz.
One of the main characteristics of a superheterodyne radio is that it has a local

oscillator that tracks the tuning for the incoming RF signal. So the tunable RF filter
and the oscillator are tied in some relationship. Usually, this relationship ensures
that no matter which station is tuned to in the oscillator, it changes accordingly
such that the difference between the oscillator frequency and the tuned RF signal
frequency is constant.

Thus, if the RF signal to be tuned is 540 kHz, the local oscillator is at 995 kHz, the
RF signal to be tuned is at 1,600 kHz, and the local oscillator is at 2,055 kHz. In