GB1584738A

GB1584738A - Television tuner

Info

Publication number: GB1584738A
Application number: GB44442/77A
Authority: GB
Original assignee: Indesit Industria Elettrodomestici Italiana SpA
Current assignee: Indesit Industria Elettrodomestici Italiana SpA
Priority date: 1976-10-25
Filing date: 1977-10-25
Publication date: 1981-02-18
Also published as: DE2746931A1; FR2371821B1; FR2371821A1

Description

(54) TELEVISION TUNER (71) We, INDESIT INDUSTRIA ELETTRO DOMESTICI ITALIANA S. p. A., an Italian Body Corporate, 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, to be particularly described in and by the following statement: The present invention relates to a device for tuning a television receiver and, in particular, to the circuitry for selecting the required television channel and converting the received signal from the frequency of the selected channel to the fixed frequency at which the signal amplifiers circuits operate.

Such a selection and conversion circuit arrangement is commonly known as a "tuner". There are many different types of tuners available on the market which, though they differ as to circuit arrangement, all have one thing in common: 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. Tuners can be classified 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 and UHF bands or only UHF or CATV or combinations of CATV and VHF or CATV, VHF and UFH bands. Obviously the design of tuners in each group differ considerably.

Such a wide range of tuners is inevitable, however, if we consider that the same television set has to be marketaable in countries broadcasting over different bands.

In Great Britain, for example, it is sufficent for the tuner to receive signals transmitted on the UHF band whereas, in Italy, it has also 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 device 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 television receiver tuning device for receiving television channels in the VHF, CATV and UHF bands and to convert the received signal to a lower frequency, the device including an amplifier circuit for regulating the amplitude of the received signal and supplying it to a first mixer circuit; a variable-frequency oscillator circuit for providing the first mixer circuit with a reference oscillation; and a second mixer circuit arranged to receive the signal from the first mixer circuit and a reference oscillation coming from a second, fixed-frequency oscillator; the arrangement being such that, in use, the received signals are converted by the first mixer circuit to a first intermediate frequency fl in an intermediate frequency band lower than and adjacent to the UHF television band and that the second mixer circuit converts the signal from the first intermediate frequency fl to a second intermediate frequency f, lower than the frequency of the first television channel belonging to the VHF band.

The invention will now be described, by way of a non-limiting example, with reference to the attached drawings in which: Figure 1 is a block diagram of one arrangement of tuning device embodying the present invention; Figure 2 shows the circuit arrangement of the blocks shown in Figure 1; Figure 3 represents the block diagram of one possible variation of the tuning device shown in Figure 1; and Figure 4 shows the circuit arrangement of the blocks shown in Figure 3.

In Figure 1, reference 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 to the input of three radio-frequency amplifiers 4, 5 and 6 having different pass-bands.

Amplifier 4, for example, has a band width ranging from 470 MHz to 900 MHz, suitable only for amplifying signals on the UHF band. Amplifier 5 has a band width ranging from 47 MHz to 139 MHz for amplifying signals on bands I and II and for CATV channels S1 to S5. Finally, amplifier 6 has a band-width ranging from 139 MHz to 300 MHz for amplifying signals on CATV channels S5 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 respective terminals 7, 8 and 9 for switching amplifier operation according to the desired channel. The output of amplifiers 4, 5 and 6 are connected to the input of a signal translating circuit 10 having two terminals 11 and 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 of 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 sent 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 radio-frequency signal received by antenna 1 is sent to translating circuit 2, which can be a normal PIN diode attenuator circuit and receives the control signal coming from the signal processing circuits connected to the video detector downstream. The function of this circuit is to prevent the amplifier stages downstream in the circuitry from becoming saturated in the case of very strong signals being received. Depending on which pass band the signal being received belongs to one of the amplifiers will be activated by a signal received at one of the activating terminals 7, 8, 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 device inside 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 frequency used in television receivers. In this way, there is no longer any need for a variably-tuned 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, at one end, receives the signal converted to the first intermediate frequency and, at the other, 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. In this way, only one circuit operates at variable frequency, that is, oscillator 14, and a single varicap can be used instead of the three used on known tuners.

Figure 2 shows one possible circuit arrangement of the embodiment of Figure 1. 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 via capacitor 21 to a normal attenuator circuit consisting of three PIN diodes 22, 23 and 24, then via capacitor 25 to be made 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 comprises a high-pass LC filter 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 band width 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, comprise transistors 38, 39 and 40 and associated parts.

Each of these transistors is connected in the common-base configuration (capacitors 41, 42 and 43 appear as short-circuits at the signal frequencies in question) and has a resistive biasing network comprising three resistors 44, 45 and 46; 47, 48 and 49; 50, 51 and 52. Capacitors 53, 54 and 55 are also short-circuits for the signal 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 LC filter 56, 57, 58 at the input of transistor 38; a high-pass LC filter 59, 60, 61, 62 at the output of transistor 38; a band-pass LC filter 63, 64 at the input of transistor 39; a band-pass LC filter 65, 66, 67, 68 at the output of transistor 39; a band-pass LC filter 69, 70 at the input of transistor 40; and a band-pass LC filter 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 and comprising PIN diode 76 which receives a first control signal coming from the video detector via inductor 77 and a second control signal through resistor 78.

Translating circuit 10 also includes two filter capacitors 79 and 80 and a biasing resistor 81. Translating circuit 10 sends the signal to the first mixer circuit 13, comprising transistor 82, which is in common base configuration and also receives the signal coming from the variable-frequency oscillator at its emitter via capacitor 83.

The mixer circuit also includes three biasing resistors 84, 85 and 86, two filter capacitors 87 and 88, a high-pass output LC filter 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 includes transistor 97, biasing resistors 98, 99 and 100 and filter capacitors 101, 102, 103 and 138. The oscillator circuit also includes a positive feedback capacitance 104 and a varicap diode 105 which, together with a resonant line 106, fixed the frequency of the oscillator.

The capacitance of varicap diode 105 and, consequently, the frequency of oscillator 14, is controlled by means of a d.c. biasing from a +V voltage source via resistor 107.

The oscillation produced in the oscillator circuit is then sent, via capacitor 108 and resistor 109, to the first mixer circuit, i.e.

circuit 13 in Figure 1.

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

The second mixer also comprises two LC filters composed of inductors 119 and 120 and capacitors 121 and 122 respectively.

The former 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, for example, the frequency of fixed oscillator 16. The collector for 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 made available at the output terminal 126 from which it is sent to the I.F. amplifiers downstream in the receiver.

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 biasing resistors 132, 133 and 134. The fixed-frequency oscillator also includes a feed-back 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 bandpass amplifier circuits, the supply terminal + B can also act as the activator terminal marked 7, 8 or 9 in Figure 1.

For an explanation of the operation of the circuits shown in Figure 2, refer to the explanation given for block diagram 1.

The Figure 2 circuits have been drawn round with dotted lines to show the corresponding Figure 1 blocks.

The television receiver tuning device described with reference to Figures 1 and 2 has one small drawback, from a certain point of view. As only one variablefrequency oscillator circuit is provided, which must be able to sweep the whole frequency range from the lowest channel on the VHF band to the highest on the UHF band, the control signal supplied to to the oscillator for varying the frequency oscillation must be variable within very small limits to enable the user to tune in accurately to the required channel. If the variable-frequency oscillator consists, for example, of a V.C.O. (voltage controlled oscillator), the control voltage must be capable of fine adjustment. As the control voltage is usually supplied to a varicap diode which, at most, can withstand a voltage of 30V, tuning variations perceptible by the user correspond to variations in control voltage of a few MV. Apart from the problem of supplying a varicap with such small variations in voltage for tuning purposes, there is also the problem of noise interfering with control voltage and affecting tuning. In all cases, even with the same control voltage variation range, an increase in the range of variation of the V.C.O.

frequency makes the tuning operation more critical.

The following is a description of a tuning device, which overcomes the aforementioned problem.

Number 201 in Figure 3 indicates an antenna connected to the input of a signal translation circuit 202 which also has a terminal, 203, for checking signal attenuation. The output of circuit 202 is connected to the input of three radio-frequency amplifiers, 204, 205 and 206, with different bandpass characteristics. Amplifier 204, for example, has a pass band ranging from 470 to 900 MHz, that is, only suitable for amplifying signals on the UHF band.

Amplifier 205 has a pass band ranging from 47 to 139 MHz, suitable for signals on bands I and II and channels S1 to S5 on the CATV band. Finally, amplifier 206 has a pass band ranging from 139 to 300 MHz for signals on channels Ss to S9 on the CATV band and for television band III.

The three amplifiers, 204, 205 and 206, are also provided with traps tuned to unwanted frequency ranges on the above bands, as well as respective terminals 207, 208 and 209 for activating the circuit by means of which the operation of the three amplifiers can be switched according to the required channel. The outputs of the three amplifiers, 204, 205 and 206 are connected to the input of a signal translating circuit, 210, which has two terminals 211 and 212 for regulating the attenuation introduced by the circuit into the translated signal.

The output of circuit 210 is connected to a first input of a mixer circuit, 213, which also has a second input connected to a variable-frequency oscillator circuit 214.

The output of mixer circuit 213 is connected to a first input of a mixer circuit, 215, which also has a first output terminal at which the received television signal is made available, converted to the fixed intermediate frequency at which the signal amplifier circuits downstream from the tuner operate. Mixer circuit 215 also has a second output terminal at which a control signal is available, dependent upon the amplitude of the signal processed by mixer 215, which is applied to terminal 212 of translator circuit 210 to prevent excessive amplitude of the processed signal from causing distortion or intermodulation. A fixed-frequency oscillator circuit 216 is, in turn, connected to a second input of mixer circuit 215.

Oscillator circuit 214 comprises three variable-frequency oscillators of the V.C.O.

type, indicated in the Figure by numerals 217, 218 and 219. Each is capable of supplying a persistent oscillation which is sent as a reference signal to mixer circuit 213. Furthermore, each has an activating terminal, 220, 221 and 222, respectively, which receives an activating signal for the oscillator.

The three oscillators, 217, 218 and 219, are activated according to the required television channel. If this belong to bands I or II or channels S1 to Ss on the CATV band, oscillator 217 is activated. If it belongs to band II or channels Ss to Sl9 on the CATV band, oscillator 218 is activated.

Finally, if it belongs to the UHF band, oscillator 219 is activated.

All three oscillators, 217, 218, and 219, have terminals 223, 224 and 225 to which the control voltage is applied which establishes the frequency of the signal produced by that V.C.O.

The circuit shown operates as follows: The radio-frequency signal received by antenna 201 is sent to translator circuit 202, which may comprise a classical PIN-diode attenuator circuit and which receives the control signal coming from the signal processing circuits associated with the video detector downstream. The function of this circuit is to prevent the amplifier stages downstream from becoming saturated in the case of very strong signals being received. Depending on whether the signal to be received belongs to one of the three pass bands of the three amplifiers, 204, 205 and 206, one of the amplifiers is activated by a signal coming to one of the activating terminals, 207, 208 or 209, which is supplied from the keyboard by the user when selecting the channel. If necessary, the received signal is also attenuated by translator circuit 210.

When control terminal 212 of translator circuit 210 receives the signal from mixer circuit 215, it operates an automatic gain control inside the tuner so as to provide a quicker, more accurate amplitude check of the signal processed inside the tuner and greater protection against distortion and intermodulation than that afforded by automatic gain controls which act on all the receiver amplifier circuits between the antenna and video detector.

The signal is then converted, in the first mixer circuit 213, to a first intermediate frequency of 426 MHz, then, in the second mixer circuit 215, to an intermediate frequency of 36 MHz which is the standard frequency used in television receivers. In this way, there is no longer any need to fit a variably tuned band filter at the input of mixer 213, in that since the first intermediate frequency is 426 MHz the image frequency produced during the conversion falls outside the receiving band and is automatically cut out. With regard to the second conversion performed by mixer 215, which receives at one end, the signal converted to the first intermediate frequency oscillation coming from oscillator quency and, at the other, a fixed-frequency (380 MHz) reference oscillation coming from oscillator 16, in this case, there are no problems as regards image frequency.

In fact, by working with a fixedfrequency signal, there is no difficulty in cutting out the image frequency produced during this second conversion by means of a highly selective fixed-frequency filter.

The three voltage controlled oscillators, 217, 218 and 219, are activated when the three amplifiers, 205, 206, and 204, are activated respectively, that is, when signals are received belonging to one of the following three bands: 1) band I, band II, channels Sl-Ss of the CATV band; 2) band III, channels S6-S19 of the CATV band; 3) UHF band.

With a first intermediate frequency of 426 MHz, the frequency ranges of the three oscillators, 217, 218 and 219, will be: oscillator 217 : 476 - 568 MHz; oscillator 218 : 568 - 726 MHz;oscillator 219 : 896 - 1,278 MHz Consequently, there are no problems with regard to the oscillators in that the frequency range to be covered by each is small and there is no need to vary the control voltage of the V.C.O.s by small amounts for fine variations in tuning. This solves the criticial tuning problems which can be posed by the device described in Figures 1 and 2.

Figure 4 shows a possible circuit for oscillator 214 shown in Figure 3.

The function of the three oscillators, 217, 218 and 219, is performed by two transistors with associated passive circuits. PNP transistor 226 performs the function of both oscillator 217 and 218 by switching a number of passive elements associated with it, whilst the function of oscillator 219 is performed by PNP transistor 227. Transistor 226 has its collector grounded by means of inductor 228, its emitter connected to activating terminal 229 by means of resistor 230, and its base grounded by means of an RC unit consisting of resistor 231 and capacitor 232 connected in parallel.

The base is also connected to terminal 229 by means of resistor 233 while terminal 229 is grounded by means of capacitor 234.

The collector of transistor 226 is grounded by means of condenser 235 and diode 236 connected in series, the series connection of resistor 237 and capacitor 238 being connected parallel to diode 236. A second.

activating terminal 239 is connected at the junction of resistor 237 and capacitor 238.

A control terminal 240 is connected through the series of a resistor 241 to the cathode of a varicap diode 242, whose anode is connected to the collector of transistor 226, while the emitter of transistor 226 is connected to the cathode of varicap diode 242 via capacitor 243. The cathode of varicap diode 242 and control terminal 240 are grounded by means of two condensers, 263 and 244, respectively. The emitter of transistor 227 is connected, at one end, to activating terminal 245, through resistor 246, and, at the other, to ground, via capacitor 247. The base is grounded by means of an RC unit, consisting of resistor 248 and capacitor 249, and connected to activating terminal 245 by means of resistor 250. Activating terminal 245 is, in turn, connected to ground through condenser 251. The collector of transistor 227 is grounded by means of resonant line 252 and connected to a second control terminal 253 by means of varicap diode 254 and resistor 255, connected in series. In addition to being connected to resistor 255, the cathode of varicap diode 254 is connected, at one end, to ground via capacitor 256 and, at the other, to the emitter of transistor 227 via capacitor 257. Finally, control terminal 253 is also grounded via capacitor 258.

The circuit described operates as follows: Transistor 227, the base of which is grounded as already mentioned, constitutes the UHF oscillator. The frequency of the oscillator is established by the resonant circuit comprising line 252, varicap diode 254 and condenser 256. The feedback required to sustain oscillation is performed by capacitors 247 and 257. The function of resistors 246, 248 and 250 is to bias transistor 227 while resistor 255 serves to supply the d.c.

voltage present at control terminal 253 to the varicap diode. Capacitors 249, 251 and 258 act as by-passes to short-circuit. the signal to the frequency of the oscillator.

The function of activating terminal 245 is quite simply to activate or deactivate by supplying or cutting off the supply voltage to transistor 227.

Transistor 226 which is in the grounded base configuration, performs the functions of oscillator 218 in Figure 3 when the supply voltage for the transistor is present at activating terminal 229 and there is no voltage at activating terminal 239. In this way, the frequency of the oscillation pro duced is established by the resonant circuit comprising inductor 228, varicap diode 242 and capacitor 263. The oscillation is maintained by the feedback via capacitor 243.

When transistor 226 is called up to perform the function of oscillator 217 of Figure 3, a d.c. voltage is present at activating terminal 239 to make diode 236 conductive. In this way, catpacitor 235 is placed in parallel to inductor 228 so as to shift the frequency range of the oscillator to the range of variation required of oscillator 217. Resistors 230, 231 and 233 bias transistor 226 while resistors 237 and 241 serve to supply the d.c. voltage required for operating diodes 236 and 242.

Capacitors 232, 234 and 238 act as by-passes for the oscillator-frequency signals. The output signals of the two oscillators are picked up by means of a magnetic coupling consisting of a loop situated near the oscillators which, for the sake of simplicity, has not been shown in the Figure. The advantages of the above embodiment will be clear from the description given; in particular, using the circuits shown, the possibility of obtaining a single tuner for receiving television channels broadcasted over a wide range of bands without, at the same time, rendering the tuning operation critical.

Furthermore, by performing the function of oscillator 214 with three separate oscillators, it is possible to delimit accurately the television channel bands the tuner can tune into so as to prevent the reception of unwanted channels such as those in the areas dividing television bands I, II, III and UHF bands. A further advantage of the tuning device described is that it enables a higher rejection of image frequency than has been possible up to now as well as a reduction in intermodulation and distortion in the case of very strong signals or signals very close to another highintensity station being received.

Another advantage is the possibility of providing a tuner with minimum adjusting requirements. In fact, the use of a single varicap and the automatic gaincontrol device inside the tuner means that parts with real values differing considerably from nominal ones can be used with no risk of jeopardising operation.

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.

One of the many variations, for example, is 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 or one even higher than the last channel on the UHF band (roughly 1 GHz) can be used.

Another variation involves the use of two variable-frequency oscillators instead of three for the VHF and UHF bands. This solution does not make the tuning operation any more critical in that the frequency variation range of the single VHF oscillator is always less than that of the UHF oscillator which, in all cases, is always the highest.

Attention is drawn to our copending application No. 43318/77 (Serial No.

1584737) which describes and claims features of the above described apparatus.

WHAT WE CLAIM IS: - 1. A television receiver tuning device for receiving television channels in the VHF, CATV and UHF bands and to convert the received signal to a lower frequency, the device including an amplifier circuit for regulating the amplitude of the received signal and supplying it to a first mixer circuit; a variable-frequency oscillator circuit for providing the first mixer circuit with a reference oscillation; and a second mixer circuit arranged to receive the signal from the first mixer circuit and a reference oscillation coming from a second, fixedfrequency oscillator; the arrangement being such that, in use, the received signals are converted by the first mixer circuit to a first intermediate frequency fl in an intermediate frequency band lower than and adjacent to the UHF television band and that the secon

Claims

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

duced is established by the resonant circuit comprising inductor 228, varicap diode 242 and capacitor 263. The oscillation is maintained by the feedback via capacitor 243.

When transistor 226 is called up to perform the function of oscillator 217 of Figure 3, a d.c. voltage is present at activating terminal 239 to make diode 236 conductive. In this way, catpacitor 235 is placed in parallel to inductor 228 so as to shift the frequency range of the oscillator to the range of variation required of oscillator 217. Resistors 230, 231 and 233 bias transistor 226 while resistors 237 and 241 serve to supply the d.c. voltage required for operating diodes 236 and 242.

Capacitors 232, 234 and 238 act as by-passes for the oscillator-frequency signals. The output signals of the two oscillators are picked up by means of a magnetic coupling consisting of a loop situated near the oscillators which, for the sake of simplicity, has not been shown in the Figure. The advantages of the above embodiment will be clear from the description given; in particular, using the circuits shown, the possibility of obtaining a single tuner for receiving television channels broadcasted over a wide range of bands without, at the same time, rendering the tuning operation critical.

Furthermore, by performing the function of oscillator 214 with three separate oscillators, it is possible to delimit accurately the television channel bands the tuner can tune into so as to prevent the reception of unwanted channels such as those in the areas dividing television bands I, II, III and UHF bands. A further advantage of the tuning device described is that it enables a higher rejection of image frequency than has been possible up to now as well as a reduction in intermodulation and distortion in the case of very strong signals or signals very close to another highintensity station being received.

Another advantage is the possibility of providing a tuner with minimum adjusting requirements. In fact, the use of a single varicap and the automatic gaincontrol device inside the tuner means that parts with real values differing considerably from nominal ones can be used with no risk of jeopardising operation.

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.

One of the many variations, for example, is 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 or one even higher than the last channel on the UHF band (roughly 1 GHz) can be used.

Another variation involves the use of two variable-frequency oscillators instead of three for the VHF and UHF bands. This solution does not make the tuning operation any more critical in that the frequency variation range of the single VHF oscillator is always less than that of the UHF oscillator which, in all cases, is always the highest.

Attention is drawn to our copending application No. 43318/77 (Serial No.

1584737) which describes and claims features of the above described apparatus.

WHAT WE CLAIM IS: - 1. A television receiver tuning device for receiving television channels in the VHF, CATV and UHF bands and to convert the received signal to a lower frequency, the device including an amplifier circuit for regulating the amplitude of the received signal and supplying it to a first mixer circuit; a variable-frequency oscillator circuit for providing the first mixer circuit with a reference oscillation; and a second mixer circuit arranged to receive the signal from the first mixer circuit and a reference oscillation coming from a second, fixedfrequency oscillator; the arrangement being such that, in use, the received signals are converted by the first mixer circuit to a first intermediate frequency fl in an intermediate frequency band lower than and adjacent to the UHF television band and that the second mixer circuit converts the signal from the first intermediate frequency fl to a second intermediate frequency f2 lower than the frequency of the first television channel belonging to the VHF band.
2. A device according to claim 1, wherein a desired channel is tuned into by means of a single variable-capacity device which forms part of the variable-frequency oscillator circuit.
3. A device according to claim 1, wherein the variable-frequency oscillator circuit comprises at least one first and one second variable-frequency oscillator which are activated according to whether the required channel is on the VHF or UHF band.
4. A device according to claim 1, wherein the variable-frequency oscillator circuit comprises three variable-frequency oscillators which are activated according to the frequency band to which a selected television channel belongs.
5. A device according to claim 4, wherein two of the oscillators are formed by a single oscillator having switchable elements for determining the frequency of the oscillation generated.
6. A device according to any one of the

preceding claims, wherein the variablefrequency oscillator is of voltage controlled type.
7. A device according to any one of the preceding claims, wherein means are provided to prevent the reception of signals belonging to unwanted frequency bands.
8. A device according to claim 7, wherein said means include activating means to enable the said variable-frequency oscillator circuit to generate oscillations belonging only to the required television channel bands.
9. A device according to any one of the preceding claims, wherein the amplifier circuit includes a first attenuator circuit operable to attenuate the signal according to a first control signal obtained from a video detector.
10. A device according to claim 9, wherein the amplifier circuit includes a second attenuator circuit which is operable according to the said first control signal and a second control signal from the said second mixer circuit.
11. A device according to claim 9, wherein the said first attenuator circuit includes at least one PIN diode.
12. A device according to claim 10, wherein the second attenuator circuit includes at least one PIN diode.
13. A device according to any one of the preceding claims, wherein the second mixer circuit includes means for detecting the amplitude of the signal present at its output and for supplying the or a second control signal.
14. A device according to any one of the previous claims, wherein the said amplifier circuit includes one or more band-pass amplifying stages.
15. A device according to claim 14, wherein the amplifier circuit includes at least two band-pass amplifying stages and that the stages are arranged to be activated one at a time according to the television channel required.
16. A device according to any one of the preceding claims, wherein the frequency of the output of the variable-frequency oscillator is equal to the sum of the said received signal frequency and the said first intermediate frequency.
17. A device according to any one of the preceding claims, wherein the frequency of the said fixed-frequency oscillator is equal to the difference between the said first intermediate frequency and the said second intermediate frequency.
18. A device according to any one of the preceding claims, wherein the said first intermediate frequency is equal to about 426 MHz.
19. A device according to any one of the preceding claims, wherein the second intermediate frequency is equal to about 36 MHz.
20. A television receiver tuning device constructed and arranged to operate substantially as hereinbefore described with reference to Figures 3 and 4 of the accompanying drawings.
21. A television receiver incorporating a device according to any one of the preceding claims.