GB2219157A - Frequency generators - Google Patents

Description

! ' 1 )' 9157 Dual standard frequency translation generator, in particular

for television grid networks The invention relates to the generation of a reference frequency having the very high precision required for driving a television transmitter or re-transmitter, in patticular in a grid network, or else usable in cable or satellite transmissions in the VHF, UHF and microwave frequency ranges.

Coverage of a large territory by means of a large number of television chains involves the use of a grid network formed of transmitters and re-transmitters using, for each chain, frequencies which are offset with respect to the frequency assigned to the corresponding channel by relatively small values, called "frame offsets".

This known method avoids mutual interference between transmitters belonging to different chains and which operate on the same channel. It involves the generation of a reference frequency, from which the transmitter frequencies will be synthesized, having a relative accuracy of about 10-9 in each station. Now, although the large stations may have very precise reference frequencies generated for example by a rubidium oscillator, the cost of such an oscillator is prohibitive for- the small retransmitters which only cover a village or a valley.

Such a reference frequency can be generated from the - 2 signal received at the station. The difficulty lies in the fact that this signal is very much attenuated and further modulated by the video signal and by the synchronization signals, and the latter have cut-offs which make the operation of the control loops required for obtaining a very precise frequency haphazard.

A first object of the invention is to overcome this difficulty and the second is to make possible the use of the same reference frequency generator circuit, either for the French standard, or for the European and American standards, for which the respective intermediate frequencies have different frequencies and reverse modulation spectra.

A first feature of the dual standard generator of the invention consists in its comprising successively a fixed narrow band filter, advantageously a quartz filter, adapted for eliminating the video signal from the carrier, a first phase locked loop which finishes eliminating the modulation and synchronization components and a second phase locked loop acting after a frequency division and introducing the time constant required for controlling the pilot oscillator which will produce the reference frequency.

According to a second feature of the invention, said generator comprises a frequency changer providing beating, with or without spectrum reversal, depending on the standard used, between the intermediate frequency to be processed and a fixed auxiliary frequency equal to the arithmetic mean of the intermediate frequencies corresponding to the two above standards.

Other features, as well as the advantages of the invention will be clear from the following description.

In the accompanying drawings:

Figure 1 is a general diagram of a television station in a grid network, which uses the generator of the invention, the preferred embodiment of which is 11 1 - 3 illustrated in figure 2.

In figure 1, a television re-transmitter station has been shown schematically, comprising a receiver 1 followed by a demodulator 2 and a re-transmitter 3.

The composite intermediate frequency Fi at the output of the receiver, formed by the complete television signal, is applied to a processing circuit 4 which delivers a frequency of 10 MHz having a relative precision of the same order as that of the transmitter. This frequency is used as reference frequency in a synthesizer 5 which generates the local oscillator frequency of receiver 1, i.e. the difference between the antenna frequency and Fi: it will be noted that this frequency is only available when the loop has come into service: at the beginning of operation, the local oscillator of the receiver is set to a close but not controlled frequency.

The 10 MHz reference frequency may further be applied directly as reference frequency to a synthesizer 6 which generates, with the suitable shift with respect to Fi, the local oscillator frequency of the transmitter 3, which makes it possible to provide, with respect to the frequency from demodulator 2, the frequency translation required for re-transmission.

However, it will be advantageous to add to synthesizer 6 a second processing circuit 7 identical to circuit 4 and receiving Fi, which willproduce a reference frequency which may be compared with that generated by circuit 4, so as to ensure correct operation of the circuits by ascertaining the equality of the two reference frequencies.

In f igure 2, the detail of circuit 4 or 7 has been shown schematically.

Frequency Fi is applied to a band pass f ilter 8 which transmits both the 32.7 MHz frequency corresponding to the French standard and the 38.9 MHz frequency corresponding to the European and American standards, to a frequency - 4 changer 9. The latter further receives a frequency of 35.8 MHz (arithmetic mean between the two above frequencies) generated by a synthesizer formed of a first frequency divider 10, a phase-frequency comparator 11 whose output is connected to the control input of an oscillator 12 through a band pass filter 13 and a divider 14 dividing the frequency by 2 connecting the output of oscillator 12 to the second input of comparator 11. The output of oscillator 12 is further connected to an input of the frequency changer 9.

The input of divider 10 is connected to the output of the output pilot oscillator 18 of the circuit through a shaping circuit 19.

The output frequency of 3.1 MHz (i.e. 35.8 - 32.7 for the French standard, 38.9 - 35.8 for the European standard) is applied to an input of a frequency changer 21 through a filter 22 having a sufficiently narrow band to eliminate the video spectra of signal Fi. It will advantageously be a quartz filter with for example a pass band of 6 kHz.

The frequency changer 21 further receives a frequency of 3.2 MHz advantageously obtained from the output frequency of oscillator 18 by means of a synthesizer of the same type as that which generates the 35.8 MHz frequency and formed of elements 10a to 14a.

The 100 kHz frequency from frequency changer 21 is applied through a bandpass filter 23 to a phase locked loop 24 with a sufficiently large time constant to eliminate the modulation and synchronization components from signal Fi which were not eliminated by the filter 22. This loop comprises an oscillator 16 controlled by the output signal, fIltered at 20, of a phase-frequency comparator 17 which receives both the output frequency from oscillator 16 and the-100 kHz frequency at the output of filter 23. It is equivalent to an extremely narrow band filter.

1 I.

1 Z The 100 kHz frequency thus restored at the output of oscillator 16, af ter division by 100 in divider 25, is compared in a phase-frequency comparator 26 with a 1 kHz frequency obtained by dividing the output frequency of oscillator 18 by 1000 by means of a divider 27. The output signal of comparator 26 is applied after filtering at 29 to the frequency control input of the pilot oscillator 18.

The latter is a quartz oscillator having a frequency stability of the order of 10-6. Because of the phase locked loop which controls it (second loop 27-26-29), the precision of the 10 MHz frequency collected at the output of amplifier 30 will be of the order of that of the signal received, namely for example 10-9 to 10- 11.

At 31-32-33 an accessory circuit has been shown intended to receive a 10 MHz frequency from a rubidium (or equivalent) oscillator when such an oscillator exists in the station. This frequency is shaped at 31, divided by 100 and 32 and again filtered at 33. The 100 kHz frequency collected at the output of filter 33 is applied directly to an input of the first loop 24, where it is substituted for the frequency from filter 23.

It will be noted that the first loop 24 comprises an output 240 connected to an input 260 of the comparator 26. This latter is adapted so that the disappearance of the output frequency of the f irst loop 24 (in the case of a temporary cut-off of Fi) results in maintaining the frequency of the second loop at the value which it had before the cut-of f. This precaution prevents the second loop from diverging, in the case where the elimination of the synchronization signals by filter 8 and the first loop 24 was not perfect, or in the case of transmission cutoff.

It should be understood that the synthesis of the local oscillation frequencies of the receiver and of the transmitter of the station by the respective synthesizers 5 and 6 is provided conventionally, taking into account 1 - 6 the frame offsets, namely d and d' respectively.

By way of example, for a reception frequency of 623.25 MHz +/- d corresponding to channel 40 in France, the synthesizer 5 will have to generate a frequency of 655.95 MHz +/- d', whereas for a transmission frequency of 710-55 MHz corresponding to channel 55, the synthesizer 6 will have to generate a frequency of 743.25 MHz +/- d'. It is these frequencies which are generated from the reference frequency of 10 MHz delivered by the circuit of figure 2.

It will be further noted that the second frequency change (at 21) uses a fixed frequency of 3.2 MHz, higher than the frequency of the narrow band filter 22 (namely 3.1 MHz), so that it is a subtractive beat which is filtered at 23.

For the European standard, we have thus successively the filtering at 22 of a beat which keeps the direction of the spectrum and a beat which reverses the sign of the spectrum, whereas for the French standard, we have successively filtering, at 22 then at 23, of two beats which reverse the sign of the spectrum.

The result is that from f ilter 23 the two loops may operate whatever the standard.

Claims (9)

1. Generator for generating a reference frequency usable for synthesizing the local oscillator frequencies of a transmission or re-transmission station, parti cularly in a television grid. network, said generator starting from an intermediate extracted frequency from the signal received by the station, characterized in that it comprises successively a fixed narrow band filter (22), advantageously a quartz filter, adapted for eliminating the video signal from said intermediate frequency, a first phase locked loop (24) which finishes eliminating the modulation and synchronization components from the intermediate signal (Fi) and a second phase locked loop (27-26-29) acting after a frequency division (at 25) and introducing the time constant required for controlling the pilot oscillator (18) which will produce the reference frequency.

2. Generator according to claim 1, characterized in that it comprises successively a first frequency changer (9) which transposes the intermediate frequency (Fi) before its passage through said filter (22), by means of a first auxiliary frequency and a second frequency changer (21) which transposes the frequency from said filter (22) by means of a second auxiliary frequency higher than the frequency from said filter (22).

3. Generator according to claim 2, characterized in that said first and second auxiliary frequencies are obtained by frequency synthesis from the output frequency of the pilot oscillator (18).

4. Generator according to claim 1, 2 or 3, characterized in that the second phase locked loop comprises a phase-frequency comparator (26) which receives an inhibition signal from the first loop (24) in case of temporary cut- off of the output signal of the first loop, - 8 adapted for then maintaining the output frequency of the second loop at its prior value.

5. Generator of a reference frequency usable for synthesizing the local oscillator frequencies of a re- transmission station in a television grid network, said generator starting from an intermediate frequency extracted from the signal received by the station, characterized in that it comprises a frequency changer (9) which translates the intermediate frequency (Fi) by means of an auxiliary frequency (35.8 MHz), the arithmetic mean of the intermediate frequencies corresponding respectively to the French (32.7 MHz) and European (38.9 MHz) standards.

6. Generator according to claim 2, characterized in that said auxiliary frequency is the arithmetic mean (35.8 MHz) of the intermediate frequencies corresponding respectively to the French (32.7 MHz) and European (38.9 MHz) standards.

7. Re-transmission station forming part of a television 20 grid network, characterized in that it comprises, for synthesizing the respective local oscillator frequencies of the receiver (1) and of the transmitter (3) from an intermediate frequency (Fi) extracted from the signal received, two reference frequency generators (4 and 7) according to one of claims 1 to 6.

8. Generator substantially as described herein with reference to the accompanying drawings.

9. Re-transmission station substantially as described herein with reference to the accompanying drawings.

Published 1989 atThe Patent Office, State House, 88171 High HolbornLondon. WCIR 4TP. Further copies maybe obtained from The Patentofftoe. Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent, Oor.L 1/87 r