DE1249360B - Constant bandwidth tuning circuit - Google Patents

Description

ESREPUBLIC OF GERMANY

CHES

PATENT OFFICE

ÄDSLEGESCHRIFT

Int. Cl .:

H03h

HO3j
German class: 21 a4- 29/01

Number: 1 249 360

File number: W 38959IX d / 21 a4

Filing date: April 12, 1965

Opened on: September 7, 1967

The presently built television receivers almost all have a tuning circuit with switchable Inductance to select channels 2 to 13 in the VHF range. The channel selector with switchable inductance has the advantage of a relatively simple structure, since it is generally only one light modified parallel resonant circuit acts. This voting circle also has the advantage of being constant Bandwidth independent of the frequency that is theoretically required for all channels. The channel selector with switchable inductance, however, has the major disadvantage that mechanical switching contacts have to be switched on the different inductances must be used in the tuning circuit.

It would of course be very advantageous if there was continuous tuning without switching different inductance values would be possible. Up until now it has not been possible to find a changeable one Build inductance with sufficient tuning range in the entire VHF band. Namely there the center frequency of channel 2 is 57.5 MHz and the center frequency of channel 13 is 213.5 MHz a ratio of at least 13.5 between the maximum value and the minimum value of the inductance im Voting circle can be achieved. Such a strong change in the inductance values can only be achieved today by switching on different coils for the different channels can be achieved. The use of changeover contacts but is expensive, and in addition, the changeover contacts pollute proportionally short operating times and wear out, causing reception problems. The channel selector with switchable inductor is therefore unreliable and requires constant and frequent maintenance Repairs.

Rotary capacitors with a capacitance ratio of 13.5 and more are relatively easy to construct. The use of a variable capacitor as a tuning element would therefore be continuous Enable voting without switching over in the entire VHF range. When using capacitive tuning however, the bandwidth of the tuning circuit is normally proportional to the square of the resonance frequency. Since, for theoretical reasons, the same bandwidth should prevail in all channels, as a result for practical reasons, capacitive tuning cannot be used. For example In a capacitive tuning circuit at 213.5 MHz the bandwidth is about 14 times as large as at 57.5 MHz assuming a constant load. Because of the saving of the changeover contacts would be so constant capacitive tuning is very important

Constant bandwidth tuning circuit

Applicant:

Westinghouse Electric Corporation,

East Pittsburgh, Pa. (V. St. A.)

Representative:

Dipl.-Ing. G. Weinhausen, patent attorney,

Munich 22, Widenmayerstr. 46

Named as inventor:

Office P. Arntsen, Murrysville, Pa. (V. St. A.)

Claimed priority:

V. St. v. America April 14, 1964 (359 561)

wishes if the bandwidth change problem could be solved.

These considerations also apply to a known constant bandwidth tuning circuit with a tunable parallel resonance circuit, which via coupling elements on the one hand with an antenna circuit and on the other hand is connected to a transistor amplifier, the coupling element between the resonant circuit and the transistor consists of a coil.

It is true that this coupling coil has a special Step-up transformation of the input resistance that occurs at the high frequencies of the frequency range of the transistor in the resonant circuit has an approximately constant bandwidth in the frequency range achieved, but the circuit assumes that the bandwidth of the tuning circuit itself remains constant, that it is again a tuning circuit with continuously or step-wise variable inductance acts, which has the disadvantages described above.

A constant bandwidth tuning circuit for a transistor amplifier is also known which consists essentially of a capacitively tunable series resonant circuit. This voting circuit, which can only be used for transistors in an emitter-base circuit, shows more in the broadcasting sector or less constant bandwidth, but is in the much larger tuning range for a television input circuit not applicable. In addition, the tuning circuit of this known circuit must necessarily be lossy because its series loss resistance to the input resistance of the transistor should be adapted.

709 640/185

The object of the invention is to provide a constant bandwidth tuning circuit for a transistor amplifier to create that is free from these drawbacks and limitations.

The inventive tuning circuit of constant bandwidth with a tunable parallel resonant circuit, the via coupling members on the one hand with an antenna circuit and on the other hand with a Transistor amplifier is connected, is characterized in that the parallel resonance circuit is capacitive is matched and that both coupling elements show inductive behavior and are dimensioned so that the Equivalent resistances of the antenna circuit and the transistor at the connection point of the parallel resonance circuit are matched to one another and are proportional to the square of the frequency.

By means of the measures specified, it is possible to create a capacitively tunable tuning circuit for To build transistor amplifiers that have an essentially constant bandwidth throughout the television sector possesses, as will be explained in detail below.

The invention is explained below with reference to the drawing. In here is

F i g. 1 shows a basic circuit diagram of the invention,

F i g. 2 an equivalent circuit diagram with more details,

F i g. 3 shows another embodiment of the invention and

F i g. 4 is a circuit diagram of the entire tuning device according to the invention.

F i g. 1 shows very schematically a tuning device which, for. B. is intended for a television receiver. The receiving antenna 10 is connected to an input coupling circuit 20. A tunable parallel resonant circuit 30 is connected to the output of the coupling circuit 20. The tuning circuit 30 contains a variable capacitor C ₁ parallel to an inductance L ₁ and is grounded on one side. A transistor coupling circuit 40, which feeds the signals to a transistor high-frequency amplifier 50, is connected to the output of the tuning circuit. The output signals are picked up at terminal T _{0 of this transistor amplifier.}

If the point of the tuning circuit 30 indicated by the arrow A is considered, maximum energy transfer is obtained when the parallel resistance in direction B is equal to the parallel resistance in direction C. The adjustment also almost agrees with that for the best signal-to-noise ratio. The circular quality Q of the tuning circle can be written as follows:

Since C _{1 is} proportional to - ^ r ₂ , we get: R _p is proportional to _λΐΜ . So if the bandwidth AW

Here R _{v is} the total parallel resistance at tuning circuit 30 from direction B and direction C and W is the resonance frequency of tuning circuit 30. If the bandwidth of the tuning circuit is denoted by Δ W, then on the other hand the following applies

^U ~ AW '

If you put this in the first equation, the result is

¹ -RC

is to remain constant, then R _v must change proportionally to the square of the frequency. The parallel resistances seen in the direction B and C in FIG. 1 should be twice as large as R _v in the above derivation and should change proportionally to W ² if constant bandwidth Δ W is prescribed. Furthermore, the resistance in direction B should be the same as the resistance in direction C at point A at all frequencies in order to achieve optimal adaptation.

So that the resistance R _v occurring at the tuning circuit 30 is proportional to the square W ^{z of} the tuning frequency, the input coupling circuit 20 must be designed so that the ohmic component of the input impedance of the antenna 10 at the tuning circuit 30 appears proportional to W ² . Furthermore, the

ao coupling circuit 40 be designed so that the ohmic component of the input resistance of the transistor amplifier 50 appears on tuning circle 30 proportional to the square of the tuning frequency. aside from that should both be transferred to the voting group

a5 carry resistance components as equally as possible be.

F i g. 2 shows a schematic circuit which fulfills the above conditions. The ohmic component of the input impedance of the antenna 10 is represented by a resistor R ₁ . The resistor R ₁ is connected to a transformer TF , the primary winding L _{v of which has} a grounded center tap. The secondary winding L _{s of} the transformer TF is grounded on the one hand and connected in series with a choke coil L ₂ on the other hand. The other end of the choke coil L ₂ is connected to the Abs immkreis 30. The resistance component R ₁ of the antenna 10 appears _s at the secondary coil L as transformed resistive component R ₁ '. The losses in C ₁ and L ₂ should be small compared to the power transmitted from the antenna 10 to the transistor amplifier 50. For the sake of simplicity, the components C ₁ and L _{2 are} regarded as lossless in the following. This results in the apparent parallel resistance in direction B at point A.

- R ₁ +

W ² L ₂

If R ₁ is made small by stepping down and L _{2 is selected to be} large, the apparent resistance at tuning circuit 30 in direction B is approximately proportional to the square W ^{2 of} the resonance frequency. This is the required condition for maintaining a constant bandwidth Δ W.

The circuit to the left of line A including tuning circuit 30 can thus be seen as an antenna coupling circuit which has a constant bandwidth because the ohmic component of the antenna is transmitted to the tuning circuit in such a way that the ohmic resistance at this point is equal to the square of the resonance frequency changes. A capacitor C _{3 shown in} dashed lines can be attached parallel to the DROSSEIL ₂ so that the band ireteAW can be reduced somewhat at high frequencies in the vicinity of the channel 13.

Assuming that the input transistor of the high frequency amplifier 50 is used in common base will, as shown in FIG. 4 has the transistor

an input impedance that has an inductive and an ohmic component. The equivalent circuit diagram of this input impedance is shown in FIG. 2 shown in the dash-dotted frame as a parallel connection of an inductance L ₄ and a resistor R ₂ . A transistor coupling circuit 40 is inserted between the tuning circuit 30 and the parallel connection of L ₄ and R _z , which consists of the parallel connection of a choke coil L ₃ and a capacitor C ₂ .

In order to have constant bandwidth for the entire circuit of FIG. 2, the resistance component R ₂ must also be transferred to the tuning circuit 30 in such a way that it increases with the square of the resonance frequency. In order to see that this is the case in the present case, one first thinks away from the capacitor C ₂ . Then the parallel resistance at point A, seen in direction C, can be written as follows:

Λ η C - Λ! ~ Γ - ^ ~ ί;

r + 2

L ₃

L ₃ L ₄

- +2

It can be seen from this that, given a small ohmic resistance R _2, the apparent resistance R _P c in direction C is approximately proportional to the square if suitable values are selected _{for L 3} and L _4. Since the above conditions are not always completely fulfilled and the input impedance of the transistor changes with the frequency, ie the inductive component L ₄ and the ohmic component R ₂ depend on the frequency, it may be necessary to _{connect the capacitor C 2 in} parallel to the inductance L. ₃ so that R _p c can be approximated as closely as possible to R _v b over the entire frequency range. Since the sign of the reactance of the capacitor C ₂ is opposite to the sign of the reactance of the inductance L ₃ , the reactance of L ₃ increases overall with increasing frequency.

From the above it follows that the _{equivalent resistance corresponding to the resistance -R 2} at the tuning circuit 30 increases as desired with the square of the resonance frequency, so that an essentially constant bandwidth results in this direction as well. The circuit to the right of the line A with the tuning circuit 30 can be referred to as a load coupling circuit, which results in an essentially constant bandwidth independent of the tuning frequency of the parallel circuit 30.

There is also adaptation at point A, since the apparent resistances to the right and left of this point are equal to one another. On the other hand, if the correct matching is present at point B, correct matching is also present at point D between resistor R ₁ and primary winding L ₂ if transformer TF and choke coil L _{2 are} lossless. This requirement applies in the vicinity of the resonance frequency W.

F i g. 3 shows a modification of the antenna coupling circuit to the left of line A in FIG. 2. By circuit analysis in a known manner it can be shown that the transformer TF with the coils L ₁ and L ₂ can be transformed into a single transformer TF '. This transformer TF ' has a relatively loose coupling (ie the coupling factor is less than one) between the primary winding Lp, which has a grounded center tap, and the secondary winding Lj ₂ . A tuning capacitor C ₁ ', which is used to tune the resonant circuit, is located parallel to the secondary winding L _12. The loose coupling between the primary winding L _v ' and the secondary winding L ₁₂ is such that the coupling factor depends somewhat on the transformation constants which were used to achieve the desired results in the relevant frequency range. The circuit according to FIG. 3 is completely equivalent to that of FIG. 2, except that the components are arranged differently and have different values. Here, too, the resistance component A _{1 of} the antenna is transformed to the input of the tuning circuit, which now consists of the capacitor C ₁ 'and the inductance L' ₁₂ , in such a way that the bandwidth remains essentially constant. The coupling circuit 40 and the transistor amplifier 50 can be added unchanged in the direction C in order to achieve a complete tuning device according to FIG. 1 and 2 to form.

F i g. Figure 4 shows an embodiment of a complete television tuning device according to the invention. The radio frequency signals are received by the antenna 10, which can be a normal television antenna with an input resistance of 300 ohms. The signals are sent to the primary winding L _{v of} the transformer TF via an antenna line, which can also have 300 ohms. The resistance component R _{1 of} the impedance of the antenna 10 is indicated by a dashed line parallel to the primary winding Lp. The signals then pass through the transformer TF and the choke L ₂ to the tuning circuit 30. The capacitor C ₁ is adjustable so that the tuning circuit is tuned to at least resonance frequencies between 57.5 (channel 2) and 213.5 MHz (channel 13) without having to switch over. Since the antenna coupling circuit, the tuning circuit and the load circuit have very small losses compared to the energy passing from the antenna to the transistor, the incoming signals are passed almost undamped to the emitter electrode of a transistor TR which has a common base circuit. A stabilizing resistor R ₃ is connected in series with the emitter electrode and the choke coil L _{3 of} the output coupling circuit 43. A relatively large capacitor C ₄ is connected in parallel to the stabilization _{resistor A 3} as a coupling capacitor. The positive terminal ηϊ of a battery E is connected to the base of the transistor TR via a series resistor R-, while another series resistor A _{4 is connected} between the base and ground in order to generate the base bias. A relatively large capacitor C ₅ is connected in parallel to the series resistor .R _{4 in} order to ground the base in terms of alternating current. A load, which is shown schematically as a resistor Rl , lies between the collector electrode and the base electrode of the transistor TR. In this circuit, the transistor has an input resistance which has an ohmic and an inductive component, as shown schematically in FIG. 2 and 3 is shown.

If the variable capacitor C _{1 is} tuned to different channels, the bandwidth of the tuning circuit remains essentially constant, since the ohmic components R ₁ and R _{2 of} the antenna impedance or the transistor impedance are transmitted to the tuning circuit 30 in such a way that they correspond to the square of the Change tuning frequency W. Furthermore, since the parallel resistances to the right and left of the

Tuning circuit 30 are essentially the same, there is adaptation for all frequencies, so that maximum Energy transfer from the antenna to the transistor as well as a good signal-to-noise ratio are guaranteed. Furthermore, the antenna line is almost finished with its wave resistance, so that there is a small proportion standing waves on the antenna cable results.

Claims (3)

Patent claims: 1. Tuning circuit of constant bandwidth with a tunable parallel resonant circuit which is connected via coupling elements on the one hand to an antenna circuit and on the other hand to a transistor amplifier, characterized in that the parallel resonant circuit (30) is capacitively tuned and that both Kooolungsglieder (20, 40) show inductive behavior and are dimensioned so that the equivalent resistances (R _p b, Rpc) of the antenna circuit (10) and the transistor (50) at the connection point (A) of the parallel resonance circuit are matched to one another and are proportional to the square of the frequency. 2. Tuning circuit according to claim 1 with a transistor amplifier in base circuit, the input impedance of which by the parallel connection of an ohmic resistor (R ₂ ) and one Inductance (L ₄ ) is represented, characterized in that the two coupling _{elements consist of inductances (L 2} , L ₃ ) and with an ohmic component (R ₁ ') of the antenna circuit at the input of the first coupling element (L ₃ ) by the following relationships given are: R _v b = R _p c = where W is the angular frequency and L _{2 is} large compared to R ₁ ' and A _{2 is} small. 3. Tuning circuit according to claim 1 or 2, characterized in that parallel to one or both coupling inductances (L ₂ , L ₃ ) capacitances (C ₃ , C ₂ ) to adapt the apparent resistances are to those of the parallel resonant circuit. Considered publications:
Austrian Patent No. 232 559;
U.S. Patent No. 2,889,453. 1 sheet of drawings 709 640/18? 8.67 ® Bundesdruckerei Berlin