GB2025178A

GB2025178A - Mixing and bandswitching in a multi-band tuner system

Info

Publication number: GB2025178A
Application number: GB7921343A
Authority: GB
Original assignee: Arris Technology Inc; General Instrument Corp
Current assignee: Arris Technology Inc
Priority date: 1978-07-07
Filing date: 1979-06-19
Publication date: 1980-01-16
Also published as: JPS5511699A; GB2025178B; FR2430695B1; DE2927225C2; FR2430695A1; DE2927225A1

Abstract

To perform mixing and bandswitching in a multi-band tuner, bandswitching takes place at the IF frequency such that diodes 10, 12, preferably of the hot carrier or Schottky type, can be used to perform both mixing and bandswitching functions. The tuner has first and second oscillation signals for tuning within the UHF and VHF bands, respectively. A first diode 10, shiftable between non-conducting and conducting states, is operably connected to receive and mix the first oscillation signal and the received VHF radio frequency signal. A second diode 12, shiftable between non-conducting and conducting states, is operably connected to receive and mix the second oscillation signal with the received UHF radio frequency's signal. The diodes are conditioned, in accordance with the receipt of bandswitching signals, to render a selected one thereof conducting, while the other diode is maintained in its non-conducting state. The diode outputs are connected through a bandpass filter 40-52, including a relatively large capacitance bypass capacitor, to an amplifier circuit 54. <IMAGE>

Description

SPECIFICATION Mixing and bandswitching in a multi-band tuner system The present invention relates to apparatus for mixing and bandswitching for use in a multiband tuner system.

A particular, though not exclusive, application of the invention is to tuner systems for television receivers capable of receiving signals in both the VHF and UHF bands, the signals being converted to a common intermediate frequency (I.F.). The following section of the description will start with a discussion of present practices in television tuners for the VHF and UHF bands and lead on to an outline of practices proposed in a circuit embodying the present invention. The proposed circuit is described in greater detail subsequently.

Mixing circuits of a variety of different structures, capable of converting a band of received radio frequency signals into a corresponding band of intermediate frequency signals, are well known. In the VHF tuner portion of a modern television receiver capable of tuning both the VHF and UHF bands, a mixer circuit is utilized to convert the radio frequencies associated with each received VHF television channel to a common IF band. In the UHF tuner portion of the receiver, a separate mixer circuit is utilized to convert the radio frequencies associated with the received UHF television channel to the common IF band.A local oscillator circuit is provided to generate separate local oscillator signals to each of the mixers such that each mixer can combine the appropriate local oscillation signal with the received radio frequency signal to produce the necessary intermediate frequency signals.

Separate bandswitching circuitry is required to actuate the appropriate mixing circuit for the band being tuned. Thus, when a channel in the VHF band is chosen, the bandswitching circuit generates a bandswitching signal causing the VHF mixer to be operative. When a channel in the UHF band is selected, the bandswitching circuitry generates the necessary bandswitching signals to render the UHF mixer operational. The bandswitching signals are generated in a variety of known ways, such as by decoding the channel selection signals generated by manipulation of a channel selector.

When the bandswitching function is included in the mixer circuitry, separate components have, in the past, been required for each function. For instance, switching diodes have been provided to perform bandswitching, while hot carrier diodes or transistors have been utilized to perform the mixing function.

Obviously, it is desirable to have a circuit wherein a single component performs both the mixing and bandswitching functions for each of the tuning bands. Significant simplification of the television receiver circuitry would result if both functions could be performed by a single component. However, diodes used for mixing obviously must be capable of converting RF signal to IF signal while suppressing distortions such as cross-modulation and intermodulation and not producing excessive conversion loss. Switching diodes, because of the characteristics thereof, cannot convert RF signal to IF signal without producing excessive cross-modulation, intermodulation and conversion loss and, thus, do not make acceptable mixers.

Further, when switching diodes are used in such a manner as presently used in multiband or TV receiver, the bandswitching must short or open relevant RF and/or IF circuits.

For both short and open operation, the switching diodes exhibit some circuit loss which are detrimental especially on circuits at RF frequency.

On the other hand, hot carrier diodes normally do not perform well as bandswitching because of high capacitance when non-conducting in low capacitance RF circuits and because of high series resistance when conducting. Thus, it has been heretofore impossible to devise a circuit wherein both functions are performed with a single component.

However, by operating a hot carrier diode as an open switching diode at IF frequency and across a large RF bypass capacitor, the hot carrier diode functions adquately, as a bandswitch diode in the open circuit mode. In the meantime, the hot carrier diode as a mixer does not required a bandswitch diode in the short circuit mode to connect relevant RF circuits because it functions as both a mixer and coupling to the relevant RF circuits in the selected band and thereby eliminating the need for bandswitch diode operated in the short circuit mode.

In order to perform adequately as both a mixer and a bandswitch, a component must have very low third-order non-linearity, which is the cause of both cross-modulation distortion and many inter-modulation distortions such as "channel 6 beat", low noise level and demonstrate adequate switching characteristics at IF frequencies. A hot carrier or Shottky diode is a metal junction device which exhibits square law transconductance. Thus, such devices meet all of the necessary circuit requirements and hence are particularly well suited for this application. However, it should be understood that other types of diodes, if capable of exhibiting the necessary electrical characteristics, could be used in the circuit instead of hot carrier diodes.The now proposed circuit is herein described as utilizing hot carrier or Shottky diodes because same are known to be particularly well suited for this application.

When a hot carrier diode is utilized in the proposed circuit, large signal handling capa bility can be achieved and the necessity for additional switching diodes for UHF-VHF bandswitching is eliminated. Further, high overall conversion gain can be achieved by combining the hot carrier diode mixer and a one-stage post IF amplifier. The IF gain achievable with a one-stage post IF amplifier is sufficiently high to offset the conversion loss by the hot carrier diode and still achieve an overall conversion gain compared to a single stage mixer such as a bipolar or dual gate MOSFET.

The foregoing has outlined certain practices which are adopted in a circuit embodying the invention that is described more particularly hereinafter. Stated more generally the invention provides in one aspect apparatus for mixing and bandswitching in a multi-band tuning system of the type having means to provide first and second oscillation signals and a radio frequency signal, said apparatus being characterized by a first diode, shiftable between non-conducting and conducting states, and operably connected to receive and mix the first oscillation signal and the radio frequency signal, a second diode, shiftable between non-conducting and conducting states, and operably connected to receive and mix the second oscillation signal and the radio frequency signal, and means operably connected to said first and second diodes for conditioning a selected one of said first and second diodes to its conducting state and the other one of said first and second diodes to its non-conducting state, in accordance with the receipt of a bandswitching signal.

Preferably a bandpass filter is operably connected to receive the output of the selected diode. The filter includes a relatively high capacitance bypass capacitor. The output of the filter is applied to a one-stage post IF amplifier in order to offset the conversion loss in the diode.

The first and second diodes are preferably Shottky or hot carrier diodes. This type of diode is preferably selected because of the very low third-order non-linearity demonstrated thereby, which is the cause of both cross-modulation and many inter-modulation distortions and because of low noise level characteristics. However, other diodes exhibiting these properties may also be used.

The condition means preferably comprises means, connected to both of the diodes, for normally reverse biasing each and separate means, operably connected to each of the diodes, respectively, for forward biasing the diode associated therewith upon receipt of the appropriate bandswitching signal.

In another aspect of the invention there is provided apparatus for mixing and bandswitching in a multi-band tuning system of the type having means for providing first and second oscillation signals and a radio frequency signal, characterized by first and second diodes, means for normally reverse-biasing said diodes, means for forward biasing a selected one of said diodes in accordance with the receipt of a bandswitching signal, means for connecting said first diode to receive the first oscillation signal and the radio frequency signal, means for connecting said second diode to receive the second oscillation signal and the radio frequency signal, means for filtering the output of said selected diode and means for amplifying the output of said filter means.

In yet another aspect of the invention there is provided apparatus for mixing and bandswitching in a multi-band tuning system of the type providing first and second oscillation signals and a radio frequency signal, characterized by first and second diodes, means operably connected to said diodes and effective when actuated by bandswitching signals, to render a selected one of said diodes conducting, means for connecting the first oscillation signal and the radio frequency signal to the input side of said first diode means for connecting the second oscillation signal and the radio frequency signal to the input side of said second diode, filter means operably connected to the output side of each of said diodes, and amplification means operably connected to amplify the output of said filter means.

In a further aspect of the invention there is provided apparatus for mixing and band-' switching in a multi-band tuning system of the type providing first and second oscillation signals and a radio frequency signal, characterized by first means for mixing the first oscillation signal and the radio frequency signal, second means for mixing the second oscillation signal and the radio frequency signal, said first means being switchable, in accordance with a first bandswitching signal, between a conducting and a non-conducting state, said second means being switchable, in accordance with a second bandswitching signal, between conducting and non-conducting states, filter means connected to receive the output of the conducting one of said first and second mixing means and means for amplifying the output of said filter means.

And in a still further aspect of the invention there is provided apparatus for mixing and bandswitching in a multi-band tuning system of the type providing first and second oscillation signals and a radio frequency signal, characterized by: first and second diodes, each having a first and a second terminal, said second terminal of each of said diodes being operably connected at a first junction node; means connected to said first junction node for reverse biasing said diodes; first means operably connected for receiving the first oscillator signal and the radio fre quency signal and for applying same to said first terminal of said first diode; means connected to said first terminal of said first diode for forward biasing said first diode upon receipt of a first bandswitching signal;; second means operably connected for receiving the second oscillator signal and the radio frequency signal and for applying same to said first terminal of said second diode; means connected to said first terminal of said second diode for forward biasing said second diode upon receipt of a second bandswitching signal; filter means operably connected to said first junction node; and amplification means operably connected to said filter means for amplifying the output thereof.

In all the foregoing other and further aspects of the invention the first and second diodes are preferably hot carrier or Schottky diodes or any other diodes exhibiting similar properties as already discussed above.

The invention and its practice will now be further described with reference to the single figure of the accompanying drawing which shows a mixing and bandswitching circuit-the proposed circuit referred to above for a multi-band tuner system of a television receiver.

As shown in the figure, a pair of hot carrier or Shottky diodes 10, 12 are provided in order to accomplish mixing for the VHF band and UHF band, respectively, and to achieve the necessary bandswitching function. Diode 10 has its anode terminal 1 0a connected to receive the VHF local oscillator signal output of a local oscillator (not shown) for tuning in the VHF band, as well as to the output of an R.F. amplifier (not shown) utilized to amplify the received radio frequency signals. The anode terminal 1 2a of diode 12 is connected to receive the UHF local oscillator signal output from a local oscillator (not shown) to achieve tuning in the UHF band and to the output of the R.F. amplifier utilized to amplify the received radio frequency signals. The cathode terminal 1 Ob of diode 10 and the cathode terminal 1 2b of diode 12 are connected to a common node 14.

The VHF local oscillator signal is connected to anode terminal 1 0a through a capacitor 16. The amplified radio frequency signal is coupled to anode terminal 1 0a by means of a transformer 18, having a primary winding 1 8a connected between a radio frequency amplifier output and ground. The secondary winding 1 8b is connected between anode terminal 1 0a and a means for forward biasing diode 10 to its conducting state.

The UHF local oscillator signal and the amplified radio frequency signal are connected to anode terminal 1 2a of diode 12 by a transformer 20. Transformer 20 comprises a pair of primary windings 20a' and 20a", respectively, connected to receive the UHF local oscillator signal and the amplified radio frequency signal. The secondary winding 20b of transformer 20 is connected between anode terminal 12a and a means for forward biasing diode 12 into its conductive state.

A means for reverse biasing both diodes 10 and 12 is connected to node 14 and thus to the cathode terminals 1 Ob and 12b of the diodes. The reverse biasing means comprises resistors 22 and 24. Resistor 22 is connected between a voltage source (not shown) and node 14, whereas resistor 24 is connected between node 14 and ground. This resistor combination, in conjunction with the power source, provides a positive potential of, for example, + 2.0 volts to the cathode of each diode in order to normally reverse bias the diodes into the non-conducting state.

The means for forward biasing diode 10 comprises a pair of resistors 26, 28. Resistor 26 is connected between a node 30 and a source (not shown) of a bandswitching signal for the VHF band. Resistor 28 is connected between node 30 and ground. This resistor combination in conjunction with the received VHF bandswitching signal provides a positive potential of, for example, + 2.3 volts, to the anode terminal 1 0a of diode 10.

The means for forward biasing diode 12 comprises a pair of resistors 32, 34. Resistor 32 is connected between a node 36 and a source (not shown) of a bandswitching signal for the UHF band. Resistor 34 is connected between node 36 and ground. This pair of resistors, in conjunction with the UHF band switching signal source, provides a positive potential of, for example, + 2.3 volts, at node 36 and therefore to the anode terminal 12a of diode 12.

It will now be appreciated that both diodes 10 and 12 are normally maintained in the non-conducting state because of the positive potential applied on the cathode terminals thereof by the reverse biasing means. However, when a channel in either the VHF or UHF band is selected, the appropriate bandswitching signal for the band in which the selected channel is situated is received by the appropriate forward biasing means such that the diode performing the mixing function for the selected band is rendered conductive. For instance, if a channel in the VHF band is selected, the bandswitching signal for the VHF band is received at the forward biasing means associated with diode 10.This forward biasing means serves to render diode 10 conductive by applying a signal to the anode terminal thereof which is more positive than the signal applied to the cathode thereof by the reverse biasing means. The local oscillator signal for tuning in the VHF band is received from the local oscillator and the R.F. signal is mixed therewith in diode 10. The output of diode 10 is connected to node 14.

It should be appreciated that during the time when diode 10 is rendered conducting, diode 12 is held in its non-conducting state by the reverse biasing means, the forward biasing means associated with diode 12 having not been actuated because of the absence of the UHF bandswitching signal. The bandswitching signal for the VHF band and the band switching signal for the UHF band are mutually exclusive. Thus, only one of these signals is present at any one time such that only one of the diodes is rendered conductive at a time.

If a channel within the UHF band is now selected, the bandswitching signal for the VHF band terminates and the bandswitching signal for the UHF band is present. Because of the lack of positive potential applied to anode terminal 1 0a of diode 10, the reverse biasing means maintains diode 10 in the nonconducting state. Since a positive potential of magnitude greater than that generated by the reverse biasing means is applied to the anode terminal 12a of diode 12 from the forward biasing means associated with diode 12, diode 12 is conditioned to its conducting state.

The UHF local oscillator signal and amplified radio frequency signal mixed in diode 12 can therefore be applied to node 14.

Node 14 is connected to node 38 which is the input for a wide bandpass filter comprising capacitors 40, 42, 44 and 46, as well as inductors 48, 50 and 52. Capacitor 40 acts as a bypass capacitor for the IF output terminal of the conducting diode at both the radio frequency and oscillator frequency for the UHF band and as a partial bypass for the VHF band. Capacitor 40 has a relatively large capacitance, for example 33Pf. The reverse biased diode capacitance of either of the diodes is less than 0.5 Pf and is thus negligible compared to the 33Pf capacitance of capacitor 40. It should also be noted that the reverse biased resistance of the diodes is over 10 K ohm and thus extremely high as compared to the IF output impedance of the conducting diode which is in the area of between 100-500 ohms throughout the band.

The output of the bandpass filter is applied to the input terminal of a single stage post IF amplifier 44 in order to achieve a high overall conversion gain. The IF gain achievable with a one-stage post IF ampifier is sufficiently high to offset the conversion loss by the hot carrier diode and still achieve higher overall conversion gain compared to a single stage mixer, such as a bipolar or dual gate MOSFET.

The present invention has been described and discussed in relation to mixing and bandswitching circuits in a multi-band tuning system for a dual band television receiver. However, by a simple duplication of parts, the circuit can be adapted to be utilized with a multi-band receiver capable of receiving a number of different bands.

The circuit described employs a diode, preferably of the hot carrier type, to perform the mixing and band-switching functions for each tuning band. The appropriate biasing means are provided to render conducting the appropriate diode in accordance with the selected band. The IF output terminal of the conducting diode is connected through a bandpass filter, having a relatively large capacitance bypass capacitor, to a one-stage post IF amplifier in order to achieve high overall conversion gain.

Switching is performed at the IF frequency and thus at a significantly different impedance level than is customary in order to achieve bandswitching which is free from any spurious interaction between the bands. Mixing in the hot carrier diode eliminates cross-modulation and many inter-modulation distortions because of the low third-order non-linearity characteristics of these devices. Thus, large signal handling capability is achieved, the necessity for additional switching diodes for bandswitching is eliminated and higher overall conversion gain is produced by the described circuit.

Claims

1. Apparatus for mixing and bandswitching in a multi-band tuning system of the type having means to provide first and second oscillation signals and a radio frequency signal, said apparatus being characterized by a first diode, shiftable between non-conducting and conducting states, and operably connected to receive and mix the first oscillation signal and the radio frequency signal, a second diode, shiftable between non-conducting and conducting states, and operably connected to receive and mix the second oscillation signal and the radio frequency signal, and means operably connected to said first and second diodes for conditioning a selected one of said first and second diodes to its conducting state and the other one of said first and second diodes to its non-conducting state, in accordance with the receipt of a bandswitching signal.

2. The apparatus of Claim 1, characterized in that said first and second diodes are Shottky diodes.

3. The apparatus of Claim 1 or 2, characterized in that said conditioning means comprises means for applying signals of given magnitude to said first and second diodes to render same conducting and non-conducting, respectively.

4. The apparatus of Claim 3, characterized in that said conditioning means further comprises first means for biasing said first diode into a conducting state upon receipt of a first bandswitching signal and second means for biasing said second diode into a conducting state upon receipt of a second bandswitching signal.

5. The apparatus of any of Claims 1 to 4, further characterized by filter means operably connected to receive the output of said selected diode and means for amplifying the output of said filter means.

6. The apparatus of any one of Claims 1 to 4, further characterized by bypass capacitance means operably connected between ground and the output terminal of each said first and second diodes.

7. The apparatus of Claim 6, characterized in that the capacitance of said bypass capacitor means is greater than the reverse biased capacitance of either of said first and second diodes.

8. The apparatus of Claim 6 or 7, characterized by filter means operably connected to receive the output of said selected diode and means for amplifying the output of said filter means.

9. Apparatus for mixing and bandswitching in a multi-band tuning system of the type having means for providing first and second oscillation signals and a radio frequency signal, characterized by first and second diodes, means for normally reverse-biasing said diodes, means for forward biasing a selected one of said diodes in accordance with the receipt of a bandswitching signal, means for connecting said first diode to receive the first oscillation signal and the radio frequency signal, means for connecting said second diode to receive the second oscillation signal and the radio frequency signal, means for filtering the output of said selected diode and means for amplifying the output of said filter means.

10. The apparatus of Claim 9, characterized in that said reverse biasing means is connected to the cathode terminal of each of said diodes.

11. The apparatus of Claim 9 or 10, characterized in that said forward biasing means is connected to said anode terminal of each of said diodes.

12. The apparatus of Claim 11, characterized in that said connecting means is connected to the anode terminal of each of said diodes.

13. The apparatus of any one of Claim 9 to 12, characterized in that said filter means comprises bypass capacitance means operably connected between ground and a terminal of each of said first and second diodes.

14. The apparatus of Claim 13, characterized in that the capacitance of said bypass capacitor means is greater than the reverse biased capacitance of either of said first and second diodes.

15. Apparatus for mixing and bandswitching in a multi-band tuning system of the type providing first and second oscillation signals and a radio frequency signal, characterized by first and second diodes, means operably connected to said diodes and effective, when actuated by bandswitching signals, to render a selected one of said diodes conducting, means for connecting the first oscillation signal and the radio frequency signal to the input side of said first diode, means for connecting the second oscillation signal and the radio frequency signal to the input side of said second diode, filter means operably connected to the output side of each of said diodes, and amplification means operably connected to amplify the output of said filter means.

16. Apparatus for mixing and bandswitching in a multi-band tuning system of the type providing first and second oscillation signals and a radio frequency signal, characterized by first means for mixing the first oscillation signal and the radio frequency signal, second means for mixing the second oscillation signal and the radio frequency signal, said first means being switchable, in accordance with a first bandswitching signal, between a conducting and a non-conducting state, said second means being switchable, in accordance with a second bandswitching signal, between conducting and non-conducting states, filter means connected to receive the output of the conducting one of said first and second mixing means and means for amplifying the output of said filter means.

17. Apparatus for mixing and bandswitching in a multi-band tuning system of the type providing first and second oscillation signals and a radio frequency signal, characterized by: first and second diodes, each having a first and a second terminal, said second terminal of each of said diodes being operably connected at a first junction node; means connected to said first junction node for reverse biasing said diodes; first means operably connected for receiving the first oscillator signal and the radio frequency signal and for applying same to said first terminal of said first diode; means connected to said first terminal of said first diode for forward biasing said first diode upon receipt of a first bandswitching signal; ; second means operably connected for receiving the second oscillator signal and the radio frequency signal and for applying same to said first terminal of said second diode; means connected to said first terminal of said second diode for forward biasing said second diode upon receipt of a second bandswitching signal; filter means operably connected to said first junction node; and amplification means operably connected to said filter means for amplifying the output thereof.

18. The apparatus of any one of Claims 9 to 17, characterized in that said first and second diodes are Shottky diodes.

19. Apparatus for mixing and bandswitching in a multi-band timing system substantially as hereinbefore described with reference to the accompanying drawing.