Elektor Electronics 5/98

The history of licence-exemptuse of radio transmittersstarted a few decades ago withthe private use of low-powertransmitters for radio-con-trolled models. Much later, CBradio in the 27-MHz band waslegalised and type-approvedradios for this band werefreely available. However,the use of type-approvedradio-control transmitters aswell as that of CB rigs was tied toa licence fee and some administration.And then, the mere disappearance ofthe dreaded paperwork and, possibly,a test, to get your licence was hailed asa great liberalisation. The result of thisde-regulation on part of the govern-ment authorities was a boom in thesales of CB radios. A few years later, theovercrowded CB band was extendedwith quite a few channels, highertransmit power was allowed, as well asFM (frequency modulation).

Although small sections of the40 MHz and 173 MHz bands (the latterexclusively in the UK) have long beenavailable for low-power devices like‘stage microphones’ and remote con-trols (for example, for garage doors),nothing was allowed without paying alicence-fee and proof that the equip-ment was type-approved.

Many of the regulations, but not thetype-approvals, have been relaxed orchanged radically over the past fewyears. Initially, licence-exempt remotecontrol systems appeared for the CB27 MHz (11-m) band. The real activitydid not start however until two smallsections of the 70-cm band, 418 MHzand 433 MHz, were ‘released’ for low-power type-approved devices (LPDs),also called short-range devices (SRDs).

In Europe, the national radio regula-tion authorities (in the UK : Radiocom-munications Agency, RA) have theirown say about the use of the SRDbands. In the UK, the 418 MHz sectionmay be used for telemetry transmittersand receivers, telecommand and in-building security equipment, while the433-MHz section is only available forin-vehicle equipment including radiokeys. This is in contrast with manyother European countries, where thesection around 433 MHz is much widerand also available for all of the afore-mentioned applications, and evenvoice communications using 10-mWFM handhelds.

In the UK, the specification withnumber MPT1340 is applicable to allLPDs using the 418 MHz and433 MHz sections of the 70-cm band.The Radiocommunications Agency(RA) is an Executive Agency of the DTI(Department of Trade and Industry)responsible for the allocation, mainte-nance and supervision of the UK radiospectrum. The RA can be contacted atthe following address: Radiocommu-nications Agency, New King’s BeamHouse, 22 Upper Ground, LondonSE1 9SA. Tel. (0171) 211 0211, fax 211

In the UK, two smallsections of the 70-cm

band, around418 MHz and

433 MHz, have beenavailable for some

time for licence-exempt wireless com-

munication usingtype-approved low-

power devices(LPDs), also called

short-range devices(SRDs). In this articlewe look at the use of

these frequencybands, and some

general design con-siderations for SRD

modules.

34

418/433 MHzshort-range communicationlicence-exempt data transmission

and remote control systems

0507. Internet: www.open.gov.uk/ radiocom.

Document number I-ETS 300 220describes the type-approval require-ments for 418/433-MHz SRDs. Accord-ing to the RA, new equipment can onlybe type approved to this standard pro-vided parameter limits stated inMPT 1340 are met.

E X - I S M F R E Q U E N C I E SA long time ago, the current SRD bandsection at 433 MHz was part of aslightly larger section reserved for ISM(industrial, scientific and medical)equipment producing RF radiation.Mainly as a result of pressure fromlicensed radio amateurs who use thispart of the band on a shared and/orsecondary basis, the use of ISM equip-ment has been phased out, and theband section is no longer identified assuch, at least not in the UK. Severalother ISM frequency bands are definedin the UK, including 167 MHz, 83 MHzand 40 MHz, all subject to strict regu-lations, the most essential of whichbeing very low ERP (effective radiatedpower) levels.

The exact frequency allocation ofthe 418 MHz and 433 MHz SRD bandsis shown in Figures 1a and 1b. Itshould be noted that the channel divi-sion and channel widths have beenadopted by SRD manufacturers, therebeing no strict RA regulation in thisrespect.

It is expected from radio amateursusing the 70-cm band to accept theactivity of low-power SRD users in thispart of the band and not cause inter-ference. Likewise, SRD users have tolive with interference caused by radioamateurs, or prevent interference byusing low transmission rates, surecodes, high redundancy and selectivereceivers. All of this is, of course, in thehands of the manufacturers of SRDs,because the users are not allowed tomake changes to type-approved equip-ment.

A N E W S R D B A N DMeanwhile, because they are so small,the 418 MHz and 433 MHz SRD bandshave become quite overcrowded. Anew band, around 886 MHz, is ‘identi-fied’ by the relevant authorities for useby SRDs (Figure 2), with reference toCEPT Recommendation T/R 70-03. Inthis band, it is planned to reserve sev-eral channels exclusively for securityapplications. Some channels in the pro-posed frequency range are, however,still in use for analogue cordless tele-phone sets of the CT2 generation.

For all SRD bands, the intention hasalways been to arrive at unified regu-lations. In the UK, however, the Radio-communications Agency “has notadopted CEPT recommendationTR 01-4 which allows general low-

35Elektor Electronics 5/98

1 2 3 4 13

14

15

16

433.

720

MH

z

434.

120

MH

z

LPD/SRD Band

Range:Width:Channels:Raster:

433.72 MHz to 434.12 MHz400 kHz1625 kHz

Specification: MPT 1340For Vehicle Radio Keys only.Licence-exempt, max. 10 mW ERP

980038 - 11a

1 2 3 4 5 6 7 843

3.72

0 M

Hz

434.

120

MH

z

LPD/SRD Band

Range:Width:Channels:Raster:

417.9 MHz to 418.1 MHz200 kHz825 kHz

Specification: MPT 1340Low-power general telemetry, telecommand and alarms.Licence-exempt, max. 0.25 mW ERP

980038 - 11b

1 2 4 53 77

78

79

80

868.

000

MH

z

870.

000

MH

z

Future SRD band

Range:Width:Channels:Raster:

868 MHz to 870 MHz2 MHz8025 kHz

CEPT Recommendation T/R 70-03 refers.

980038 - 12

1

Figure 1. Frequencyallocation and (man-ufacturer-proposed)channel division ofthe two 70-cm SRDbands available inthe UK.

Figure 2. Plans areafoot to open the886-MHz band forSRD use.

2

Visit our Web site at http://ourworld.compuserve.com/homepages/elektor_uk

power devices tooperate in thisband”. Hopefully,the 886 MHz SRDband will be gracedby cross-Europeanstandards, andreceive an ETS(European Telecom-munication Stan-dard).

L P DM O D U L E SIn this country,RadioMetrix andRadioTech are themain suppliers ofready-made, type-

approved receivers and transmittersfor short-range communications in the418/433 MHz bands. In this context, weshould also mention the activities ofthe LPRA, the Low-Power Radio Asso-ciation, who publish an interesting andhighly topical newsletter, as well asmaintain a fine Internet web site atwww.lpra.org.uk

The ready-made, UK type-approved LPD modules fromRadioMetrix and RadioTech come in avariety of frequencies and transmitpowers, depending on your applica-tion and country of use. Modules arealso available for digital communica-tions between, say, a PC and a printer,the radio link effectively acting as avery long RS232 cable. All LPD mod-

ules we have seen so far contain SMDparts to keep the overall size as smallas possible.

The simplest versions of SRDs usedto rely on an amplitude-modulatedtransmitter (Figure 3) and an associ-ated regenerative receiver (Figure 4).Note that such systems are probablyno longer allowed under RA specifica-tion MPT 1340. The transmitter consistsof a one-transistor oscillator. Modula-tion is obtained by applying the datasignal to the base of the transistor. Asingle surface-acoustic wave (SAW)resonator is used as the frequency-determining element. A highly similarcircuit for experimental use was pub-lished in Elektor Electronics July/August1993, page 54. Note, however, that thisdesign is based on frequency modula-tion (FM) using two varicap diodes,while the SAW has a fine-tuningadjustment.

The receiver shown in Figure 4 alsocontains just one transistor. It is biasedto act as a regenerative oscillator, inwhich the received antenna signalcauses the transistor to switch to highamplification, thereby automaticallyarranging the signal detection. Next,the ‘raw’ demodulated signal is ampli-fied and shaped-up by opamps. Theresult is a fairly clean digital signal atthe output of the receiver. The logic-high level is at about 2/3 of the supplyvoltage, i.e., between 3 V and 4.5 V.

The range of the simple systemshown in Figures 3 and 4 is muchsmaller than that of more expensiveunits, mainly because of the low trans-mit power (approx. 1 mW) and the rel-

36 Elektor Electronics 5/98

T1

R3

10

0Ω

R4

20

0Ω

R2

6k

8

R1

4k7

C3

3p

C4

7p

C1

470pC5

470p

L2

68nH

X1

433,92 MHz

DATA IN

ANT.

980038 - 13

L1

27nH

3

Figure 3. Circuit dia-gram of an ultra-sim-ple 433-MHz SRDtransmitter usingamplitude modulation(not type-approved byRA).

IC1bIC1a

R15

47k

R10

100k

R11

100k

R18

10Ω

R7

12

k

R8

68

0Ω

R6

18

k

R5

20

k

R9

5k

6

R14

3M

C5

33p

C6

1n

C8

1n

C7

2p

T1D2

D1

1N

C4

2p

L1

2µH2

C9

1µ

C21

47µ

R12

6M8

D3

1N4148

C10

4µ7

4148

2x

ANT.

DATA OUT

*

C12

10µ

980038 - 14

VCC

*1,5 Wdg.

*1,5 wdgn.

*1.5 turns

*1,5 spires.

4

Figure 4. Circuit of asimple AM receivermodule using a singletransistor in the(regenerative) RF sec-tion (not type-approved by RA). Insome cases, there’san additional pream-plifier stage using asecond transistor.

ative insensitivity and wide-bandnature of the receiver. Moreover, ampli-tude-modulated noise is not sup-pressed in any way.

For more demanding applications,FM (frequency modulation) is the obvi-ous alternative. Block diagrams of anLPD-type 433-MHz FM transmitterand its associated receiver are shownin Figures 5 and 6 respectively.

The transmitter is automaticallyactuated by means of pulse edge detec-tion, and uses an accurately defined 4-ms time slot to transmit, as soon asdata pulses are detected at the input.When the data signal is removed, thetransmitter automatically returns tostandby mode afterabout 200 ms. As in thelower-spec transmitter,the frequency stability isderived from a SAW res-onator. The main advan-tage of these resonatorsis their low cost. On thedown side, they are subject to rela-tively large production tolerances, andtheir temperature stability is a far cryfrom that of a quartz crystal.

To keep the bandwidth of the fre-quency modulated transmitter outputsignal within limits, the frequency devi-ation is limited (±2.5 kHz to ±20 kHz,depending on the SAW type and man-ufacturer). The input data rate is alsolimited (low-pass filter). The upshot isthat the highest data rate of the FMmodules is about 10 kBit/s (using a high-est modulation frequency of 5 kHz).

The antennas used for SRDs are tra-ditionally produced in the form of aprinted-circuit board track, while ⁄-lambda flexible anten-nas are also seen occa-sionally.

The FM receivermodule shown in theblock diagram (Figure 6)is a superheterodynedesign. Here, too, anSAW resonator is used in the oscillatorto ensure frequency stability. All of theintermediate-frequency (IF) filteringcan be done with low-cost 10.7-MHzceramic filters. Because of the possiblefrequency offset caused by the SAWresonator, a fairly large bandwidth(approx. 280 kHz) is required anyway.Most ready-made SRD receiver mod-ules are compatible with 5-V or 3-V sys-tems.

Thanks to miniaturisation, SRD

modules with evenhigher specifications

are not necessarily larger, but dearerand more complex.

Higher-spec transmitters achievebetter frequency stability thanks to theuse of a crystal-controlled synthesiser,while the harmonics suppression isalso better as a result of extensive fil-tering at the output.

Likewise, high-end SRD receivermodules are usually double-conversiontypes using synthesiser tuning and nar-row-band IF filters. The technology

used to manufacture these modules isthe same as found in handhelds for the70-cm amateur radio band.

D A T A T R A N S M I S S I O NFor simple data transmission applica-tions, such as a remote control link,you need a suitable encoder at thetransmitter side, and a matchingdecoder at the receiver side. Speciallydesigned integrated circuits are avail-able like the MM57410N fromNational Semiconductor, theMC145026/MC145028 from Motorola,

37Elektor Electronics 5/98

data inputswitching threshold

hysteresisFM

modulatorRF output

stage

pulse edgerecognition

Integratedantenna

supply regulationRF carrier enable

IN Ub = 5 - 12V980038 - 15

Figure 5. Block dia-gram of a typical FMtransmitter module forone of the 70-cm SRDbands.

5

selectionRF input stage

oscillator

AF amplifierpulse shaper

IF amplifierFM demodulator

mixer

Ub = 5V OUT 980038 - 16

Figure 6. FM receivermodules may be super-heterodynes or evendouble-conversiondesigns.

6

Visit our Web site at http://ourworld.compuserve.com/homepages/elektor_uk

or the HE8 andHT12 from Hei-land Electronic(D-48351 Ever-swinkel, Germany.Tel. +49 2582-7550,

fax +49 2582-7887).An example of an addressable sig-

nal transmission link for the 433 MHzSRD band is shown in Figure 7 (note:this application may not be allowed inthe UK). The encoder IC type HT12Esupplies its data directly to the modu-lation input of the transmitter. Simi-

larly, the HT12D decoder IC is founddirectly at the output of the receiver.On the DIP switches in the encoderyou set the same address as in thereceiver to be addressed (multiplereceivers may be addressed by a singletransmitter).

In addition to the receiver address,four data bits may be applied to theinput. Here, these four bits come frompush-button switches. By applying thetransmit-enable signal (/TE), theencoder is prompted to supply a 12-bitserial word (consisting of 8 address

bits, and 4 data bits) to the transmitter.The decoder receives the 12-bit wordand extracts the first 8 bits as anaddress, and the remaining 4 bits asdata. The four data bits only appear atthe output if the received addressmatches the DIP switch setting in thedecoder. The 4-bit dataword is firstlatched and then used to control exter-nal devices (here, LEDs are used).

To make sure the transmissionarrives securely at the decoder, theencoder transmits the 12-bit serialword four times each time the /TEinput is activated. The decoder with-holds the data until three identical, suc-cessive, copies have been received. TheVT output then flags the availability ofvalid data.

This process is very well suited toslow data transmission. For higher datarates, a microcontroller is an obviousalternative to special encoder/decoderICs. Note, however, that opting for amicrocontroller (like a PIC) almostalways means that you have to writeyour own software aimed at achievingsecure and reliable transmissions.

If data is to be exchanged betweenequipment having a serial interface,the first solution that comes to mind isoften one as adopted in the project‘Long-distance IrDA link’ published inElektor Electronics May 1997.

For more demanding telemetryapplications, special data modems areemployed in combination with high-end 418-MHz SRD modules. A trans-mission protocol is then used toimprove the data security. Often, theAX.25 protocol is employed, a spin-offof the X.25 protocol which has been inuse for several years for amateurpacket radio.

M O D U L A T I O NT E C H N I Q U E —A B O T T L E N E C KWhile most data transmission modulesapproved for SRD use are usually saidto use ‘FM’, in practice the actual mod-ulation method is FSK (frequency shiftkeying). Though simple from a designand technology point of view, FSK isburdened by a large bandwidthrequirement which is the chief cause ofthe relatively short distances that can becovered. Assuming a receiver band-width of 25 kHz (at –36 dBm) is beingused for data transmission, then thehighest achievable data rate using FSKwould be a measly 500 bits per second!Consequently, professional applicationsof SRD modules call for special modu-lation techniques like GMSK (Gaussian

Minimum Shift Keying) whichreduce the bandwidth require-ment by a factor of 15 and more,while considerably improving thetransmission security.

(980038-1)

SW-DIP8

HT12E

OSC2

OSC1

DOUT

18

A0

A1

A2

A3

A4

A5

A6

A7 10D0

11D1

12D2

13D3

14TE

15

16

171

2

3

6

7

8

4

5

9

9

10

11

12

13

14

15

16

1

2

3

6

7

8

4

5

1M

HT433-1/T

OUTIN

5V

START

DATA

AD

DR

ES

S

SW-DIP8

HT12D

OSC2

OSC1

VT

18

A0

A1

A2

A3

A4

A5

A6

A7 10D0

11D1

12D2

13D3

14DIN

15

16

171

2

3

6

7

8

4

5

9

9

10

11

12

13

14

15

16

1

2

3

6

7

8

4

5

47k

5V

DATA

AD

DR

ES

S

56

0Ω

56

0Ω

56

0Ω

56

0Ω

HT433-1/R

OUT IN

MP

980038 - 17

7

Figure 7. Applicationcircuit for an address-able digital link usingan SRD and frequencymodulation (FM).

8

Figure 8. A professionaldata modem designedfor use with a 70-cmSRD (not type-approvedby RA).

38 Elektor Electronics 5/98