GB1584737A

GB1584737A - Television tuning device

Info

Publication number: GB1584737A
Application number: GB4331877A
Authority: GB
Original assignee: Indesit Industria Elettrodomestici Italiana SpA
Current assignee: Indesit Industria Elettrodomestici Italiana SpA
Priority date: 1976-10-25
Filing date: 1977-10-18
Publication date: 1981-02-18
Also published as: FR2368828B1; IT1071537B; FR2368828A1; DE2747691A1

Description

(54) TELEVISION TUNING DEVICE (71) We, INDESIT INDUSTRIA ELETTRODOMESTICI ITALIANA S.p.A., an Italian Company of Str. Piossasco Km 17, Rivalta, Turin, Italy, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, ta be particularly described in and by the following state ment:- The present invention relates to a tuning device for a television receiver.

There are many different types of television tuners available on the market which, though they differ as to circuit arrangement, they generally all have one thing in common, namely they are very complicated especially with regard to component assembly and adjusting of the finished circuit. Another feature common to known types of tuners is that they offer very little versatility and differ considerably from one another as regards operation.

In fact, there is such a wide range of tuners available on the market that they can be classified in a number of ways, for example, according to the television channels they are designed to receive. If classified in this way, we find there are tuners for receiving channels on the VHF + UHF bands or UHF or VHF only or CATV or combinations of CATV + VHF or CATV + VHF + UHF bands. Obviously, the tuners in each group differ considerably with regard to design.

Such a wide range of tuners is inevitable, however, if it is considered that the same television set has to be marketable in countries broadcasting over different bands. In Great Britain, for example, it is sufficient for the tuner to receive the signals transmitted on the UHF band whereas, in Italy, it also has to be designed to receive VHF band signals and, in some countries, such as Belgium, where cable broadcasting has become widespread, also CATV band signals.

Given the complexity of the tuner devices and the difficulties encountered both with regard to manufacture and design, it has not yet been found possible to produce a single tuner which is both economical and capable of meeting all demands.

According to the present invention, there is provided a tuning device for tuning a television receiver to a desired television channel in any one of two or more bands and for converting the frequency of the received signal to a lower frequency band suitable for processing by the remaining receiver circuitry, the device comprising a first frequency converter circuit for receiving the desired television signal and a first reference oscillation from a variable-frequency oscillator and adapted to supply a frequency converted output signal, the variable frequency oscillator, including an oscillator having a transistor and frequency determining components including a variablecapacitance device, the variable capacitance device being a varicap diode whose capacitance is controlled by d.c. biasing; and a second converter circuit for receiving the converted signal at an output of the first converter circuit and a second reference oscillation from a fixed-frequency oscillator to provide a signal in said lower frequency band at its output wherein the arrangement is such that, in use: (a) for a television signal in a lower frequency band, frequency conversion is effected using the basic frequency of said oscillation and, for a television signal in a higher frequency band, frequency conversion is effected using a second harmonic of said variable frequency oscillator; (b) the same frequency determining components are used irrespective of the frequency of the oscillation produced; and (c) the manner of connection of the variable frequency oscillator and the first frequency converter is independent of the frequency of the television signal.

The invention will be further described, by way of a non-limited example, with reference to the attached drawings in which: Figure 1 is a block diagram of one embodiment of tuning device according to the present invention; and Figure 2 represents the circuit arrangement of the blocks shown in Figure 1.

In Figure 1, numeral 1 indicates an antenna connected to the input of a signal translating circuit 2 fitted with a terminal 3 for controlling signal attenuation. The output of circuit 2 is connected in parallel to the input of three radio-frequency amplifiers 4, 5 and 6 having different pass-bands. Amplifier 4, for example, has a pass-band ranging from 470 MHz to 900 MHz, and as such is suitable only for amplifying signals on the UHF band. Amplifier 5 has a passband ranging from 47 MHz to 139 MHz for amplifying signals on bands I and II and for CATV channels S1 to S,, while amplifier 6 has a pass-band ranging from 139 MHz to 300 MHz for amplifying signals on CATV channels S6 to S19 and band III. All three amplifiers 4, 5 and 6 have traps tuned for cutting out unwanted frequency ranges within the above bands as well as a respective terminal 7, 8, 9 for switching amplifier operation according to the desired channel.

The outputs of amplifiers 4, 5 and 6 are connected to the input of a signal translating circuit 10 having two terminals 11, 12 for controlling the attenuation introduced into the translated signal by the circuit. The output of circuit 10 is connected to the first input of a mixer circuit 13 which also has a second input connected to a variable frequency oscillator 14, The output cf mixer circuit 13 is connected to a first input of mixer circuit 15 which also has a first output terminal to which the received television signal is sent, converted to the intermediate fixed frequency at which the signal amplifier circuits downstream from the tuner operate. Mixer circuit 15 also has a second output terminal to which a control signal is sent for controlling the amplitude of the signal processed by mixer 15 and which is applied to terminal 12 of translating circuit 10 to prevent distortion or intermodulation caused by excessive amplitude of the processed signal. Finally, a fixed-frequency oscillator circuit 16 is connected to a second input of mixer circuit 16.

The circuit operates as follows: The radiofrequency signal received by antenna 1 is applied to translating circuit 2, which can be a normal PIN diode attenuator circuit, which receives the control signal coming from the signal processing circuits connected to the video detector downstream in the receiver circuitry. The function of this circuit is to prevent the amplifier stages downstream from becoming saturated when very strong signals are received. Depending on which of the passbands of amplifiers 4, 5 and 6, the signal being received is in the appropriate one of the amplifiers will be activated by a signal received at one of the activating terminals 7, 8 or 9 and which is sent out by the operator selecting the channel on the keyboard. If necessary, the received signal is also attenuated by translating circuit 10.

When control terminal 12 of translating circuit 10 receives the signal of mixer circuit 15, it operates an automatic gain control in the tuner to provide a better, quicker amplitude check of the signals processed inside the tuner so as to give added protection against distortion and intermodulation to that provided by the automatic gain control device which acts on all the receiver amplifying circuits between the antenna and the video detector.

The signal is then converted in the first mixer circuit 13 to a first intermediate frequency of 426 MHz and then, in the second mixer circuit 15, to an intermediate frequency of 36 MHz which is the normal intermediate frequency used in television receivers. In this way, there is no longer any need for a variablytuned band filter at the input of mixer 13. As the first intermediate frequency is 426 MHz, the image frequency produced during conversion falls outside the reception band and is automatically cut out.

No image frequency problems arise during the second conversion by mixer 15 which receives the signal converted to the first intermediate frequency and a fixed-frequency (390 MHz) reference oscillation from oscillator 16.

By keeping to a fixed-frequency signal, the image frequency produced during the second conversion can easily be cut out by means of a highly selective fixed-frequency filter.

For the reasons already mentioned, for UHF and VHF use, variable-frequency oscillator 14 must operate so as to provide mixer circuit 13 with a reference signal ranging in frequency from 476 MHz to 1278 MHz for receiving the lowest channel on the VHF band and the highest channel on the UHF band respectively.

As it would be difficult to provide a single oscillator with such a performance and costly to use two separate oscillators or provide for switching of the circuits determining the frequency at which the oscillator operates, the problem has been solved, for receiving UHF band channels, by using the second harmonic of the oscillation produced by the oscillator whereas the basic frequency of the oscillator still falls within the VHF band.

Variable-frequency oscillator 14 is therefore built using a single oscillator circuit comprising a single transistor and a single tuned circuit comprising, in turn, a single varicap diode.

The frequency range within which the basic frequency of the oscillation produced by variable-frequency oscillator 14 must fall has therefore been fixed at 448 MHz to 726 MHz.

Allowance has had to be made for the oscillator falling below 476 MHz for receiving the first channel on the VHF band as, to receive the first UHF band channel using the second harmonic of the oscillator, the basic frequency must be 896 - MHz, 2 that is, 448 MHz.

The frequency range covered by variable oscillator 14 is therefore 448-726 MHz for VHF band reception and 89d1452 MHz for UHF band reception.

By fixing the first intermediate frequency at 426 MHz, the theoretical frequency range required for receiving all the VHF, CATV and UHF channels is 476--726 MHz and 896-1278 MHz which means all requirements can be met.

The following equation may be used for establishing the optimum level of the first intermediate frequency as regards the frequency range within which basic frequency and the second harmonic of variable oscillator 14 must fall: fj=fu --2 2 fv nsn in which fi = the first intermediate frequency, cumin= the minimum frequency of the UHF band required and fv nnin = the minimum frequency of the VHF band required. If fu inin = 470 MHz and fv min = 50 MHz, fi would work out at 370 MHz. Two risks would be run using this frequency; the image frequency of a UHF signal falling between 790 and 852 MHz could be received in VHF (between 50 and 112 MHz); and the second harmonic of the intermediate frequency (740 MHz) could be received in UHF.

To overcome both these risks, it is ensured that the first intermediate frequency level is selected so that: funuax ft 2 2 in which fu n,lax is the maximum frequency of the UHF band being received.

Therefore, if fumSx=852 MHz (channel 68), fi 2 426 MHz.

Figure 2 shows an electric circuit arrangement of how the embodiment shown in block form in Figure 1, can be carried into practice.

Numeral 19 indicates an input terminal corresponding to the input terminal of the translating circuit 2 shown in Figure 1. The signal coming from the antenna passes through capacitor 21 to a conventional attenuator circuit consisting of three PIN diodes 22, 23 and 24, then through capacitor 25 to be available at the output of signal translating circuit 2.

Translating circuit 2 comprises five resistors, 26, 27, 28, 29 and 30, an inductor 34 which act as a biasing and control network for the PIN diodes and four capacitors 31, 32, 33 and 36 which act as a by-pass. Translating circuit 2 also consists of a high-pass filter LC composed of inductor 20 and capacitor 21 which, together with the trap filter composed of inductor 35 and capacitor 37, serves to provide the correct pass-band to translating circuit 2.

The output of the latter is connected directly to the three input terminals of band-pass amplifiers 4, 5 and 6 shown in Figure 1 and which, in Figure 2, comprises transistors 38, 39 and 40 and associated parts.

Each of these transistors is connected in common-base configuration (capacitors 41, 42 and 43 being equivalents to short-circuits at regards the frequencies in question) and has a resistive biasing network comprising three resistors 44, 45, 46, 47, 48, 49 and 50, 51, 52! Capacitors 53, 54 and 55 can also be regarded as short-circuits for the frequencies in question. Connected to the input and output of the three amplifiers, comprising the three transistors 38, 39 and 40, are: a high-pass filter LC 56, 57, 58 at the input of transistor 38; a high-pass filter LC 59, 60, 61, 62 at the output of transistor 38; a band-pass filter LC 63, 64 at the input of transistor 39; a band-pass filter LC 65, 66, 67, 68 at the output of transistor 39; a band-pass filter LC 69, 70 at the input of transistor 40; and a band-pass filter LC 71, 72, 73, 74, 75 at the output of transistor 40.

The outputs of the three amplifiers are therefore connected to the input of the signal translating circuit 10 shown in Figure 1 comprising PIN diode 76 which receives a first control signal coming from the video detector through inductor 77 and a second control signal through resistor 78. Translating circuit 10 also includes two filter capacitors 79 and 80 and a bias resistor 81. Translating circuit 10 sends the signal to the first mixer circuit 13 consisting of transistor 82 connected in common-base configuration which also receives the signal from the variable-frequency oscillator on the emitter through condenser 83. The mixer circuit also comprises three biasing resistors 84, 85 and 86, two filter capacitors 87 and 88, a high-pass output filter LC 89, 90 and a twin-tuned band-pass circuit tuned to the first intermediate frequency (426 MHz) comprising capacitors 91, 94, 95 and 96 and two resonant lines 92 and 93.

The variable-frequency oscillator 14, shown in Figure 1, comprises transistor PNP 97 (type BF 479) a 1.2 Kohm resistor 98 for biasing the emitter of transistor 97 and base bias resistors 99 and 100 of 4.7 and 5.6 Kohms respectively. The oscillator circuit also includes a capacitance for creating the 2.2 pF positive feedback 104 and for oscillating transistor 97, as well as a varicap diode 105, type BB 209, which, together with resonant line 106, fixes the frequency of the oscillator. The capacitance of varicap diode 105 and, consequently, the frequency of oscillator 14, is controlled by means of d.c. biasing received from a + V voltage source through the 56 Kohm resistor 107.

The circuit also includes: a capacitor 103 of 22 pF which, together with the maximum capacitance of varicap diode 105, establishes the minimum frequency at which the oscillator is to operate, acts as a load impedance for transistor 97 and determines the amount of positive feedback produced by capacitor 104; two 820 pF capacitors 101 and 102 which filter the +B supply voltages and +V varicap biasing voltages; a 820 pF capacitor 138 for grounding the base of transistor 97 for a.c.

operation of the transistor.

The oscillations produced in the oscillator circuit is applied to the first mixer (circuit 13 in Figure 1) through capacitor 108 and resistor 109.

The signal converted to the first intermediate frequency (426 MHz) and available at the output of the first mixer is then sent to the second mixer (circuit 15 in Figure 1) comprising transistor 110, filter capacitors 111, 112, 113 and 114 and bias resistors 115, 116, 117 and 118.

The second mixer also consists of two LC filters composed of inductors 119 and 120 and capacitors 121 and 122 respectively. The former LC filter 119, 121 is tuned to the second intermediate frequency, that is, 36 MHz (the standard intermediate frequency used on television receivers) and the latter filter 120, 122 to a frequency forbidden for radiation (e.g. the frequency of fixed oscillator 16). The collector of transistor 110 is connected to a diode 123 which detects the amplitude of the signal on the collector, and, by means of the low-pass filter comprising capacitor 124 and resistor 125, produces a d.c.

signal for controlling the attenuation introduced by translating circuit 10.

The converted signal is finally available at the output terminal 126 from which it is sent to the I.F. amplifiers downstream in the receiver circuitry.

The 390 MHz reference oscillation required for the second conversion is supplied to transistor 110 via capacitors 127 and 128 by a fixed-frequency oscillator (oscillator 16 in Figure 1) comprising transistor 129, filter capacitors 130 and 131 and bias resistors 132, 133 and 134. The fixed-frequency oscillator also includes a feedback capacitor 135 and a resonant circuit tuned to 390 MHz frequency comprising capacitor 136 and resonant line 137.

All the circuits mentioned up to now are supplied from the terminals marked +B. In the case of the three band-pass amplifier circuits, the supply terminal +B can also act as the activating terminal marked 7, 8 or 9 in Figure 1.

For an explanation of the operation of the circuits shown in Figure 2, see the explanation given for block diagram 1. The Figure 2 circuits have been shown as enclosed by dotted line to show the corresponding blocks of Figure 1.

The advantages of the above embodiment will be seen clearly from the above description, in particular, the possibility of providing a tuning device with minimum adjusting requirements. In fact, the use of a single varicap, which overcomes the well-known problem of selecting and matching three separate diodes, and the automatic gain control in the tuner means that parts with actual values differing considerably from calculated ones can be used with no risk of jeopardizing operation.

Another advantage is that by using the circuits mentioned in the description, a single tuning device can be used for receiving all types of television channels regardless of the band. Yet another advantage is that it enables a much higher rejection of image frequency than has been possible up to now as well as a reduction of intermodulation and distortion in the case of very strong signals or signals very near another strong station being received.

To those skilled in the art, it will be clear that variations can be made to the circuits described by way of example without, however, departing from the scope of the present invention as defined in the appended claims.

One of the many variations, for example, is in the selection of the first intermediate frequency and fixed-frequency oscillator. It has no effect on the operation of the circuit if a first intermediate frequency other than 426 MHz is used. Any one of the frequencies between the last television channel on band III and the first on the UHF band may be used.

If a different first intermediate frequency is selected, changes must also be made accordingly to the frequency range within which basic frequency and the second harmonic of the oscillation produced by variable-frequency cscillator 14 are allowed to vary.

Attention is drawn to our copending application No. 44442/77 (Serial No. 1,584,738) which describes and claims features of the above described apparatus.

WHAT WE CLAIM IS: 1. A tuning device for tuning a television receiver to a desired television channel in any one of two or more bands and for converting the frequency of the received signal to a lower frequency band suitable for processing by the remaining receiver circuitry, the device comprising a first frequency converter circuit for receiving the desired television signal and a first reference oscillation from a variable-frequency oscillator and adapted to supply a frequency converted output signal, the variable frequency oscillator, including an oscillator having a transistor and frequency determining components including a variable-capacitance device, the variable-capacitance device being a varicap diode whose capacitance is controlled by d.c. biasing; and a second converter circuit for receiving the converted signal at an output of the first converter circuit and a second reference oscillation from a fixed-frequency oscillator to provide a signal in said lower frequency band at its output wherein the arrangement is such that, in use: (a) for a television signal in a lower frequency band,

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. positive feedback produced by capacitor 104; two 820 pF capacitors 101 and 102 which filter the +B supply voltages and +V varicap biasing voltages; a 820 pF capacitor 138 for grounding the base of transistor 97 for a.c. operation of the transistor. The oscillations produced in the oscillator circuit is applied to the first mixer (circuit 13 in Figure 1) through capacitor 108 and resistor 109. The signal converted to the first intermediate frequency (426 MHz) and available at the output of the first mixer is then sent to the second mixer (circuit 15 in Figure 1) comprising transistor 110, filter capacitors 111, 112, 113 and 114 and bias resistors 115, 116, 117 and 118. The second mixer also consists of two LC filters composed of inductors 119 and 120 and capacitors 121 and 122 respectively. The former LC filter 119, 121 is tuned to the second intermediate frequency, that is, 36 MHz (the standard intermediate frequency used on television receivers) and the latter filter 120, 122 to a frequency forbidden for radiation (e.g. the frequency of fixed oscillator 16). The collector of transistor 110 is connected to a diode 123 which detects the amplitude of the signal on the collector, and, by means of the low-pass filter comprising capacitor 124 and resistor 125, produces a d.c. signal for controlling the attenuation introduced by translating circuit 10. The converted signal is finally available at the output terminal 126 from which it is sent to the I.F. amplifiers downstream in the receiver circuitry. The 390 MHz reference oscillation required for the second conversion is supplied to transistor 110 via capacitors 127 and 128 by a fixed-frequency oscillator (oscillator 16 in Figure 1) comprising transistor 129, filter capacitors 130 and 131 and bias resistors 132, 133 and 134. The fixed-frequency oscillator also includes a feedback capacitor 135 and a resonant circuit tuned to 390 MHz frequency comprising capacitor 136 and resonant line 137. All the circuits mentioned up to now are supplied from the terminals marked +B. In the case of the three band-pass amplifier circuits, the supply terminal +B can also act as the activating terminal marked 7, 8 or 9 in Figure 1. For an explanation of the operation of the circuits shown in Figure 2, see the explanation given for block diagram 1. The Figure 2 circuits have been shown as enclosed by dotted line to show the corresponding blocks of Figure 1. The advantages of the above embodiment will be seen clearly from the above description, in particular, the possibility of providing a tuning device with minimum adjusting requirements. In fact, the use of a single varicap, which overcomes the well-known problem of selecting and matching three separate diodes, and the automatic gain control in the tuner means that parts with actual values differing considerably from calculated ones can be used with no risk of jeopardizing operation. Another advantage is that by using the circuits mentioned in the description, a single tuning device can be used for receiving all types of television channels regardless of the band. Yet another advantage is that it enables a much higher rejection of image frequency than has been possible up to now as well as a reduction of intermodulation and distortion in the case of very strong signals or signals very near another strong station being received. To those skilled in the art, it will be clear that variations can be made to the circuits described by way of example without, however, departing from the scope of the present invention as defined in the appended claims. One of the many variations, for example, is in the selection of the first intermediate frequency and fixed-frequency oscillator. It has no effect on the operation of the circuit if a first intermediate frequency other than 426 MHz is used. Any one of the frequencies between the last television channel on band III and the first on the UHF band may be used. If a different first intermediate frequency is selected, changes must also be made accordingly to the frequency range within which basic frequency and the second harmonic of the oscillation produced by variable-frequency cscillator 14 are allowed to vary. Attention is drawn to our copending application No. 44442/77 (Serial No. 1,584,738) which describes and claims features of the above described apparatus. WHAT WE CLAIM IS:

1. A tuning device for tuning a television receiver to a desired television channel in any one of two or more bands and for converting the frequency of the received signal to a lower frequency band suitable for processing by the remaining receiver circuitry, the device comprising a first frequency converter circuit for receiving the desired television signal and a first reference oscillation from a variable-frequency oscillator and adapted to supply a frequency converted output signal, the variable frequency oscillator, including an oscillator having a transistor and frequency determining components including a variable-capacitance device, the variable-capacitance device being a varicap diode whose capacitance is controlled by d.c. biasing; and a second converter circuit for receiving the converted signal at an output of the first converter circuit and a second reference oscillation from a fixed-frequency oscillator to provide a signal in said lower frequency band at its output wherein the arrangement is such that, in use: (a) for a television signal in a lower frequency band,

frequency conversion is effected using the basic frequency of said oscillation and, for a television signal in a higher frequency band, frequency conversion is effected using a second harmonic of said variable frequency oscillator; (b) the same frequency determining components are used irrespective of the frequency of the oscillation produced; and (c) the manner of connection of the variable frequency oscillator and the first frequency converter is independent of the frequency of the television signal.

2. A device according to Claim 1, wherein the transistor of the variable-frequency oscillator is connected in the common-base configuration.

3. A device according to any one of the preceding claims, wherein the arrangement is such that, in use, the frequency of the frequency converted signal at the output of the said first converter circuit falls between a highest frequency channel in said lower frequency band and a lowest frequency channel in said higher frequency band.

4. A device according to any one of the preceding claims, wherein the arrangement is such that, in use, the frequency of the said converted signal at the output of the said first converter circuit is greater than or equal to half the frequency of the highest-frequency signal to be received.

5. A device according to claim 3 or 4, wherein the arrangement is such that, in use, the frequency of the said converted signal present at the output of the first frequency converter circuit is approximately 426 MHz.

6. A device according to claim 5, wherein said fixed-frequency oscillator will supply an output of approximately 390 MHz.

7. A device according to any one of the preceding claims, and including at least one amplifier for amplifying the said received signal and applying it to the first frequency converter circuit.

8. A device according to any one of the preceding claims, and including translating and controlled-attenuation circuits for supplying the said received signal to the said first converter circuit.

9. A device according to claim 8, wherein said translating and controlled-attenuation circuits will receive at least one signal depending on the amplitude of the signal sent out from the second frequency converted circuit and/or the amplitude of the signal processed by circuits of the receiver downstream from the second frequency converter circuit.

10. A tuning device for a television receiver, such device being constructed and arranged to operate substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

11. A device according to claim 10 and incorporating circuitry substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.

12. A television receiver incorporating a tuning device according to any one of the preceding claims.