This paper provides formulas for evaluating RF spectral shape and interfering carrier distortion in connection with FM trunk radio systems having low modulation ratios Numerical results are given for 1800-speech-channel systems The curves showing interfering carrier distortion are of particular interest in connection with systems using RF multiplex (ie, systems consisting of a number of carriers each modulated with a signal built up from a large number of speech channels) Preferred carrier spacings, originally determined for 600-channel systems, have been recommended by the CCIR 9th Plenary Assembly, 1959, for use with up to 1800 channel systems It appears from the analysis that with these larger numbers of channels, a considerable amount of attenuation by filtering or by isolators will be required to give adequate protection against intermodulation distortion

RF Spectra and Interfering Carrier Distortion in FM Trunk Radio Systems With Low Modulation Ratios

This paper is concerned with intermodulation distortion in FM, FDM trunk radio systems due to a number of echoes of differing amplitudes and delay times. General formulas are developed, and applied to the case of a 960-channel system subjected to distortion due to echoes from flanges in waveguide runs. Examples considered are 100- and 200-ft runs of RG-50/U waveguide, in 10-ft sections, for use in the 6000-Mc band. The calculations are carried out with the simplifying assumption (for which there is some experimental evidence), that the individual flange reflections are equal and of the same sign. Distortion levels are computed both with equal section lengths and with section lengths made slightly unequal, so as to destroy coherent addition of the individual echoes. While the latter arrangement gives a considerable reduction, as compared with the equal length arrangement, in the peak input standing-wave ratio, the improvement in the distortion is so small as probably not to justify, in practice, any deliberate variation of the lengths.

Multiple Echo Distortion in Frequency-Modulation, Frequency-Division-Multiplex Trunk Radio Systems

http://ieeexplore.ieee.org/iel5/5/31057/01444742.pdf

A common error in FM distortion theory

With application to satellite communication, a concurrent investigation of the threshold extension to be expected in f.m. f.d.m. operation from the frequency feedback demodulator (f.m.f.b.) and the dynamic tracking filter (d.t.f.) has revealed a close similarity, both algebraically in the expressions governing a performance assessment and quantitatively in the predicted threshold extensions. This paper and a companion paper give an analysis of both types of low-threshold demodulator, show comparisons between theory and measurement for a system capacity of 240 channels and present theoretical curves showing the predicted threshold extension for a 600-channel system to be 4dB. The papers provide evidence that the COMSAT formula is unnecessarily pessimistic in its prediction of the threshold extension available for large-capacity f.m. f.d.m. satellite systems.

Frequency-feedback receiver as a low-threshold demodulator in f.m. f.d.m. satellite systems

There exists a considerable body of published work concerned with the theoretical evaluation of distortion arising from various sources in trunk radio systems. The analytical difficulties, however, are often formidable, and simplifying approximations have usually to be made. As a consequence, the theory has developed in a piecemeal fashion, different approaches having to be followed depending on the origin of the distortion. By contrast, the measurement method prescribed by international agreement is of universal application, though it is liable to suffer from the uncertainties inevitable when measurements are carried out in complex and often somewhat ill-defined situations.It is the purpose of the present paper to describe a method of computer simulation of the standard measurement technique, and to give some representative numerical results. The method is in principle capable of good accuracy, and greatly extends the range of distortion problems amenable to theoretical treatment. It is applicable to the evaluation of distortion arising from external interference sources, such as added thermal noise, as well as distortion due to echoes, filtering etc. The present approach is found to give good agreement with existing theory where this may be expected to apply.

Evaluation of distortion in f.m. trunk radio systems by a Monte Carlo method

Although comprehensive sets of curves are available giving the radio-frequency spectra of waves phase modulated with band-limited white noise (simulating f.d.m. telephony signals), the corresponding problem for frequency modulation turns out to be much more intractable. In the present paper the frequency-modulation problem is attacked using a method employed in an earlier paper, in conjunction with numerical devices for improving the convergence rate of very slowly converging series. The results are presented as a set of curves covering the range of modulation parameters likely to be encountered in trunk radio systems. This information is necessary for the evaluation of system bandwidth and interference due to unwanted carriers. To bring the analysis within reasonable bounds, the minimum modulating frequency has been taken as zero. It is known that unless the ratio between r.m.s. frequency deviation and maximum modulating frequency is large, the shape of the central portion of the spectrum changes substantially as the minimum modulating frequency departs from zero. However, for practical ranges of parameters there is evidence indicating that the shape of the spectrum tails (which is of particular importance in connection with distortion problems) is not greatly sensitive to variations in the minimum modulating frequency.

R.G. Medhurst

Papers

R.F. spectra of waves frequency modulated with white noise