WO2001084709A1

WO2001084709A1 - Bandswitichable double tuned circuit

Info

Publication number: WO2001084709A1
Application number: PCT/GB2001/001897
Authority: WO
Inventors: Paul Jeffrey-Read; Takashi Maruyama
Original assignee: Alps Electric (Uk) Ltd.
Priority date: 2000-05-04
Filing date: 2001-05-01
Publication date: 2001-11-08
Also published as: GB2362049A; AU2001250546A1; GB2362049B; GB0010665D0

Abstract

A band switching double-tuned circuit for receiving analogue or digital television broadcasts is discribed. It is adapted to receive VHF low band and high band signals and comprises inductive coupling means for each band switching diode (D3, D4) connected at the input and output side and in series with a high band inductor connected to ground. The junction of each switching diode (D3, D4) and high band tuning inductor (L1, L4) are connected to a high band coupling inductor (L6) in order to inductively couple the input to the output. For low band RF response (L2, L5) are aligned, whereas for high band (L1) and (L4) are aligned. (L1) and (L4) can be physically aligned for high band, (by operator manipulation) without having to consider the image frequency. To set the correct image frequency for high band reception, inductor (L6) must be correctly aligned for optimum image rejection.

Description

BANDS ITICHABLE DOUBLE TUNED CIRCUIT

The present invention relates to a tuning circuit. More particularly, the present invention relates to a double tuned circuit adapted to receive both low band VHF signals and high band VHF signals.

Double tuned circuits are used in applications where high selectivity is required; for example in a television or video cassette receiver, or in any such device designed to receive analogue (PAL, NTSC or SECAM), or digital (DTTN or DCTN) television broadcasts. The task of the double tuned circuit is to pass a desired frequency while rejecting unwanted frequencies that may be present in a given signal. A typical circuit consists of two tuned circuits that can be coupled inductively, magnetically or capacitively. Capacitive coupling is not normally employed where tuning over a very large frequency range is required, as is the case with the present circuit.

Figure 1 shows schematically a NHF tuner for receiving NHF low band and NHF high band broadcast signals. The tuner includes an input for an aerial 1 , connected to an input circuit 2 for selecting a desired RF frequency.

An interface 9 provides low band (LB), high band (HB) and tuning (TU) voltage selection; LB or HB is selected by a DC voltage, and the receiving frequency is controlled by the TU voltage. The interface also allows automatic gain control (AGC) of the amplifier 3. The amplifier is, for example, a field effect transistor (FET) or other such suitable amplifier device.

The signal amplified by the FET is fed to the double tuned circuit 4, and the circuit is switched to the required band via the interface 9. The resonant frequency of the input circuit 2 is determined by the tuning voltage (TU).

The output of the double tuned circuit 4 and the output of a local oscillator 7 are both fed into an RF mixer 5.

An intermediate frequency (IF) output 10 from the mixer 5 can be equal to both the sum, and difference, of the desired RF and the local oscillator frequency, LO. The desired signal, RF = LO-IF, but undesired RF signals can be present at RF = LO + IF and will be present at the output from the RF mixer 5. This unwanted signal causes interference in the IF signal and is commonly known as the Image Frequency. It is desirable for the tuner to have sufficient image rejection capability to overcome this problem. A common solution is to employ an Image Frequency trap within the tuner. This trap will be placed before the mixer stage in the signal path.

The filter 6 receives the IF signal from the mixer 5 from where it is output to a second amplifier stage 8. In general a further demodulator stage will also be present, but for simplicity this is not shown.

Figures 2 to 4 show several examples of double tuned circuits which are able to tune within a single frequency band. Their primary task is to pass a desired frequency RF, and reject all other unwanted frequencies (eg Image Frequency) that may be present. The circuits shown consist of two tuned circuits coupled by either an inductor or a magnetic field.

Common to all three circuits are variable capacitance diodes Dl, D2 and bypass capacitors Cl, C2. To tune the circuit to a specific frequency, a tuning voltage must be applied to the terminals TU.

Dl and D2 are in reverse bias mode and their respective capacitance's vary according to a changing tuning voltage. The resistors Rl and R2 are of sufficiently high magnitude that the flow of the RF signal is blocked from the circuit.

Capacitor C7, which is not standard in a double tuned circuits, is employed as a frequency trap at the Image Frequency of the circuit.

At the input of each of the circuits of Figures 2 to 4, capacitor Cl and variable capacitance diode Dl are in parallel with an inductor LI. These form a first tuned circuit. The second tuned circuit is formed at the output by C2, D2 and an inductor L4; C2 and D2 are in parallel with L4.

The tuned circuits will have resonant frequencies which are determined by the respective values of the inductors LI and L4 and the variable capacitance diodes Dl and D2. The inductance of LI and L4 can be altered by adjustments to the windings of each; this will alter the resonant frequency of each tuned circuit. In practice, suitable adjustments are made to LI and L4 until the two resonant frequencies are aligned sufficiently to produce a double tuned response.

A magnetically coupled double tuned circuit is shown in Figure 2. Here, there is no physical connection between the inductors LI and L4. The respective tuned circuits of LI and L4 are situated with respect to one another in such a way that they magnetically couple to one another. Thus, the RF signal is able to pass through the circuit.

Figure 3 shows a double tuned circuit where there is no magnetic coupling between the input and output tuned circuits. In this case, LI and L4 are situated sufficiently remote from one another in order that stray magnetic coupling is minimised. Coupling is achieved inductively via an inductor L6.

Figure 4 is a double tuned circuit that also utilises inductive coupling. However, in place of inductors LI and L4 there are inductors L2, L3 and L5. Inductor L3 is in series with both the input circuit inductor L2, and the output circuit inductor L5. In the input tuned circuit L2 and L3 are connected in parallel with Cl and Dl. Likewise, in the output tuned circuit, L3 and L5 are in parallel with C2 and D2. The magnitude of inductor L3 will be small compared with the magnitudes of inductors L2 and L5. As such, L3 will have only a small influence on the overall series inductance of the two tuned circuits. Nevertheless, it is inductor L3 which produces the requisite coupling for the double tuned circuit.

A presently available double tuned circuit is shown in Figure 5. This circuit also includes two variable capacitance diodes Dl and D2. It additionally includes band- switching diodes D3 and D4. With the circuit configured as shown, the tuner can switch to receive either low band or high band signals, as follows:

High Band Reception

Band-switching diodes D3 and D4 are set to forward bias modes, thus effectively removing L2, L3 and L5 from the circuit by connecting LI and L4 to ground. The circuit is now equivalent to that shown in Figure 2, and the input and output tuned circuits are magnetically coupled.

Low Band Reception

With diodes D3 and D4 in reverse bias mode they become high impedance circuit elements. The diodes are reverse biased (as opposed to no bias) to reduce the stray capacitance of the diodes. LI and L4 are not shorted to ground and are therefore connected in series with inductors L2 and L5 respectively. Inductor L3 is in series with L2 and L5, and is connected to ground.

The inductance values of LI and L4 are small compared to the values of L2 and L5;

LI and L4 exert only a small inductive influence. The circuit when switched for low band frequencies is equivalent to that shown in Figure 4. Equivalent components to the circuit of Figure 5 are given the same reference numeral.

In the prior circuit of Figure 5, inductors LI and L4 must be correctly physically aligned with respect to one another in order that the required radio frequency (RF) response of the circuit is obtained; this is generally achieved physically by making subtle adjustments to the inductor loops during the manufacture of the circuit; an operator physically manipulates the loops until the correct response is achieved.

Another problem arises due to the magnetic coupling between LI and L4. In order that the image frequency of the circuit is correctly set, the gap between LI and L2 must also be adjusted, again by physical manipulation by an operator. However, this is difficult to accomplish without effecting the inductance which in turn alters the circuit resonant frequency, and consequently disturbs the RF response discussed above.

It takes considerable practice for production line operatives to gain the skill required to effect both adjustments within a production time limit to optimise the RF response and the image frequency setting. Consequently, consistency in alignments of these circuits is difficult to achieve.

The present invention provides a double tuned circuit that can be switched to receive low band and high band NHF signals, and which can be independently optimised for both RF response and image frequency.

According to an aspect of the present invention there is provided a double tuned circuit comprising: a first capacitor and a first variable capacitance element connected in series between a signal input line and a ground line; a first switching element connected in series to a first inductor, the first switching element and the first inductor connected in parallel with said first capacitor and said first variable capacitance element; a second capacitor and a second variable capacitance element connected in series between a signal output line and said ground line; a second switching element connected in series with a second inductor, the second switching element and the second inductor connected in parallel with said second capacitor and said second variable capacitance element; a third inductor and a fourth inductor connected in series between said signal input line and said ground line; a fifth inductor connected between a junction between said third and said fourth inductors, and said signal output line; a sixth inductor connecting a junction between said first switching element and said first inductor, with a junction between said second switching element and said second inductor; wherein the double tuned circuit is tuned for high band NHF signals when the first and the second switching elements are switched on, and the double tuned circuit is tuned for low band NHF signals when the aforesaid switching elements are switched off, and the signal input line and a signal output line are inductively coupled when tuned for high band NHF or low band NHF by the sixth inductor and the fourth inductor respectively.

Preferably, the first and second variable capacitance elements are variable capacitance diodes.

Preferably, the first and second switching elements are diodes.

Preferably, the first and second switching elements are transistors or any such other discreet switching device.

According to another aspect of the present invention there is provided a double tuned circuit comprising: a first capacitor and a first variable capacitance element connected in series between a signal input line and a ground line; a first switching element connected in series to a first inductor, the first switching element and the first inductor connected in parallel with said first capacitor and said first variable capacitance element; a second capacitor and a second variable capacitance element connected in series between a signal output line and said ground line; a second switching element connected in series with a second inductor, the second switching element and the second inductor connected in parallel with said second capacitor and said second variable capacitance element; a third inductor and a fourth inductor connected in series between said signal input line and said ground line; a fifth inductor connected between a junction between said third and said fourth inductors, and said signal output line; wherein the double tuned circuit is tuned for high band NHF signals by magnetic coupling between the first and second inductors, or by inductive coupling between the third, fourth and fifth inductors when turned for low band NHF signals.

According to another aspect of the present invention there is provided a double tuned circuit comprising: a first capacitor and a first variable capacitance element connected in series between a signal input line and a ground line; a first switching element connected in series to a first inductor, the first switching element and the first inductor connected in parallel with said first capacitor and said first variable capacitance element; a second capacitor and a second variable capacitance element connected in series between a signal output line and said ground line; a second switching element connected in series with a second inductor, the second switching element and the second inductor connected in parallel with said second capacitor and said second variable capacitance element; a third inductor and a fourth inductor connected in series between said signal input line and said ground line; a fifth inductor connected between a junction between said third and said fourth inductors, and said signal output line; a sixth inductor connecting a junction between said first switching element and said first inductor, with a junction between said second switching element and said second inductor; a third switching element connected in series to an eighth inductor, the third switching element and the eighth inductor connected in a parallel with said first capacitor and said first variable capacitance element, and connected in parallel with said first switching element and said first inductor; a fourth switching element connected in series to a ninth inductor, the fourth switching element and the ninth inductor connected in parallel with said second capacitor and said second variable capacitance element, and connected in parallel with said second switching element and said second inductor; and a seventh inductor connected in series between the third switching element and the fourth switching element, and connected in series between the eighth and ninth inductors; wherein the double tuned circuit is tuned for low band NHF, mid band NHF, or high band NHF by inductive or magnetic coupling of the signal input line and the signal output line.

According to the invention there is further provided a band switching double-tuned circuit for receiving analogue or digital television broadcasts and adapted to receive NHF low band and high band signals, said circuit comprising inductive coupling means for each band, and switching devices connected at the input and output side and in series with a high band running inductor connected to ground, whereon the junction of each switching device and high band tuning inductor are connected to a high band coupling inductor.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows a schematic block diagram of a NHF tuner incorporating a double tuned circuit;

Figure 2 shows a magnetically coupled double tuned circuit for tuning within a single frequency band; Figure 3 shows an inductively coupled double tuned circuit for tuning within a single frequency band;

Figure 4 shows an alternative inductively coupled double tuned circuit for tuning within a single frequency band;

Figure 5 shows presently available double tuned circuit for low band and high band NHF tuning;

Figure 6 is a preferred embodiment of a double tuned circuit;

Figure 7 shows another embodiment of a double tuned circuit; and

Figure 8 shows a further embodiment of a double tuned circuit.

A preferred embodiment of a double tuned circuit is shown in Figure 6. This circuit is also switchable between low and high band frequencies. In Fig 6, components corresponding to components of Fig 5 are accorded like reference numerals. High Band and Low Band reception is achieved as follows:

High Band Reception

Selecting high band tuning is achieved by making terminal HB positive with respect to terminal LB. This operation sets diodes D3 and D4 to forward bias due to biasing resistors R3 and R7.

Resonance will occur in the input tuned circuits contaming Cl, Dl and LI and will also occur in the output tuned circuit containing C2, D2 and L4. L2 and L5 are large in comparison with LI and L4 and can be neglected in the input and output tuned circuits in high band operation. Consequently, the input and output resonant circuits are inductively coupled via inductor L6. The circuit in high band operation is equivalent to that which is shown in Figure 3.

Low Band Reception

Diodes D3 and D4 are set to reverse bias mode by making terminal LB positive with respect to terminal HB. This operation will increase the impedance of the diodes greatly by reducing their capacitance.

Consequently, LI, L4 and L6 can be neglected when the circuit is set for low band signals. Resonance is between L2 and L5 and parallel capacitors Cl and C2 in respective input and output circuits. In this instance inductive coupling is achieved via inductor L3. An equivalent circuit is shown in Figure 4.

Setting Image Frequency and RF response

For low band RF response L2 and L5 are aligned, whereas for high band LI and L4 are aligned. LI and L4 can be physically aligned for High Band, (by operator manipulation) without having to consider the image frequency.

To set the correct Image Frequency for high band reception, inductor L6 must be correctly aligned for optimum image rejection. However, when aligning L2 and L5 for RF response in low band there will be consequent changes to the Image Frequency set for high band.

To counter this, L2 and L5 are aligned for optimal low band response first, then LI and L4 and L6 are adjusted for high band response and high band image rejection last. This method negates having to shift the image trap frequency again, since no further adjustments of L2 and L5 are necessary. Although there is an Image Frequency for low band, no image trap alignment is required since the trap location is at a higher frequency than Image Frequency. Given the high 'Q' factor of the circuit when switched for low band tuning, there is sufficient attenuation and the image trap can be ignored.

The main advantages of the circuit of Figure 6 can be summarised as follows: ξ Both low band and high band tuning utilises inductive coupling. ξ LI and L4 can be aligned for high band reception without needing to consider the image frequency. ξ Image frequency alignment requires adjustment of L6 only ξ Alignment consistency and time taken to align is improved ξ Only two variable capacitance diodes and two band-switching diodes are required; prior art circuits which can switch between low and high band, and which utilise inductive coupling in both modes would require additional diodes.

Figures 7 and 8 show alternative embodiments of a double tuned circuit.

The circuit shown in Figure 7 is identical to that shown in Figure 6 save for the absence of inductor L6. This circuit can operate in the high band regime by either magnetically coupling LI and L4, or by inductively coupling the input circuit of L2 with the output circuit of L5 via coupling inductor L3.

Figure 8 shows another double tuned circuit which is the same as the circuit in Figure 6 with two additional switching diodes D5 and D6, and additional inductors L7, L8 and L9. This circuit can switch between three NHF bands: low band, high band and mid band.

Claims

CLAΓMS

1. A double tuned circuit comprising: a first capacitor and a first variable capacitance element connected in series between a signal input line and a ground line; a first switching element connected in series to a first inductor, the first switching element and the first inductor connected in parallel with said first capacitor and said first variable capacitance element; a second capacitor and a second variable capacitance element connected in series between a signal output line and said ground line; a second switching element connected in series with a second inductor, the second switching element and the second inductor connected in parallel with said second capacitor and said second variable capacitance element; a third inductor and a fourth inductor connected in series between said signal input line and said ground line; a fifth inductor connect between a junction between said third and said fourth inductors, and said signal output line; a sixth inductor connecting a junction between said first switching element and said first inductor, with a junction between said second switching element and said second inductor; wherein the double tuned circuit is tuned for high band NHF signals when the first and the second switching elements are switched on, and the double tuned circuit is tuned for low band NHF signals when the aforesaid switching elements are switched off, and the signal input line and a signal output line are inductively coupled when tuned for high band NHF or low band NHF by the sixth inductor and the fourth inductor respectively.

2. A double tuned circuit as claimed in claim 1, wherein the first and second variable capacitance elements are variable capacitance diodes.

3. A double tuned circuit as claimed in claim 1 or 2, wherein the first and second switching elements are diodes.

4. A double tuned circuit as claimed in claim 1 or 2, wherein the first and second switching elements are transistors.

5. A double tuned circuit comprising: a first capacitor and a first variable capacitance element connected in series between a signal input line and a ground fine; a first switching element connected in series to a first inductor, the first switching element and the first inductor connected in parallel with said first capacitor and said first variable capacitance element; a second capacitor and a second variable capacitance element connected in series between a signal output line and said ground line; a second switching element connected in series with a second inductor, the second switching element and the second inductor connected in parallel with said second capacitor and said second variable capacitance element; a third inductor and a fourth inductor connected in series between said signal input line and said ground line; a fifth inductor connected between a junction between said third and said fourth inductors, and said signal output line; wherein the double tuned circuit is tuned for high band NHF signals by magnetic coupling between the first and second inductors, or by inductive coupling between the third, fourth and fifth inductors when tuned for low band NHF signals.

6. A double tuned circuit as claimed in claim 5, wherein the first and second variable capacitance elements are variable capacitance diodes.

7. A double tuned circuit as claimed in claim 5 or 6, wherein the first and second switching elements are diodes.

8. A double tuned circuit as claimed in claim 5 or 6, wherein the first and second switching elements are transistors.

9. A double tuned circuit comprising: a first capacitor and a first variable capacitance element connected in series between a signal input line and a ground line; a first switching element connected in series to a first inductor, the first switching element and the first inductor connected in parallel with said first capacitor and said first variable capacitance element; a second capacitor and a second variable capacitance element connected in series between a signal output line and said ground line; a second switching element connected in series with a second inductor, the second switching element and the second inductor connected in parallel with said second capacitor and said second variable capacitance element; a third inductor and a fourth inductor connected in series between said signal input line and said ground line; a fifth inductor connected between a junction between said third and said fourth inductors, and said signal output line; a sixth inductor connecting a junction between said first switching element and said first inductor, with a junction between said second switching element and said second inductor. a third switching element connected in series to an eighth inductor, the third switching element and the eighth inductor connected in a parallel with said first capacitor and said first variable capacitance element, and connected in parallel with said first switching element and said first inductor; a fourth switching element connected in series to a ninth inductor, the fourth switching element and the ninth inductor connected in parallel with said second capacitor and said second variable capacitance element, and connected in parallel with said second switching element and said second inductor; and a seventh inductor connected in series between the third switching element and the fourth switching element, and connected in series between the eighth and ninth inductors; wherein the double tuned circuit is tuned for low band NHF, mid band NHF, or high band NHF by inductive or magnetic coupling of the signal input line and the signal output line.

10. A double tuned circuit as claimed in claim 9, wherein the first and second variable capacitance elements are variable capacitance diodes.

11. A double tuned circuit as claimed in claim 9 or 10, wherein the first, second, third and fourth switching elements are diodes.

12. A double tuned circuit as claimed in claim 9 or 10, wherein the first, second, third and fourth switching elements are transistors.

13. A band switching double-tuned circuit for receiving analogue or digital television broadcasts and adapted to receive NHF low band and high band signals, said circuit comprising inductive coupling means for each band, and switching devices connected at the input and output side and in series with a high band tuning inductor connected to ground, wherein the junction of each switching device and high band tuning inductor are connected to a high band coupling inductor.

14. A double tuned circuit substantially as hereinbefore described with reference to and as illustrated by any of figures 6, 7 or 8.