will be a cosine table. It will be appreciated, however, DIGITAL RADIO MODULATOR that a sine table can be used in an equivalent manner.
Not only is a single look-up table provided but also a BACKGROUND OF THE INVENTION "folding" technique is preferably adopted whereby it is
1. Field of the Invention 5 necessary to store only the cosines of one quadrant, for The invention relates to a digital radio modulator and example 0° to 90°. Thus, preferably there is provided a
in particular a phase modulator. register arrangement which takes the combination
2. Description of the Prior Art phase numbers and which increments or decrements, by Direct digital modulation techniques have the advan- a number corresponding to 90% a phase offset which is
tage of eliminating a number of analogue components ^ added to the combination phase numbers. The modulus
which are inherently less accurate than digital compo- of the combination phase numbers is preferably such
nents and more susceptible to changes in characteristics that 0°; 90°; 180° and 270° are represented in the two
because of temperature changes and ageing. most significant bits (MSBs) of the number applied to
Phase modulation can be effected by synthesising the look-up table by 00; 01; 10 and 11 respectively,
separate in-phase and quadrature (sine and cosine) com- 15 Preferably the MSBs are used to control negation of the
ponents of (a) intermediate frequency (IF) carrier sig- table input and output according to the quadrant in
nals and (b) modulation signals; mixing respective car- which the phase angle lies. A preferred feature of the
rier and modulation signals; and summing the mixer cosine look-up table is that its output is in a least signifi
outputs. This is an established analogue technique and cant bit (LSB) biased 2's complement format. In other
proposals have been made to use a precisely analogous 20 words, there is an implicit J LSB added to each value at
digital technique—see for example EP-A-0377180. the output. This reduces the capacity required of the
Such a system, however, requires high speed high look-up table in terms of bits per word and fully utilises capacity digital storage and processing with conse- the digital to anal0gue convenor, quently many integrated circuit chips. Chip count can modulation phase generator is programmed to be reduced by very large scale integrated circuit 25 provide modulation phase numbers having regard t0 the (VLSI) techniques. An example of Progress in this di- data ^ ^ m acCordance with a particular modurection ,s described m U| Pat No. 5,008 900 where a lation scheme ^ hase modulator scheme can be finite impulse response (FIR) chip is used m conjunc- k hase or Frequency shift Keying Uonwitftamgi^ mtermeaiatetrequency ^^Kmpto (FSK) and in particular Minimum Shift Keying (MSK) perform digital IF synthesis and modulaton. Again, 30 feKJ Gaussian uinimvm shift Keying however, the in-phase and quadrature components for -r • n t. j i j? the IF and modulating signals are derived and applied f Mf,K>- Typically such systems produce several d,fto a modulator in a manner analogous to the analogue Phased ^f1^ for ea^h data Reprocess. Thus, the requirement for digital memory ca- *>re' f a ^P1^ rate wluchu1S a multlPle of the pacity is hardly reduced and the VLSI chips are very 35 data symbol rate and the phase number rates correspond
expensive. The present invention provides an improve- to toe sampling rate.
ment The invention further provides a method of digital
radio phase demodulation using components of the
SUMMARY OF THE INVENTION above-described modulator.
According to one aspect of the invention there is 40 In using the modulator as a demodulator a digital provided a digital radio phase modulator comprising a siS^ is derived from the received radio signal by a digital carrier phase generator for generating a se- sampling analogue-to-digital convertor. The modulaquence of IF phase numbers which together synthesise tion phase generator is set to produce zero output and an intermediate carrier frequency signal; a modulator the intermediate frequency carrier signal from the carphase generator which receives digital modulator data 45 rier phase generator, before application to the look-up symbols and which generates corresponding modulator ^ie.is alternately switched, at the sampling frequency, phase numbers; an arithmetic-logic unit which com- between in-phase and quadrature components. The bines the IF phase numbers with the modulation phase cosine table consequently gives alternate sine and cosine numbers to produce a sequence of combination phase outputs and this is multiplied in a multiplier by the outnumbers; a look-up table which determines the sines 50 put from the A/D convertor. The output from the muland/or cosines of the combination phase numbers to tiplier is applied to an image rejection/selectivity digital give a trigonometric digital output and a digital to ana- filter and the output of the filter constitutes the received logue convertor for converting the trigonometric digi- data symbols.
tal output to analogue form. It is to be understood that The invention is applicable to a time division multiple
the trigonometric digital output may be the sines or the 55 access (TDMA) system. This requires pulsed operation
cosines or a combination of sines and cosines of the of the transmitter to give bursts of transmission corre
combination phase numbers. sponding to respective time slots. The intermittent oper
The arrangement of the invention takes advantage of ation of the transmitter causes spectral widening. In
the flexibility of digital manipulation to combine the order to alleviate this it is proposed to amplitude moducarrier and modulation signals before trignometric con- 60 late the output from the look-up table in the transmis
version. Thus, separate conversion of the IF and modu- sion mode. The amplitude modulation is such as to
lation signals is not required and significant savings in increase the amplitude of the signal progressively at the
components can be achieved. Also, because of the com- start of a burst and decrease it progressively at the end
bination of phase numbers before trignometric conver- of the burst.
sion it is possible to synthesise the modulated signal 65 The invention has application in mobile radio applica
from the cosine or sine conversion products alone. tion and particularly, for example, in radio telephones.
Thus, a single look-up table can be provided and in A difficulty in such systems is to maintain frequency
the embodiment of the invention to be described, this registration of the transmit and receive frequencies of