Chapter 3 Direct (TRF) Radio Receivers
3.1.The Simplest Radio Receiver
3.1.1. Input Circuit
3.1.2. The Antenna
3.1.3. The Ground
3.1.4. Other Components
3.2. The Simplest Amplified Radio Receiver
3.3. Simple Radio Receiver with TDA7050 IC
3.4. Simple Radio Receiver with LM386 IC
3.5. Radio Receiver with Increased Sensitivity Audio Amplifier
3.6. Universal Audio Amplifier
3.7.Receiver with HF Amplifier
3.8. The Audion - Direct Receiver with Drain Detector
3.9.1. Reaction - Type Receiver
3.9.2. Direct SW Receiver for AM, AM-SSB & CW Signals
3.10. Miniature Receiver with ZN414 (ZN414Z) IC
3.11. Pocket Receiver with ZN414 & LM386 IC’s
3.12. Miniature Receiver with ZN415E IC
3.13. Receiver with ZN415 & LM386 IC’s
3.14. Mini Receiver with ZN415 & TDA7052 IC’s
3.15. Direct (TRF) FM Receivers
3.15.1. The Simplest FM Receiver
3.15.2. The Simplest FM Receiver with Audio Amplifier
3.15.3. FM Receiver with One Transistor and Audio Amplifier
3.15.4. FM Receiver with (just) One Transistor
3.1. The Simplest Radio Receiver
Each radio receiver must have a reception
antenna. It is an electrical conductor, where voltages of various
frequencies and amplitudes are being induced, under the influence of
electromagnetic fields from various radio transmitters. Besides these
voltages, those induced by EM fields that are created by various disturbance
sources (such as electrical motors, various household appliances spark-plugs
of an automobile and all other devices where electrical current is being
switched on/off during work) are also present in the antenna, as well as
those from fields originating from outer space or the Earth’s atmosphere.
Basic roles that a radio receiver has are:
a. To separate the signal (voltage) of the radio station that it is tuned at
from the multitude of other voltages, whilst suppressing (weakening) all
other signals as much as possible,
b. amplifies the extrapolated signal and take out information from it and
c. reproduces that information, i.e. restores it into its’ original shape.
Even the simplest radio, the one we are discussing in this chapter, must be
able to accomplish all these tasks. The electronic diagram of one such
device is given on Pic.3.1. It is the famous (years ago) Detector Radio
Receiver or shortly, Detector. The signal selection (separation) and voltage
amplification are performed in the oscillatory circuit that is

made of the capacitor C and coil L, the
separation of information (speech or music) from the AM station signal in
the detector that comprises the diode D, capacitor C2 and resistance of the
headphones, and information restoring in the very headphones.
Main advantages of this device lie in its extreme simplicity and the fact
that it requires no additional energy sources for its’ operation. All the
energy required it draws from the antenna, which therefore has to be at
least a dosen metres long for proper operation. It is also useful to have a
good ground. One can do without it but the reception with it is truly
better, especially considering the distant and small-power transmitters.
3.1.1. Input Circuit
The capacitor that takes the signal from
the antenna (so-called coupling capacitor) C1, variable capacitor C and coil
L form the input circuit of the radio receiver. Its main role is to separate
the signal of station the receiver is tuned at from multitude of voltages
(having various frequencies and amplitudes) existing in the antenna, amplify
that signal and turns it over to the detector.
In order to better understand the requests that are to be fulfilled during
the practical realization of input circuit, it is necessary to know basic
characteristics of circuit made of capacitor C and coil L. It is called ‘The
oscillatory circuit’ and is shown on pic.3.2-a. The amount of its impedance
(resistance to AC current) between points A and B, which is marked with ,
depends on the frequency, as it is shown on the diagram on pic.3.2-b. The
most important characteristic of this circuit is its resonance frequency,
being given by the Thomson’s formula:

As one may notice, the resonance frequency
depends on the capacitance of the capacitor C and inductivity of coil L, and
changes if one of them change. In our receiver, a variable capacitor is
used, that can change its capacitance from Cmax to Cmin, therefore changing
the resonance frequency in boundaries from
to

The area between fd (lower boundary frequency) and fg (upper boundary
frequency) is the reception area of the input circuit, as shown on
pic.3.2-b. On this picture, carrier frequencies of four radio transmitters
are being marked with fs1, fs2, fs3 and fs4. The resonance frequency of the
oscillatory circuit is set (by means of C) to be equal to the carrier
frequency of the second station: fs2. In that case, the impedance ZAB -
frequency dependance is shown in continuous line. As one can see, the
impedance ZAB for all received signals whose carriers have frequencies less
than fs1 and greater than fs3 is less than 20 kOhms, while for the station
that is tuned it is equal to 200 kOhms. Let us now imagine that the parallel
oscillatory circuit is connected with the antenna and ground, as shown on
pic.3.1-b. Imagine, also, that there are (only) four voltages in the
antenna, that have the same amplitude and are created by four radio

transmitters, having carrier frequencies of fs1, fs2, fs2 and fs4. Since
these voltages spread between the antenna and the ground, four currents will
flow through the oscillatory circuit: Is1, Is2, Is3 and Is4. The voltages
that are created by them in the oscillatory circuit, between points A and B,
are equal, acc. to Ohm’s Law, to the product of current and impedance:
UAB=I*ZAB. Acc. to pic.3.2-b, for Is2, impedance of the circuit is ZAB=200
kOhms, and for currents Is1 and Is3 it is 10x smaller. That means that the
voltage that is being created in the oscillatory circuit by the station that
transmits on frequency fs2 will be ten times greater than the voltages being
created by stations transmitting on frequencies fs1 and fs3. This is how
selection of one station is performed, by means of the oscillatory circuit.
Transition to some other station is performed by changing the capacitance of
capacitor C, as long as the resonance frequency of the oscillatory circuit
does not become equal to the carrier frequency of that station. If its
frequency happens to be fs4 (acc. to pic.3.2-b), the impedance of the
oscillatory circuit for that case is shown in dashed line, which causes that
on the circuit output voltage of the station that transmits on frequency fs4
is acquired, while other stations’ signals are suppressed.
At first glance, everything is just the way it should be: Parallel
oscillatory circuit extrapolates one and suppresses all other stations.
Unfortunately, the reality isn’t so simple. First of all, radio transmitters
operate with various output (emission) powers and on various geographic
distances from the receiver, therefore making the voltages that their
signals create in the reception antenna very different in amplitude. It is
clear that stronger signals will “cover” the weak ones, thus disabling their
reception. E.g. if radio transmitter that emits on the frequency fs1 is
geographically much closer to our radio receiver that the transmitter
operating on fs2, the voltage the former creates in the reception antenna
can be even 200 times greater than the one created by the latter. The
oscillatory circuit will do its job as previously described, but on its ends
the voltage of the first transmitter will still be greater (20x) than that
of the transmitter the receiver is tuned at, and normal reception won’t be
possible. There are also other problems whose solving will not be discussed
herein, and readers that are interested in those can read a book “Radio
Receivers”, written by Momir Filipovic, issued by the National Textbook
Publishing Company from Belgrade, Yugoslavia.
To conclude this chapter, we may say that the simplest radio receiver can
cover only signals of the local and powerful radio transmitters.
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