Double tuned and band-switchable RF circuit with balanced output signals

Abstract: A band switchable double tuned circuit (16) having a balanced secondary is presented. A primary tunable tank circuit with a plurality of first inductances (28, 30) is tuned to a first frequency by a first capacitor (36, 40) coupled in parallel with the first inductances (28, 30). A secondary tank circuit with a plurality of second inductances (32, 34, 62) inductively coupled to ones of the first inductances (28, 30) has a second capacitor (38, 42) coupled in parallel with the second inductances (32, 34, 62) for tuning the secondary to the first frequency. The DC reference potential (50) for the secondary circuit is coupled to a nodal junction of the plurality of second inductances (32, 34, 62) with the secondary tank circuit providing a balanced signal output. At least one of the plurality of first inductances (28, 30) and the second inductances (32, 34, 62) are switchable for bandswitching the tuning of the primary and secondary tank circuits to a second frequency. The output signal from the balanced secondary tank circuit is coupled to the input of a balanced mixer (22) via constant bandwidth inductances (44a, 46a; 44b, 46b) without a mixer input balance transformer.

Description: The present invention relates to television tuners, and more particularly to double tuned circuits for converting a single ended RF signal to a balanced RF signal. Frequency selective tuned circuits are often used before and after RF amplifying stages of a television tuner. Current practice involves the use of a double tuned filter in the signal path before a mixer to further improve frequency selection and to provide better image rejection. An example of a television receiver with double tuned circuits coupled to a mixer is described, for example, by J. Y. Ma in US patent 3,962,643 which issued 8 June 1976. In this case the mixer is unbalanced and comprises a single mixer transistor coupled at the base thereof to an output of a doubled tuned RF circuit and to the output of a local oscillator transistor. Such a simple (unbalanced) mixer may be adequate for many purposes but balanced mixers are available (e.g., four quadrant multipliers or "Gilbert Cells") which offer improved mixing performance. However, typical active - balanced mixers require balanced input signals. It is desirable to drive the mixer in an optimum manner in order to maximize conversion efficiency, and reduce unwanted harmonics and image frequencies. It is common to drive a mixer, e.g., a doubly balanced mixer, with a transformer. An example of a balanced diode ring mixer driven by a transformer is described in an article entitled "Front-End For FM Receiver" published in ELEKTOR ELECTRONICS, vol. 14 (1988), No. 152 at page 69. However, a transformer is a relatively expensive component and difficult to optimize. The mixer of the present embodiment is a Gilbert cell mixer wherein the input circuit is arranged in a common base configuration. Such an arrangement provides a very consistent low input impedance with optimum gain, wide bandwidth, good stability and low noise when the signal input is driven from a balanced source. To this end, it is desirable to drive the mixer from a transformerless balanced source and to maintain this balanced drive when bandswitching between frequency bands. An example of a known double tuned circuit for driving a mixer without a coupling transformer is described by Lammermann and Rehak in their article entitled "PAL-SECAM-Tuner-ZF-Kombination für das CUC-Farbfernsehgerate-Chassis" published in GRUNDIG TECHNISCHE INFORMATIONEN, vol. 29, no. 3, 1982, Fürth, DE, pages 146-151. In this apparatus the secondary of a double-tuned circuit is AC coupled via a pair of DC blocking capacitors to respective inputs of subsequent mixer circuitry. To provide a DC reference for operation of the secondary tuning diodes, one end of the secondary winding is coupled to ground via an isolation inductor which blocks the AC (RF) signal but allows DC to pass to provide a DC reference for the tuning (varactor) diodes. One problem with Lammermann et al. arrangement is that the outputs coupled from the double tuned circuit are at different voltage levels (one being at ground, the other being at a tuning voltage level of a series varactor diode) and so the circuit is not DC balanced. This DC unbalance requires AC coupling to the subsequent mixer and so the mixer must be provided with its own DC bias circuitry. It is, therefore, an object of the invention to provide a double tuned arrangement which provides a balanced output signal with respect to a predetermined reference potential of the secondary tank circuit. A double tuned arrangement according to the present invention, which is defined in the appended claims, comprises a primary tank circuit (28,30,36) having an input and a secondary tank circuit (32,34,62,38) inductively coupled to the primary tank circuit and having an output. An inductance means (32,34,62) in the secondary tank circuit comprises first (32,34) and second (62) parts in series, wherein the inductance of the first part (32,34) and that of the second part (36) are substantially equal and the circuit elements are arranged for providing a balanced output signal with respect to a reference potential at the secondary tank circuit. Reference is made to the following drawings wherein like members have been assigned like designations, and wherein: Fig. 1 is a block diagram of a portion of a typical tuner of a television apparatus. Fig. 2 is a schematic diagram of the double tuned circuit of Fig. 1 according to the prior art. Fig. 3 is a schematic diagram of a double tuned circuit of an RF tuner according to aspects of the present invention. FIG. 1 shows a typical arrangement of a television tuner. The RF signal path typically includes, in the order named, an antenna 10 or other television signal source, a tuned circuit 12, an RF amplifier 14, and a tuned circuit 16 which desirably is double tuned in order to provide sufficient adjacent channel signal rejection. The output signal from tuned circuit 16 is coupled to a mixer 22 where it is mixed with a signal from local oscillator 24 for converting the RF signal to an IF signal 25. Tuned circuit 12, double tuned circuit 16, and local oscillator 24 are tuned by a tuning voltage (VT) to a selected channel or frequency. Tuning voltage (VT) typically is applied to varactor diodes. Bandswitching often is also provided for tuned circuit 12, double tuned circuit 16, and local oscillator 24, i.e., bandswitching between the VHF low frequency band of channels 2-6 (United States) and the VHF high frequency band of channels 7-13 (United States). Referring now to FIG. 2, there is shown a schematic diagram of a typical prior art arrangement for the double tuned circuit 16. Circuit 16 receives a signal from single ended RF amplifier 14, and couples the double tuned signal to the balanced input of a mixer 22. RF amplifier 14 typically is a MOSFET transistor 26 wherein the signal from tuned circuit 12 is coupled to gate electrode G1 with an RF AGC signal (not shown) being coupled to gate electrode G2. The source electrode S of transistor 26 is grounded and an amplified signal is available at the drain electrode D. In the lower frequency band, the signal from drain electrode D is applied to a double tuned circuit 16 comprised of: 1) a primary circuit of series connected inductances 28, 30 coupled between drain electrode D to an AC reference point and DC reference potential 50, and 2) a secondary circuit of series connected inductances 32, 34 coupled between a nodal point N1, and AC reference point and DC reference potential 50. There is a coefficient of mutual inductance "M" between inductances 28, 32 and between inductances 30, 34. The primary circuit is tuned to the desired frequency by varactor diode 36 coupled across inductances 28, 30 and the secondary circuit is tuned to about the same frequency by varactor diode 38 coupled across inductances 32, 34. The capacitance of varactor diodes 36, 38 is changed by application voltage Vt applied to their respective cathodes which are coupled to signal ground through respective capacitors 40, 42. The double tuned single ended output signal from secondary node N1 is coupled to mixer 22 through series connected inductors 44, 46 and balun transformer 48. the value of inductors 44, 46 are chosen to provide a constant bandwidth with frequency over the appropriate frequency band, as disclosed in U.S. Patent No. 3,628,152 of Carlson. One side of the primary of balun transformer 48 is coupled to reference point 50 with the secondary winding providing a balanced signal to the input terminals of mixer 22, which can be a single or doubly balanced mixer. Bandswitching is accomplished by PIN diodes 51, 52 coupled across respective inductances 30, 34. Diodes 51, 52 are switched by bandswitching control voltage BS1 applied to their respective anodes, which are signal grounded by respective capacitors 54, 56. In the switched "on" conductive state, diodes 51, 52 signal bypass respective inductances 30, 34, thus lowering the total inductance in the respective primary and secondary circuits, and raising the tuned frequency. When bandswitching, the inductive value of constant bandwidth inductances 44, 46 is accordingly changed by the series connection of PIN diode 58 and capacitor 60 coupled across inductance 44, said diode 58 also being switched by bandswitching voltage BS1. It should be noted that the tuning varactor diodes 36, 38 require a DC return path back to the DC reference potential 50 for their respective tuning voltage Vt and this DC return path is provided by their respective tuned inductances 28,30 and 32,34. In a similar manner bandswitching diodes 51, 52, and 58 require a DC return path for their respective bandswitching voltage BS1, with the return path for diode 51 being inductance 30, inductance 34 being the return path for diode 52, and inductances 44, 32, 34 being the return path for diode 58. An alternate DC return path for diode 58 is inductance 46 and the primary of balun transformer 48. A balun transformer, for providing a balanced drive to the mixer, is a relatively expensive component and the transformer can be ill behaved making performance optimization difficult across the wide tuning band. A better performance across the tuning band and more consistent performance from unit to unit can be provided by avoiding use of transformer 48 and direct driving mixer 22. This is particularly desirable since the mixer of the present embodiment is a doubly balanced Gilbert cell mixer, i.e., an MTI 3001T integrated circuit manufactured by the Philips Company of the Netherlands. This mixer is driven in a grounded base configuration at about a 50 ohm impedance across a wide band of frequencies. However, with the elimination of transformer 48, conversion is now provided by double tuned circuit 16. Reference is now made to Fig. 3 wherein like members shown in Fig. 2 have given like numeral designations. In the lower frequency band, the signal from drain electrode D is applied to double tuned circuit 16 comprised of: 1) a primary circuit of series inductances 28, 30 coupled between drain electrode D to AC reference point and DC reference potential 50, and 2) a secondary circuit of inductances 32, 34 and 62 coupled between nodal points N1 and N2. There is a coefficient of mutual inductance "M" between inductances 28, 32, and between inductance 30 and at least one of the secondary inductances 34 and 62. The inductance values of inductances 32 and 34 are the same as in the circuit arrangement shown in Fig. 2. However, the inductance of inductance 62 is equal to the sum of the inductance of series connected inductances 32 and 34. The primary circuit is tuned to the desired frequency by varactor diode 36 coupled across series connected inductances 28, 30 and the secondary circuit is tuned to about the same frequency by varactor diode 38 coupled across series connected secondary inductances 32, 34 and 62. In this arrangement, a double tuned balanced signal is provided between nodal points N1 and N2. The DC reference potential 50 is coupled to a nodal point N3, which is the junction of secondary inductances 34 and 62. Since the inductance of inductance 62 equals the sum of the inductance of inductances 32 and 34, connection of reference potential 50 is at the AC reference point for the balanced system and the AC grounding of node N3 does not change the AC signal characteristics of the secondary of double tuned circuit 16. The tuning of double tuned circuit 16 is accomplished by changing the capacitance of varactor diodes 36, 38 by application of tuning voltage Vt applied to their respective cathodes, which are AC coupled across their respective primary and secondary inductances by respective capacitors 40, 42. The balanced double tuned output signal from nodes N1, N2 is coupled to mixer 22 through respective series connected constant bandwidth inductors 44a, 46a, and 44b and 46b. The sum of the inductance of series inductances 44a,46a and 44b,46b are chosen for providing a signal of constant bandwidth with frequency, over the appropriate frequency band, to mixer 22. Such a constant bandwith arrangement is disclosed in U.S. Patent No. 3,628,152 of Carlson. Bandswitching is accomplished by PIN diodes 51 and 52 coupled across respective inductances 30, and 34, 62. Switching diodes 51, 52 are switched by bandswitching voltage BS1 applied to their respective anodes, with the AC circuit being completed by respective capacitors 54, 56. In the switched "on" conductive state, diodes 51, 52 signal bypass respective inductances 30, 34 and 62, thus lowering the total inductance in the respective primary and secondary circuits, and raising the tuned frequency. Since the signal from nodes N1, N2 is a balanced signal, changes made in one signal path must also be made in the other signal path in order that the signal remain balanced. The values of the sum of inductances 44a, 46a, and of 44b, 46b are accordingly changed by the switching "on" of respective PIN diodes 58a, 58b by bandswitching voltage BS1. Respective capacitances 60a, 60b complete the diode circuit. Tuning varactor diodes 36, 38 require a DC return path for tuning voltage Vt, and bandswitching diodes 51, 52, 58a and 58b require a DC return path for their respective bandswitching voltage BS1. This will now be discussed. In the bandswitched mode, the balanced signal at node N2 is isolated from DC ground return 50 by inductances 34 and 62, which are in an AC parallel arrangement. This prevents the signal at node N2 from being short circuited to ground while still providing a DC return path for the diodes 38, 52, 58a and 58b. This AC isolation of DC ground return 50 by the parallel connection of inductances 34 and 62 is effective even though inductance 30 of the primary circuit is signal short circuited by bandswitching diode 51. This is because the coefficient of mutual inductance "M" between inductance 30 and inductances 34, 62 goes to zero since there is no signal current flowing through inductance 30. Thus, an AC short circuit of inductance 30 is not reflected from the primary to the secondary circuit. In this way, the DC return path for diode 38 is through inductances 32 and 34, the DC return path for diode 52 is through inductance 34, the DC return path for diode 58a is through inductances 44a, 32, and 34, and the DC return path for diode 58b is through inductances 44b, and 62. As an additional matter, for the bandswitched higher frequency mode, the AC reference point of the balanced secondary signal shifts to a virtual node within inductance 32.

Claims: A double tuned arrangement for RF signal processing, including: a primary tank circuit (28, 30, 36) coupled to a source (14) of an unbalanced RF input signal; a secondary tank circuit (32, 34, 62, 38) inductively coupled to said primary tank circuit and including a first (32), a second (34) and a third (62) inductance connected in series having two end nodes (N1, N2), wherein for the purpose of band switching a third node between said first and second inductances can be AC short circuited with that one (N2) of said end nodes that belongs to said third inductance; the sum value of the inductance values of the first (32) and second (34) inductances being substantially equal to the inductance value of the third inductance (62), the first inductance (32) being coupled to an inductance (28) in the primary tank circuit, and at least one of the second (34) and third (62) inductances being coupled to another inductance (30) in the primary tank circuit; said values and the couplings being selected to provide mutually balanced RF signals at said end nodes (N1, N2) of the first to third inductances connected in series, regardless of said third node being AC short circuited or not AC short circuited, wherein the node (N3) between the second (34) and third (62) inductance is connected to DC and RF ground potential. Arrangement according to claim 1, wherein for the purpose of band switching: in response to a bandswitching voltage (BS1) a first diode (51) is used to AC short circuit the node between said inductance (28) and said another inductance (30) in said primary tank circuit with said DC and RF ground potential; in response to said bandswitching voltage a second diode (52) is used to AC short circuit said third node between said first (32) and second (34) inductances with that one (N2) of said end nodes that belongs to said third inductance (62). Arrangement according to claim 1 or 2, wherein first (44a, 46a) and second (44b, 46b) constant bandwidth means couple respective ones of said end nodes (N1, N2) to respective input terminals of a balanced load (22).

Inventors: PUGEL MICHAEL ANTHONY, PUGEL, MICHAEL ANTHONY
Applicants: THOMSON CONSUMER ELECTRONICS, THOMSON CONSUMER ELECTRONICS, INC.
Application year: 1995
Main classification: H04N5
IPC codes: H04N5, H03J5, H03J3