US2426187A - Pulsed carrier frequency demodulator - Google Patents

Description

Patented Aug. 26, 1947 PULSE!) CARRIER FREQUENCY DEMODULATOR Charles William Earp, London W. C. 2, England, assignor to. Standard Telephones and Cables Limited, London, England, a British company Application November 23, 1942, Serial No. 466,655 In Great Britain December 19, 1941 Claims.

The present invention relates to electrical communication systems of the kind in which the unmodulated carrier wave is of a complex waveform repetitive at some predetermined constant period and in which the intelligence bearing modulation modifies the repetitive characteristic of the carrier Wave.

In the simplest form of electrical communication, a carrier wave, which comprises a singlefrequency only, is used to bear the essential modulation which represents the intelligence to be transmitted. In other systems, a carrier wave of a complex nature is used. For example, in modulated carrier wave telegraphy the car'- rier wave comprises a modulated single frequency, or, actually, three separate frequencies. In this case, the radiation of the three frequencies would not itself convey any intelligence, which can only be transmitted as an interruption or modulation of the complex carrier.

In time modulation or pulse modulation systems, the complex carrier wave is a train of pulses, a Fourier analysis of which would show the presence of a multitude of simple carrier waves spaced over a wide frequency range.

In any such systems, the complex carrier wave is a wave which repeats itself at definite fixedv periodicity, and may be converted into a singlefrequency carrier wave which itself carries the original modulation.

In these"complex systems, i. e., systems utilising carrier waves of complex wave form, information-bearing modulation becomes evident as a modification of the exact repetitive nature of the complex carrier. For example, in the case of modulated carrier wave telegraph signal transmission, the amplitude of the complex carrier is keyed according to the wave-form of the signals to be transmitted. In the case of the modulation of a pulse train, information-bearing modulation may take the form of a modification of the amplitudes, or time of occurrence, or even the durations of successive pulses. In general, the complex carrier wave of a. communication system may be amplitude-modulated,frequencymodulated, phase-modulated, or time-modulated, or carry any combination of these modulations. As a result, the individual simple carrier waves (derived by Fourier analysis of the complex carrier wave), all bear the modulation corresponding to the information carried by the system.

It i the object of this invention-to provide simple detecting arrangements whereby the intelligence may be obtained from the modulated repetitive complex carrier wave.

According to one feature of the present invention in a system for the demodulation of a carrier wave of complex wave form repetitive at some predetermined constant period of time, and modified by the intelligence bearing modulation, the modulated. complex carrier wave or a wave derived therefrom is divided along two paths having a difference in electrical length equal to the repetition period of the carrier wave or to an exact multiple of said period, and the signals from the two paths are combined inabalanced modulator or differential detector.

According to another feature of the invention in a. system for the demodulationof a carrier wave of complex wave form repetitive at some predetermined constant frequency and modified by the intelligence bearing modulation, the modulated wave is applied to a balanced mixing device ormodulator over two paths differing in electrical length by an amount equal to' the repetition period of said carrier wave or to an exact. multiple of said period, one or both of saidpaths including a further modulator for combining the modulated wave with a carrier wave of constant frequency.

The invention will be better understood from the following description taken in conjunction with the accompanying drawings;

In Fig, 1, the signal wave from the output of I, which may bean intermediate frequency amplifier, is divided. into two portions, both ofv which are transmitted to the differential detector D which has a well known construction, over two parallel paths, one having, a delay network DN to introduce a difference in the electrical lengths ofthepaths equal to the repetition periodv of the unmodulated wave or an. exact multiple thereof.

The signal wave in a branch A may be represented as- 61 sin (21r n1ft+01) +es sin (211- mftX 03) +etc. (where n is an integer) +Noise components of indeterminate phase and frequency, which can. be Written as 2N(sin: wi l-41 After subjecting this wave to a delay of l f. seconds, the resulting signalacmrents, which. are. rotated. in phase: by exact multiple of 2%, are

where cp is linear with w. As, however, no may be of any value, may be considered to lie anywhere between and Zr.

Beating together the original wave in branch A, and the delayed or comparison wave in branch 13 in the diiferential detector D, the direct current output is due, only, to the various frequency components in one channel beating with the components of identical frequency in the other channel.

The total D. C. output may be divided into three groups, according to derivation:

(1) Signal components beating with equivalent signal components.

(2) Noise components beating with equivalent noise components.

(3) Signal components beating with noise components of identical frequency.

Let us now consider group 1 components. Here the total output at zero frequency may be written down as +k(e1 +e2 +e3 assuming a square law detector, or

+7c[f(e1)+f(ez)+,f(e3)+ .l

for any detecto law.

In either case, all signal components conspire to give a positive output.

Let us now consider group 2 components. Here the total output at zero frequency may be written down as:

where N corresponds to those particular Values of N which are on frequencies identical to signal frequencies. The D. C, component of this series =212KeN cos (0-o) Here, as in group 2, the various components are not all of the same sign, because both 0 and 1 may be of any value. The summation of noise from this cause is therefore inefiicient, and may result in zero output.

In the system shown in Fig. 1, we have assumed that the received wave is composed of frequency components which are exact multiples of frequency This assumption is, however, not necessary, the fundamental requirement for the signal being that it is composed of a multiplicity of frequency components separated by 1 cycles, or multiples of 1 cycles.

In this system of Fig. 1, however, it is necessary that the signal components shall arrive at D in similar phase from the two paths. A small displacement in the mean frequency of the signal, may of course upset this condition, whereby the signals in A and B might arrive in phase quadrature, thereby giving zero output. In cases, therefore, where the signal may not be accurately defined in absolute frequency, for example, if the signal is derived from the intermediate frequency amplifier of a radio receiver (where frequency shifts occur according to the high frequency oscillator tuning) it is desirable to make some change.

Referring now to Fig. 2, in the system there shown this difficulty has been completely avoided.

In Fig. 2 the wave source I feeds two separate signal paths to the demodulator M2 as before, and a delay network DN is inserted in one path (either path is satisfactory) as before.

In one of these paths, however, the modulator M1 is inserted, in which the signal is modulated by an oscillator S at frequency F cycles. A filter F (which may also be the delay network) selects one of the side-bands of F produced by the signal.

The two inputs to the demodulator M2 are now similar to those of Fig. 1, except that in channel B all the components of the signal and noise have been raised (or lowered) in frequency by an amount F cycles.

The output is now selected at frequency filter Fl. This A. C. output cannot be cancelled by a slight detuning of the signal frequency, this detuning of the signal now only causing a phase rotation of the output with respect to the oscillator S. If, for example, the mean frequency of the signal is raised by cycles, the phase of the output rotates through 21r radians, owing to the relative phase shift of the two paths caused by the delay network.

A second modulator, similar to M1, may also be included in the upper path. In this case, the filter Fl will have a mean pass frequency equal to the sum or differenc frequency of the modulating oscillator S and the similar modulating oscillator of the upper path.

It should be pointed out that the noise suppression depends upon the use of a filter Fl at frequency F in the output circuit. Similarly, in Fig. 1, the D. 0. component must be selected by a low-pas filter LF.

In the specification of copending application Serial No. 457,786, filed September 9, 1942, the modulating systems described in connection with Figs. 1 and 2 have been applied to systems in which a carrier wave of single high frequency has imposed upon it a modulation of constant repetitive frequency and which modulation bears no intelligence and is particularly described in relation to obstacle detection. The present invention reside in the application of the same demodulating system to communication systems in which the unmodulated carrier wave is of complex form repetitive at a predetermined frequency and modulated by an intelligence bearing modulation as regards variation in amplitude frequency or phase of the carrier wave or as a time modulation of the repetition.

The following applications to intelligence hearing modulated systems will now be considered.

(1) Modulated carrier wave telegraph transmission.

(2) Tone telegraph transmission using phase or frequency modulation.

(3) Amplitude-modulated pulse transmission.

(4) Duration-modulated pulse transmission.

(5) Time-modulated pulse transmission.

(6) Modulation (of any form) on a phase or frequency modulated carrier wave.

(7) Combination modulations.

( 1) Modulated continuous telegraph transmission Modulated continuous wave telegraphy normally consists of the keying of a carrier wave which is heavily modulated at some constant frequency, usually of the order of 500 cycles/second. This modulation is most often sinusoidal, when the total complex carrier comprises three frequencies-a simple carrier plus two sidebands spaced 500 cycles/second on each side of it. The arrangements described may be used to demodulate any form of amplitude modulation, in which the modulating wave is repetitive at a constant frequency. Such a modulating wave may be at 500 cycles, for example, and may contain many harmonics of 500 cycles. For example, over-modulation may-be used. Such a transmission comprises a. succession of short pulses of uniform time spacing.

All transmissions of this type may be demodulated with advantage in the circuits shown in Figs. 1 and 2. The circuit of Fig. 1 converts the keyed signal to a keyed direct current, and that of Fig. 2 produces a keyed pure carrier wave. In both cases, the differential delay in the parallel signal paths to the final modulator should equal the repetition period of the original complex carrier.

There are several particular advantages to the system.

(a) A complete frequency-diversity system is provided. Under all conditions of selective-fading of the signal, all the individual components of the signal conspire to produce outputs which add together exactly in phase. The signal therefore, cannot disappear unless every component of the signal fades to zero.

(1)) In many cases, a very considerable increase in signal-to-noise ratio is obtained, as compared with the radiation of the same mean power by continuous wave. In a telegraph system it is only the noise present in the absence of signal which interferes with the transmission of the message. It has been shown hereinbefore that uniformly distributed noise tends to give zero output when no signal is'present.

(0) Despite the use of a Wider frequency spectrumthan for continuous wave transmission, an equally good or better signal-to-noise ratio is obtained in the receiver. Searching for signals by an operator will be thus much facilitated. Frequency stability of transmitter and receiver becomes less important.

(2) Tone telegraph system, using phase or frequency modulation In a tone telegraph system the spaces may be as usual, represented by a complete absence of any transmission, and the marking signal may be a simple carrier-wave, frequencyor phase-modulated at a constant low frequency.

In this case, as in the case of modulated continuous wave telegraph transmission the complex carrier wave may be analysed as a series of simple carriers, spaced accurately in frequency by multiples of the low frequency modulation. This representsa particularly practical Way for distributing the energy of transmission uni formly over a wide frequency band and provides freedom from the-effects of selective fading.

(3) Amplitude-modulated pulse transmission One methodof transmitting speech or other intelligence is to modulate the amplitude of a train of pulses. The number of pulses per second must,of course, exceed the maximum frequency con-tained in the speech wave to be transmitted. Such a transmission may be represented as a largenumber of simple-carrier waves, spaced by a frequencyequal to the number of pulses per second, each simple carrier bearing the speech or other modulation. When the signals ofsuch a transmission are applied to the 'demodulating arrangements shown in Figs. .1 and 2, the output for Fig. 1 is the speech wave and for Fig. 2 a simple carrier wave modulated by the speech wave.

The arrangements described are ideal for demodulating such a transmission, giving .the maximum possible signal-to-noise ratio, and considerable freedom from the effects of fading.

(4) -Dn1atz'0n-m0dulated pulse train Speech or other intelligence may be transmitted as thickness or duration modulation of the pulses of a train of pulses occurringat some frequency greater than the highest frequency component of the speech to be transmitted.

Once more, the system of Fig. 1 may be used to demodulate such a transmission, and the system of Fig. 2 may be used to convert the transmission to a simple amplitude-modulated carrier wave.

Such a receiver is completely efficient, utilising 7 every component of the signal, and offering .a

such pulses.

high signal-to-noise ratio. It is also a more simple and practical arrangement thanarrangemerits which have been hitherto proposed.

It should be noted that in this type of modulation system there are certain limitations as to depth of modulation, as demodulation is not linear with duration of the pulse. For small pulse durations, however, this effect is negligible.

(5) Time-modulated pulse'train In this case, the complex carrier wave isalso a succession of sharply defined pulses, equally spaced in time. Speech may now be transmitted by the modulation'of the time of occurrenceof (This would normally be achieved by phase or frequency modulation of an oscil-' lator, before deriving pulses from it, according to known systems.)

This modulation system is somewhat different from the modulation systems consideredhereinbefore in that a spectrum analysis would show that the simple carrier waves which make upthe complex carrier are phase or frequency modulated by the information-bearing modulation, and not amplitude-modulated.

If the transmission is now subjected to the conversion circuit of Fig. 2, the output is a single phase or frequency modulated carrier wave. The circuit of Fig. 1 should, in order to correspond, yield a phase or frequency-modulated direct current. This, of course, has no meaning according to recognised definitions of Phase and frequency-modulation, and the system will therefore be examined in greater detail.

Let the mean (i. e., unmodulated) period between successive pulses=T.

Let the constant pulse 1ength=t.

Let T be modulated between limits -T+AT and TAT Here, therequirement is made that AT,' the.peak value of modulation shall be small compared-with one high frequency period of the pulsed .carrier wave, and that it shall not exceed .onequarter of such periods. If the transmission is such that AT is greater, it is necessary to change its .frequency'bybeating down (by known methods) to a new mean high frequency, in which one :high frequency period-is long compared with AT.

Referring again to Fig. 1, it is arranged that (a) The differential delay of the split paths=T.

(b) The high frequency phases of the two pulse trains arriving at the differential detector differ by 90, when no modulation is present.

Under these conditions, the differential detector gives zero output for zero modulation.

Modulation now increases or decreases the time between successive pulses, and therefore increases or decreases the high frequency phase difference of 90 between the pulse trains arriving at the differential detector. The output from the detector is now a succession of positive and negative D. C. pulses, whose amplitudes depend upon the amplitude of the modulating wave. A suitable low-pass filter here selects the original speech or other modulating wave.

It should be observed that when the conversion circuit of Fig. 2 is used, the output is always a frequency or phase modulated wave whose carrier is at the frequency of the oscillator S. It is not necessary to arrange that the maximum depth of modulation does not exceed one quarter of a high-frequency cycle. When this network is used, it is of course possible to reproduce the original modulating wave by the well-known use of a. frequency discriminator.

Another method for demodulation of a timemodulated pulse train using the conversion circuit of Fig. 1, is the following (in this case it is not necessary to arrange that the depth of modulation does not exceed one quarter of a high frequency cycle):

It is arranged that for zero modulation, the pulses arriving at the diiferential detector overlap in time (and are, of course, one pulse period out of step). Further, it is arranged that these pulse trains shall be D. C. pulse trains, by rectification of the original pulses either before or after subjection to the delay network. Output from the differential detector occurs only during the overlap time bet-Ween the pulses. The total output is therefore a duration-modulated pulse train, which can be filtered to yield the original modulation.

It may be mentioned here that in constant amplitude systems such as system 4 and 5, the known technique of further noise discrimination by the use of voltage limiters may be utilised. Constant amplitude pulses may be conditioned by voltage limiters either at high-frequency or direct current.

Furthermore, the circuit of Fig. 1 may easily be arranged to discriminate against noise in the absence of signal, by slightly biassing the rectifier elements of the differential detector. Only pulses which appear at the correct time, and which have sufficient amplitude, can contribute to the output.

(6) Modulation (of any form) on a periodic phase or frequency modulated carrier wave A further type of transmission suitable for demodulation in the networks of Figs. 1 and 2, would be a complex carrier consisting of a frequency-modulated oscillator. (For transmission of speech, this frequency-modulation must be at super-audible frequency.) Modulation by speech, or any other intelligence to be transmitted, would be superimposed as an amplitudemodulation of the complex carrier.

Such a transmission is directly demodulated by ,the circuit of Fig. 1, giving the original modulating wave as output. The circuit of Fig. 2 gives,

s as output, a single amplitude-modulated simple carrier wave.

(7 C0mbination-modulations After considering the above cases of signals which bear a spurious modulation periodic in form plus an information-bearing modulation, it becomes evident that many more similar systems may be devised. A simple carrier wave may, for example, be simultaneously modulated in more than one manner, for example, frequency-modulated and pulse-modulated, by the repetitive modulation and may be further modulated in any way by the intelligence-bearing modulation.

Whatever the system chosen, the circuits of the Figs. 1 and 2 may be used to simplify the reception and to remove all the spurious or repetitive modulation.

What is claimed is:

1. System for the demodulation of a carrier wave of complex wave form repetitive at some predetermined constant period of time, the repetitive characteristic of said wave being modified by the intelligence bearing modulation, comprising a balanced modulator having two input circuits and an output circuit, means for applying said wave to one input circuit of said balanced mod-- ulator, means for applying said wave to the other input circuit of said balanced modulator with a delay equal to an integral number of periods of repetition. of the complex wave form and a utilization circuit connected to said output circuit.

2. System for the demodulation of a carrier wave of complex wave form repetitive at some predetermined constant period of time, the repetitive characteristic of said wave being modifled by the intelligence bearing modulation, comprising a modulator, means for impressing said wave on said modulator over two paths, frequency changing means including an oscillator in one of said paths, means for producing a difference in transmission delay over the two paths equal to an integral number of period of repetition of the complex wave form, and a utilization circuit connected to the output circuit of said modulator.

3. System according to claim 2 further comprising a filter connected between the output circuit of said modulator and said utilization circuit, said filter having a mean pass frequency equal to the frequency of said oscillator.

4. System for the demodulation of a carrier wave of complex wave form repetitive at some predetermined constant period of time, the repetitive characteristic of said wave being modified by the intelligence bearing modulation, comprising a source of received waves, a modulator, two electrical paths extending from said source to the input of the said modulator, a frequency changer in at least one of said paths, mean for producing a difference in transmission delay over said two paths equal to an integral number of periods of repetition of the complex wave form, and a utilization circuit connected to the output circuit of said modulator,

5. System for the demodulation of a carrier wave of complex wave form repetitive at some predetermined constant period of time, the repetitive characteristic of said wave being modified by the intelligence bearing modulation, comprising a source of received waves, a differential detector having two input circuits and an output circuit, two electrical paths extending respectively from said source to said input circuit, means for producing a difference in transmission 9 delay over said two paths equal to an integral number of periods of repetition of the complex wave form, and a utilization circuit connected to the output circuit of said differential detector.

CHARLES WILLIAM EARP.

REFERENCES CITED The following references are of record in the file of this patent:

Number 5 2,212,420 2,212,173 2,340,432

UNITED STATES PATENTS Name Date Harnett Aug. 20, 1940 Wheeler et a1. Aug. 20, 1940 Schock Feb. 1, 1944 Guanella Feb. 18, 1941

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