A Simple Radio Receiver

8/3/2019 A Simple Radio Receiver

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A Simple Radio Receiver

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The diagrams, images and screen shots below detail thedevelopment of a simple modular radio receiver based

entirely on circuits and devices studied during the Part IAcourse on Linear Circuits and Devices.

This receiver may not take the market by storm! However,

we hope it will help illustrate how the circuits andprinciples we are studying all contribute to the art of

electronic circuit design.

Contents

A little history

Modern crystal sets

Development of Crystal

Receiver

Resonant LC circuit

Diode Detector Development of

Improved Receiver

Audio Amplifier Radio Frequency (RF)

Amplifier

Battery Lifetime

Output Amplifier The Complete Receiver

Photo Gallery

Circuit Diagrams

Notes and

Acknowledgments

A little historyThe first radio receivers were crystal sets, and became available in the 1920's with the

opening of Marconi's first broadcast station in Chelmsford.

A crystal set does not have a battery. It runs completely from the energy extracted from

radio waves it picks up from the antenna. A resonant LC (or tuned) circuit coupled to a

large aerial or antenna was used. Many amateur experimenters constructed crystal sets,
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often with the tuner inductor coil wound on a tubular box or a drinking glass. At this time

the semiconductor diode had not been invented, so extracting the audible modulation

signal from the transmission relied on the non-linear electrical properties of the 'crystal',typically a piece of coke or galena. In early sets a "cats whiskers" - a fine piece of wire -

was adjusted by trial and error to make a suitable contact with the crystal.

There were many limitations to the crystal set: it needed a big aerial (antenna), an earth

connection, the clumsy cats whisker, and the weak signal could only be listened to byone person at a time with headphones. Very quickly the crystal set began to be replaced

by valve radios with loudspeakers, powered by batteries.

In World War II, crystal sets were used by prisoners of war in prison camps to listen to

news from home. Much ingenuity went into improvising the necessary components.

Seethis presentation for further details of historic crystal set receivers.

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Modern crystal sets

Nowadays people only use crystal sets as a hobby. There are various different kits

available on the market.

Dave's Crystal Radio Set Page

http://www.crystalradio.net/

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Development of Crystal Receiver

The design shown here is a little more complex than strictly necessary, but some of theadaptations incorporated make it easier to develop the design, adding amplifiers and other

stages as we meet them in the course.

Resonant LC circuit
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Circuit (a) alongside shows

LC resonant circuit compriand VC1 used to select or t

required frequency and stat

The inductance used is 729

and VC1 can be varied overange 30 to 234 pF. That m

the range of resonant freque

is about 400 to 1075 kHz, wincludes a good part of the

Medium Wave band of

frequencies.

L1 is a coil of wire wound on a ferrite rod. Ferrite is a material with a

high magnetic permeability - that simply means that that to achieve agiven inductance, a smaller number of turns is required than if the coil

had an air core. That keeps its winding resistance low, and enhances theQ, which in turn means the resonant circuit has a narrower resonance

peak, and is better at rejecting unwanted frequencies.

VC1 is a variable capacitor

which the capacitance is va

rotating a knob, which contthe extent to which a set of

parallel plates are enmeshe

capacitance can never be reto zero, of course, owing to

residual capacitances due to

structure. To these figures be added any stray or parascapacitances arising from th

connecting wires, circuit bo

and other elements. These never be eliminated, but ca

construction aims to keep th

down to a few tens of pF.

The signal from the antenna (perhaps a few tens of microvolts, or hundreds for a nearbytransmitting station) is introduced to the LC circuit either through a small capacitance, or,

as in this case, by means of a second coil L2 wound on top of L1, with its other endconnected to earth. This behaves like a transformer- currents flowing in L2 generate a

changing magnetic flux which cuts L1 and induces an emf in it.

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Diode Detector
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Transmissions on Medium Wave frequencies are byAmplitude Modulation. The

amplitude of the transmitted radio signal orcarrieris modulated - made to rise and fall -

by another signal of audible frequency - for example, speech or music. The receiver mustextract ordetectthis audio signal by separating it from the carrier. One way of doinjg

this is by means of a diode detector.

The circuit below represents the detector. The key component is D1, type OA47, a pn

junction diode.

The signal is

introduced via

capacitor C7 (notethat this component

is not strictly

necessary for the

basic crystal

receiver, but isincluded for ease of

connection to lateradditions to the

design). It blocks

the flow of any DCcurrent into the

detector. Its value

is chosen so that ithas a low reactance

at the (medium

wave) frequenciesof interest, whichtherefore pass

without hindrance.

The pn junction

diode D1 allows theflow of

conventional current

from left to rightonly. Thus, the

waveform observedat the right of D1would comprise

positive-going parts

of the incomingsinusoidal signal;

negative going half-

cycles are


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effectively

removed. In this

way, informationcan be extracted

from the transmittedsignal, whoseamplitude is

modulatedby a

modulating signal

(typically an audiofrequency derived

from speech, music

etc.

Seethis presentation ( in 2005) for further information on how the detector

works.

The combination of R8 and C8 removes any remnants of the carrier wave, leaving only

the lower frequency, modulation signal. This can be applied to a sensitive earphone.

More realistically, an amplifier may be used to boost the signal to a level where it iscapable of driving a loudspeaker.

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Development of Improved Receiver

Audio Amplifier

The audio signal from the detector is at best of the order of a few mV. This is just

enough to be audible if applied to a sensitive earphone.

For comfortable listening with headphones an audio signal of about 1 volt amplitude isdesirable, so that an amplifier with a gain of about 100 times is needed.

This specification is comparable with the Design Example in Lecture 14. The circuit

below has been prepared for entry to the pSpice simulator so that its performance can be

predicted. It is almost identical to the one discussed in the lecture. The only differenceis that pSpice does not readily accommodate volume controls (implemented in the case

study using a variable potential divider, or potentiometer); instead, a fixed potential

divider (using resistors RV1 and RV2) is shown; the resistors are chosen so that 90% ofthe input signal VIN is applied to the amplifier. The values chosen for the coupling and

bypass capacitors are based on a determination of their reactance at the lowest frequency

of interest, typically 60 Hz.
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Function Value Reactance at 60 Hz Notes

Coupling capacitor 1 F 3000 Much smaller than RG1 and

RG2

Bypass capacitor 100 F 30 Much smaller than RS1 and

RS2

Note that even with such an amplifier, the performance will only be satisfactory with

strong signals from a nearby transmitter. This point is considered in the section below.

The (V) markers in the circuit above indicate the three points in the circuit at which the

output is plotted:

Input signal Output from Stage 1

Output from Stage 2


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The output plots the signals at the designated mark points as a function of time:

Input signal (V(RV1:2), green trace) is a sine wave at 600 Hz with amplitude 10

mV

Output from Stage 1 (V(J2:g), blue trace) has amplitude 100 mV approximately,and is inverted with respect to the input

Output from Stage 2 (V(C5:2), red trace) has amplitude about 1.2 V. It has the

same polarity as the input.

The overall gain is thus predicted to be about x120, about -11 for each of the stages. This

is enough to allow comfortable listening with earphones for reasonably strong signals.

pSpice can also be programmed to tabulate or plot circuit currents - for example, the

current flowing between Drain and Source in either JFET, or the total current being

supplied by the battery. These figures are useful in helping us get some idea of batterylifetime. A typical plot of battery current is shown below, and it can be seen that the

current consumption averages 1.5 mA.

In the practical implementation of the receiver, the JFETs used in the audio amplifier

turned out to have rather low mutual conductance, gm, of about 2.5 mS. Themanufacturer's data-sheet indicates that for a random sample, gm may lie between 2 and

6.5 mS - a very large spread! The simulator uses an average value of about 4 mS. Sincethe voltage gain is directly proportional to gm, the measured voltage gain was less thanpredicted, and was about -5 for each stage.

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Radio Frequency (RF) Amplifier


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For the design to work well with weak signals from more remote transmitting stations, or

with a smaller antenna, additional amplification is needed.

Since the detector works more efficiently with a larger signal applied, it makes sense toamplify the signal before it is applied to the detector. At this point, the signals being

amplified are at the carrier, or Radio Frequency, so this kind of amplifier is called an RFamplifier. The resultant block diagram is shown below.

The 2N3819 JFET will amplify over a wide range of signal frequencies, up to somehundreds of MHz, whereas in our design study the carrier frequency is approximately

1MHz (550 kHz to 1.6 MHz in fact for the Medium Wave band). As a result, a circuit

very similar to that used for the audio amplifier may be used for RF amplification. The

main difference is that smaller coupling and bypass capacitors can be used since thefrequency of operation is about 100 times as high as for the audio case; and capacitive

reactance is proportional to 1/f. The circuit used is shown below, together with the

predicted response (determined by pSpice) to a 600 kHz signal of 100 microvolts

amplitude.

As before, the (V) and (I) markers in the circuit above indicate points in the circuit at which the outpu

RF input signal from resonant LC circuit Output from Stage 1

Output from Stage 2

Current flowing into the negative terminal of the battery (i.e. net current consumption)


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With the same voltage scales in use, the input (amplitude 100 V, shown in green)appears almost flat in comparison with the two output signals. The gain achieved by each

stage is approximately 12.5, so the overall gain of the two-stage amplifier is about 150.

If we were to tabulate the performance over the full range of radio frequencies, we would

expect to find some variation. That is one disadvantage of this simple approach toreceiver design. So far as possible, modern design styles avoid dependence on frequency

by transforming the incoming carrier frequencies to a more convenient, fixed frequency

(called the intermediate frequency), in a process called mixingorheterodyning, to allowthe amplifiers to be better optimised for good performance.

As with the audio amplifier, the JFETs used in the receiver had lower gm than assumed by

pSpice, so the measured gain was rather lower than predicted.

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Battery Lifetime

The capacity of a typical 'budget' PP3 9-volt battery is about 400 mAh (milliampere-hours), which means it can (theoretically) supply a current of 1mA for 400 hours.

In the 'experiment' shown in Lecture 14-15, a 6-volt 0.5A cycle

lamp connected to a fresh PP3 burnt for about 40 minutes. This

suggests that the battery could provide approximately 40/60 x 0.5

Ah, or about 330 mAh.
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In practice, batteries can provide more mAh when discharged

slowly than if overloaded, as in this case.

Battery current is required by the RF- and audio amplifiers in the receiver design

described.

The consumption figures are (approximately):

RF Amplifier - 0.75 mA (first stage) and 0.75 mA (second stage) - see the plot

above

Audio Amplifier - 0.75 mA (first stage) and 0.75 mA (second stage)

On this basis, if used in our receiver circuit (based on two RF and two audio stages), thetotal consumption is about 3mA, and the battery lifetime would be approximately 130

hours. Not bad! But still the design is suitable for headphone reception only.

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Output Amplifier

A loudspeaker capable of being heard from a range of a few feet needs to be energised

with about 1 watt of electrical power - typically about 4 volts and a current of about 500mA (both peak figures). Although the JFET stage could possibly give sufficient voltage

gain to satisfy this need, it could never provide enough current to drive a 'speaker

directly.

For this, we need an additional amplifier with limited voltage gain, but high currentgain. JFETs are not the best choice for this purpose, and you will meet circuits much

better suited to these applications in Part IB. The design demonstrated in the lectures is

fitted with an output amplifier based on a proprietary chip, the LM386. This is in manyways like an operational amplifier (to be covered in lectures 17-18), but specially

designed to be able to deliver substantial current. This completes the development of our

modular radio - the final block diagram is shown below.


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With such an output amplifier fitted, the current consumption increases substantially. Infact, the current draw depends on the setting of the volume control, since this controls theaudio output, i.e., how much electrical energy is directed to the loudspeaker. With the

volume set high, a battery lifetime of only an hour or two must be expected. This is

consistent with observations of commercial radio designs.

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The Complete Receiver

Photo Gallery

Clickhere or on the thumbnail image to see a larger version of the picture with a key

identifying the functional blocks
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Rear view of receiver - click on thumbnail to see the original full-size picture and a key to

the functional sections (coming soon)

Two stage radio-frequencyamplifier

Diode detectorTwo stage audio frequency

amplifier

Output stages (op-amp and

LM386)Loudspeaker Volume control

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Circuit Diagrams

Audio amplifier (two stages)

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Volume control (potential divider)


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A Radio Frequency Amplifier (two stages)

The two RF stages are essentially identical and are very similar in structure to theaudio amplifier. The network comprising R7 and C5 is called a decouplingcircuit. It

prevents signals being coupled to other stages by way of the power supply leads.

With the highcapacitors (C

amplifier wit

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Output Audio Amplifier (based on operational amplifier)

MC33172 is a standard op-amp. The circuit is set to give a voltage

gain of -20, which is more than enough. The output produced by this

circuit is quite limited because the device can only provide a current ofabout 15 mA maximum at its output terminal, simply not enough to

drive a loudspeaker effectively.

The loudspeaker.

The large capacitor C17 is to

prevent a substantial DC currflowing through the speaker,

could cause long-term damag

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Audio power amplifier (based on LM386 chip) capable of driving a loudspeaker


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LM386 circuit Loudspeaker

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Notes and Acknowledgments

Virtually all the circuitry discussed above is abstracted from the lecture notes for the

course. This includes:

LC resonant circuit (Lecture 12)

RF amplifier (two stages, based on JFET - Lecture 14)

Audio amplifier (two stages, based on JFET - Lecture 14) Volume control (potentiometer - Lecture 14) Audio amplifier (based on an op-amp - Lecture 17).

The only items not strictly part of the IA curriculum are the use of the p-n junction diode

as a detector, and the power amplifier (used to drive a loudspeaker) based on the LM386IC. You will meet the latter in Part IB.

With thanks to CUED Electronic Development Group for assistance in fabricating the

design.

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David Holburn Last updated: on 18th July 2008
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A Simple Radio Receiver