GB2262674A - Modulator circuit - Google Patents

Abstract

A broadband angle modulator comprises an arrangement W1...Wn for generating a series of weighted power in the modulation function whereby to provide a substantially linear response. The modulator, e.g. for a fibre optic transmission system, comprises a multistage arrangement of product mixers (M1...Mn) and summing nodes (S1...Sn). Each mixer is fed with the weighted modulation (x(t)) and, via a delay line with the phase shifted carrier (cos(wct)). Close approximation is achieved by increasing the number of modulator stages. The mixers M1-Mn may be of the switching type. The modulator which is a phase modulator is applicable to TV transmission systems usable in cable television. <IMAGE>

Description

MODULATOR CIRCUIT This invention relates to angle modulators and in particular to a broadband modulator e.g. for frequency modulation of a laser transmitter.

With the proposed introduction of multi-channel vestigial wideband transmission of television signals over an optical fibre path, e.g. for cable television systems, a need has arisen for a modulation technique that can meet the necessary linearity and bandwidth requirements. Typically a linearity of 0.02 % and a bandwidth of 500 Mllz is required. It has been found that conventional angle modulators are unable to reach the strict linearity requirement when driven at the high modulation indices (typically between 0.1 and 1) that are required for adequate system noise performance and to prevent intermodulation modulation.

A conventional phase modulator mixes a quadrature carrier with the modulation input and then adds the in-phase carrier to the resultant. The output phase angle e obtained by this technique is given by the equation tan e = x (t) As is well known, for small values of x (t) tan e approximates to e, and so for small x (t) e-"x (t) From the above discussion it will be appreciated that the conventional modulator is linear only for small modulation angles.

Furthermore, deviation from linearity is increased by frequency multiplication so that a conventional arrangement may require multiple stages of frequency multiplication and filtering. This requires a complex and costly arrangement that is prone to spurious responses.

Frequency multiplication also amplifies the carrier phase noise and therefore does not improve the real dynamic range of the modulated signal.

One approach to this problem is described in specification No. US-A-3,996,532. In this arrangement a carrier signal is split into in-phase and quadrature components. These components are multiplied respectively with even order and odd order components of the modulating signal and are then subtracted to give a difference output signal. This gives an improvement in output linearity but requires somewhat complex circuitry for processing the odd and even order modulation components.

A further approach is described in specification No.

US-A-4,028,641. This describes an arrangement for suppressing even order and third order non-linearity. However, this arrangement requires a pair of amplitude modulators that require careful matching to achieve the desired result.

Neither of the above techniques is particularly suited to broadband applications.

The object of the invention is to minimise or to overcome these disadvantages.

According to the invention there is provided a broadband angle modulator for modulating a carrier signal with a modulation function, the modulator including means for generating series of weighted power in the modulation function whereby to provide a substantially linear response.

According to the invention there is further provided a broadband angle modulator for modulating a carrier signal with a modulation function, the modulator including a multistage (n-stage) arrangement each stage of which comprises a product mixer having an output coupled to a summing node, weighting or attenuation means one coupled to a first input of each said product mixer, means for applying a carrier signal to a second input of the mixer of the nth stage, the second inputs of the remaining product mixers being coupled each to the summing node output of the adjacent stage, and a series arrangement of delay elements one for each said stage and each arranged to apply successive phase shift of ll/2 to the carrier, the output of each said delay element being coupled to the corresponding summing node, and wherein each said weighting means attenuates the modulation function fed to the corresponding mixer by a factor corresponding to the position of that mixer in the multistage arrangement whereby to provide a modulator output comprising a power series approximation to a linear response.

An embodiment of the invention will now be described with reference to the accompanying drawings in which: Fig. 1 is a partly schematic diagram of a multistage angle modulator circuit; Fig. 2 is a phasor diagram illustrating the operation of the circuit of Fig. 1; and Fig. 3 is a schematic diagram of an optical transmission system incorporating the modulator of Fig. 1.

Referring to Fig. 1, the modulator includes a delay line comprising a series of delay elements Tl....Tn to which, in use, a carrier signal (cos(wct)) is fed. Each delay element introduces a carrier # signal delay equivalent to It/2 radians at the carrier frequency of wc.

The successive stages of the delay line thus generate the functions +sin(wct),-cos(wct), - sin (wet), + cos (wet).... corresponding to each successive delay of =/2 radians.

The modulator further includes a series connected arrangement of product mixers M1 - Mn and summing nodes Sl...Sn, there being one mixer and one summing node for each delay element. It will be appreciated that the number n of stages of the modulator will depend on the required accuracy of the approximation to linearity.

Typically three stages will be sufficient for many applications, but if closer accuracy is required then further stages can be added. The modulation signal x(t) is fed to all the mixers in parallel. In addition, the undelayed carrier cos(wct) is fed to the highest order or nth mixer Mn of the series. Each of the mixers has an associated weighting network Wl....W whereby the output of each (nth) mixer is attenuated n1 by a factor of Ir corresponding to its position in the series arrangement.

The output of each mixer is fed to the adjacent summing node whereby the signal is added to the corresponding delayed signal from the delay line. It will thus be seen that the output of the mixer comprises a corresponding number of power of x(t) multiplexed by cos(wct) or sin (wet). For example, the output Vo of a three stage modulator is given by the expression V0 = - sin (wct) - x(t) cos (wct) + 1/2! x2(t) sin (wet + 1/3! x3(t) cos (wct) This expression can be rearranged by gathering terms in cos(wct) and sin (wct) and by ignoring the minus sign which simply represents a rotation of the phasor diagram to give VO = (x(t) - x3(t) cos(wct) + (l-x2(t)) sin(wct) a o This expansion approximates to give: : V ~ sin (x(t)) cos (wct) + cos (x(t)) sin (wct) 0 =sin (wet + x(t)) This is thus the required output to provide an approximation to the wanted function sin(wct + x(t)) and is illustrated on the phasor diagram of Fig. 2.

Further modulator stages may be added to provide higher order terms with their correct coefficients to give an even closer approximation to the tan function.

Because angle modulation is relatively insensitive to level, the modulator mixers may comprise switching mixers. The system components can then all be broadband to provide a large system bandwidth. If necessary, phase errors can be corrected by the use of trimming capacitors coupled to the delay elements.

To demonstrate the efficacy of the modulator, the performance of a three stage arrangement has been calculated. The output angle e was derived as a function of x(t) which, to the ninth order, is given by e = x(t) + O(x(t))2 + 0(2(t))3 + O(x(t))4 + l/30(x(t))5 + O(x(t))6 + 1/252 (x(t))7 + O(x(t))8 + 1/648 (x(t))9 Clearly, in this series expansion the first and fifth order terms are the most significant. Using this approximation the level of the highest intermodulation products was calculated for two sinusoidal input terms of equal amplitude but different frequency and having a combined peak levels B.The results are summarised below: B Inter modulation level 1.0 - 44 dBc 0.5 -69 dBc 0.2 - 100 dBc From the above it will be seen that the performance level with B = 0.5 is comparable to that achieved by a conventional narrowband modulator with B = 0.02.

We have also investigated the relationship between performance and errors in the delay elements. Typically we have found that a delay error of e.g. 20 gives intermodulation products of -80 dBc These results demonstrate the feasibility of the arrangement.

Referring now to Fig. 3, this illustrates an optical transmission system incorporating the modulator of Fig. 1. An input broadband modulation signal (x(t)) is fed, optionally via an integrator, to the phase modulator circuit to provide an output phase modulated carrier. This output is fed via a limiter to a laser diode to provide a corresponding optical signal to be transmitted via a fibre path to a remote station.

At the remote station the phase modulator carrier is recovered via a detector diode and is fed via a preamplifier and a limiter to a demodulator whereby to recover the broadband signal.

It will of course be appreciated that the modulator described above is not limited to this particular system application.

Claims (4)

CLAIMS:

1. A broadband angle modulator for modulating a carrier signal with a modulation function, the modulator including means for generating series of weighted power in the modulation function whereby to provide a substantially linear response.

2. A broadband angle modulator for modulating a carrier signal with a modulation function, the modulator including a multistage (n-stage) arrangement each stage of which comprises a product mixer having an output coupled to a summing node, weighting or attenuation means one coupled to aa first input of each said product mixer, means th for applying a carrier signal to a second input of the mixer of the n stage, the second inputs of the remaining product mixers being coupled to the summing node output of the adjacent stage, and a series arrangement of delay elements one for each said stage and each arranged to apply successive phase shift of to the carrier, the output of each said delay element being coupled to the corresponding summing node, and wherein each said weighting means attenuates the modulation function fed to the corresponding mixer by a factor corresponding to the position of that mixer in the multistage arrangement whereby to provide a modulator output comprising a power series approximation to a linear response.

3. A broadband angle modulator substantially as described herein with reference to and as shown in Fig. 1 of the accompanying drawings.

4. An optical transmission system incorporating an angle modulator as described in claims 1, 2 and 3.