Compensating for non-linear effects in signal processing with active devices

Abstract

PCT No. PCT / GB93 / 01106 Sec. 371 Date Dec. 16, 1994 Sec. 102(e) Date Dec. 16, 1994 PCT Filed May 27, 1993 PCT Pub. No. WO93 / 26086 PCT Pub. Date Dec. 23, 1993In signal processing circuitry using active devices, non-linearity of the device causes distortion of pure tones and the generation of intermodulation products for more complex signals. The transfer characteristics of such active devices may be considered as the accumulated characteristics of input and output filter stages separated by a non-linear region. To compensate for non-linearities and reduce intermodulation products, a drive circuit comprises pre- and post-correction filters separated by a compensating amplifier. The frequency response of the pre-correction filter corresponds to that of the input filter stage but is reversed in the frequency spectrum; with conventional I.F. the transfer characteristic of the amplifier and frequency response characteristic of the post-correction filter are the respective complements, of those of the non-linear region and pre-correction filter. The circuitry may be extended to compensate for active devices having more than one non-linear region and has particular application to klystron amplifiers used in broadcast television transmitters.

Description

TECHNICAL FIELDThis invention relates to signal processing circuitry using an active device and in which inter-modulation products due to non-linearity of the active device are reduced. The invention will be described in the context of a klystron amplifier used in a broadcast television transmitter, to which it is particularly applicable, but the invention is not limited to such systems.BACKGROUND ARTA basic background description of such transmitters is to be found in a paper by C. J. Edgcombe and C. N. O'Loughlin entitled "The Television Performance of the Klystron Amplifier" published in The Radio and Electronic Engineer, September 1971. That paper describes in particular the 4-cavity klystron and the fact that it has a non-linear behaviour. As the input power is increased from zero towards the saturation value, with a single-frequency carrier wave input, the ratio of output power to input power decreases. A phase shift can also be introduced. This non-linearity is compensated by...

AI Technical Summary

Benefits of technology

Nowadays, most high power klystron TV transmitters use synchronising-pulse control to improve efficiency and the present correction method has particular application to this. Typically, sync-pulsing improves the efficiency (peak-of-sync) from 40% to 60%. During the sync periods, the beam current is increased, relative to that during the picture period when the beam is set to that value required for black level with a correctable linearity. This means that the non-linearity of the device is different between the picture and sync periods. The present correction method is preferably applied to the non-linearity of the picture periods: when the non-linearity is changed by switching-up for sync periods, the different non-linearity which is no longer corrected properly does not generally have a serious effect due to the absence of picture signal components forming intermodulation products and the output spectrum is limited to a narrow band of a few hundred kHz around the vision carrier. More serious effects will however be caused during common amplification where bearer tones are present in one image side-band during sync periods. To compensate for the imperfect correction (where the correction method is set up for the picture period) further compensation, by inverse amplitude and phase modulation, may be applied in the bearer drives for the synchronising periods.

Problems solved by technology

All active devices are non-linear to some extent.

Their non-linear effects tend to worsen with increases in output signal level and in conversion efficiency i.e., AC power out / DC power in.

In-band intermodulation products, due to the TV signal components, are rarely at a high enough level to be a subjective nuisance particularly as they are non-coherent in their nature.

If this is an output-modulated amplifier such inter-modulation effects restrict the power output and efficiency obtainable from such a device for a given specified performance for unwanted spurious emissions in the image side band.

This also restricts the efficiency to which a vision output power amplifier can be operated.

Thus, in summary, non-linearity restricts the power output and efficiency of an active device.

This is important because of running costs with high power RF amplification stages.

Non-linearity also prevents the effective utilisation of the lower sideband of TV channels due to the relatively high levels of regenerated image components.

The in-band intermodulation products severely restrict the power output and the efficiency of devices such as klystrons operating as common colour vision and sound amplifiers.

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