A Simple Radio Receiver

< Return to Part IA Linear Circuits & Devices page ...

frontA.jpg (117992 bytes) The diagrams, images and screen shots below detail the development of a simple modular radio receiver based entirely on circuits and devices studied during the Part IA course on Linear Circuits and Devices.  

This receiver may not take the market by storm! However, we hope it will help illustrate how the circuits and principles we are studying all contribute to the art of electronic circuit design.

Contents

A little history

The 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, 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 "catís 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 catís whisker, and the weak signal could only be listened to by one 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.

See this presentation for further details of historic crystal set receivers.

Back to top

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/

Back to top

Development of Crystal Receiver

The design shown here is a little more complex than strictly necessary, but some of the adaptations incorporated make it easier to develop the design, adding amplifiers and other stages as we meet them in the course.

Resonant LC circuit

Circuit (a) alongside shows the LC resonant circuit comprising L1 and VC1 used to select or tune the required frequency and station.  The inductance used is 729 uH, and VC1 can be varied over a range 30 to 234 pF.  That means the range of resonant frequencies is about 400 to 1075 kHz, which includes 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 a given inductance, a smaller number of turns is required than if the coil had an air core.  That keeps its winding resistance low, and enhances the Q, which in turn means the resonant circuit has a narrower resonance peak, and is better at rejecting unwanted frequencies.

VC1 is a variable capacitor, in which the capacitance is varied by rotating a knob, which controls the extent to which a set of small parallel plates are enmeshed.  The capacitance can never be reduced to zero, of course, owing to the residual capacitances due to its structure.  To these figures must be added any stray or parasitic capacitances arising from the connecting wires, circuit board, and other elements.  These can never be eliminated, but careful construction aims to keep them down to a few tens of pF.

The signal from the antenna (perhaps a few tens of microvolts, or hundreds for a nearby transmitting 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 end connected 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.

Back to top

Diode Detector

Transmissions on Medium Wave frequencies are by Amplitude Modulation.  The amplitude of the transmitted radio signal or carrier is modulated - made to rise and fall - by another signal of audible frequency - for example, speech or music.  The receiver must extract or detect this 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 (note that this component is not strictly necessary for the basic crystal receiver, but is included for ease of connection to later additions to the design).  It blocks the flow of any DC current into the detector.  Its value is chosen so that it has a low reactance at the (medium wave) frequencies of interest, which therefore pass without hindrance. 

The pn junction diode D1 allows the flow of conventional current from left to right only.  Thus, the waveform observed at the right of D1 would comprise positive-going parts of the incoming sinusoidal signal; negative going half-cycles are effectively removed.  In this way, information can be extracted from the transmitted signal, whose amplitude is modulated by a modulating signal (typically an audio frequency derived from speech, music etc.

See this 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 is capable of driving a loudspeaker.

Back to top


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 is desirable, 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 difference is 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% of the 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.

Function

Value

Reactance at 60 Hz

Notes

Coupling capacitor 1mF

3000 W

Much smaller than RG1 and RG2
Bypass capacitor  100 mF 30 W 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:

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

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 battery lifetime.  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.  The manufacturer'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.  Since the voltage gain is directly proportional to gm, the measured voltage gain was less than predicted, and was about -5 for each stage.

Back to top

Radio Frequency (RF) Amplifier 

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 to amplify 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 RF amplifier.  The resultant block diagram is shown below.

The 2N3819 JFET will amplify over a wide range of signal frequencies, up to some hundreds 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 the frequency 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 output is plotted:

  • 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)

With the same voltage scales in use, the input (amplitude 100 mV, 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 to receiver 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 mixing or heterodyning, to allow the 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.

Back to top

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.  

PP3andBulb.jpg (27035 bytes) 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.  

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):

On this basis, if used in our receiver circuit (based on two RF and two audio stages), the total 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.

Back to top

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 500 mA (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 current gain.  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 many ways 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.

With such an output amplifier fitted, the current consumption increases substantially.  In fact, the current draw depends on the setting of the volume control, since this controls the audio 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.

Back to top


The Complete Receiver

Photo Gallery

frontB.jpg (141076 bytes)

Click here or on the thumbnail image to see a larger version of the picture with a key identifying the functional blocks

rearB.jpg (170889 bytes)

Rear view of receiver - click on thumbnail to see the original full-size picture and a key to the functional sections (coming soon)

RF.jpg (35330 bytes)

DET.jpg (14045 bytes)

AF.jpg (53094 bytes)

Two stage radio-frequency amplifier Diode detector Two stage audio frequency amplifier

OP.jpg (81951 bytes)

LS.jpg (107571 bytes)

VR.jpg (13779 bytes)

Output stages (op-amp and LM386) Loudspeaker Volume control

Back to top

Circuit Diagrams

Audio amplifier (two stages)

Back to top

Volume control (potential divider)

A Radio Frequency Amplifier (two stages)

The two RF stages are essentially identical and are very similar in structure to the audio amplifier.  The network comprising R7 and C5 is called a decoupling circuit.  It prevents signals being coupled to other stages by way of the power supply leads. With the higher operating frequency, smaller coupling and bypass capacitors (C1, C2, C3, C4, C6) may be used than for the audio amplifier without adverse effect on performance.

Back to top

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 of about 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 current flowing through the speaker, which could cause long-term damage.

Back to top

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

LM386 circuit Loudspeaker

Back to top

Notes and Acknowledgments

Virtually all the circuitry discussed above is abstracted from the lecture notes for the course.  This includes:

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 LM386 IC.  You will meet the latter in Part IB.

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

Back to top


David Holburn    Last updated: 18:35 on 18th July 2008