Communications receiver having a switchable filter input stage

• US 4,435,841 A
• Filed: 03/01/1982
• Issued: 03/06/1984
• Est. Priority Date: 03/01/1982
• Status: Expired due to Term

• Current Assignee: Zenith Radio Corporation
• Original Assignee: Zenith Radio Corporation
• Inventors: Dobrovolny, Pierre
• Primary Examiner(s): NG, JIN FAY
• Application Number:
  US06/353,694
• Time in Patent Office: 736 Days
• Field of Search: 455/131, 455/180, 455/188, 455/190, 455/191, 455/197, 455/315, 455/316, 455/339, 455/340, 455/284, 455/286, 455/291, 455/292, 333/167, 333/168, 333/174, 333/175, 333/176, 333/178, 333/180, 333/103, 333/129, 333/132
• US Class Current: 455/180.1
• CPC Class Codes: H03H 7/01 Frequency selective two-port networks
• CPC Class Codes: H03H 7/1758 Series LC in shunt or branch path H03H7/1791 takes precedence
• CPC Class Codes: H04B 1/26 for superheterodyne receivers multiple frequency-changing H03D7/16

Abstract

A multichannel communications receiver, e.g. a CATV converter, includes a switched filter for reducing third order intermodulation distortion by limiting the number of channels processed by the receiver. In a preferred embodiment, the filter is switchable between a highpass and a lowpass configuration by a control signal supplied to two switching diodes. The input and output of the filter are coupled by a mutual inductance which partially compensates for the effect of parasitic inductance at higher frequencies.

Claims

1. A multichannel communications receiver characterized by a reduced level of nonlinear distortion comprising:

an input terminal for receiving signals broadcast over a plurality of communication channels;

channel selection means developing a first control signal representing a selected one of said plurality of channels and a second control signal representing a band of channels encompassing said selected channel, said band of channels having a bandwidth less than said plurality of channels;

an input stage coupled to said input terminal comprising a plurality of non-tunable circuit components including switchable filter means responsive to said second control signal for assuming a highpass filter characteristic or a lowpass filter characteristic, said filter means comprising an arrangement of inductors and capacitors, a first switching means and a second switching means being responsive to said second control signal for causing said inductors and capacitors to assume said highpass filter characteristic or said lowpass filter characteristic, at least one of said switching means being characterized by a parasitic reactance coupled across said filter means, said receiver comprising compensating means coupled across said filter means for reducing the effect of said parasitic reactance; and

frequency conversion means coupled to the output of said input stage and responsive to said first control signal for processing the passed band of channels so as to develop an output signal having a predetermined frequency representing only said selected channel.

2. A multichannel communications receiver according to claim 1 wherein said input stage includes a non-tuneable wide band amplifier for amplifying signals received over all of said plurality of communication channels.

3. A multichannel communications receiver according to claim 1 wherein only one of said switching means is rendered conductive in response to said second control signal for causing said filter means to assume its lowpass filter characteristic, said conductive switching means exhibiting a parasitic reactance directly coupling said filter means to reference potential.

4. A multichannel communications receiver according to claim 3 wherein said compensating means comprises means for inductively coupling the broadcast signals received at said input terminal to the output of said filter means in phase opposition to the received broadcast signals coupled across said filter means by said parasitic reactance.

5. A multichannel communications receiver according to claim 4 wherein said network comprises;

an input node and an output node;

first and second series connected capacitors connected between said input and output nodes;

first and second series connected inductors connected between said input and output nodes;

a third capacitor connected between the common junction of said first and second inductors and a reference potential; and

a third inductor connected between the common junction of said first and second capacitors and one plate of a fourth capacitor, the other plate of said fourth capacitor being connected to reference potential;

said first and second switching means comprising first and second diodes connected between the common junctions of said first and second capacitors and said first and second inductors and reference potential respectively.

6. A multichannel communications receiver according to claim 5 wherein the anode of said first diode is connected to the common junction of said first and second capacitors and wherein the cathode of said second diode is connected to the common junction of said first and second inductors.

7. A multichannel communications receiver according to claim 6 including means for coupling said second control signal to said output node and to the common junction of said third inductor and said fourth capacitor.

8. A multichannel CATV converter having an improved intermodulation distortion characteristic comprising:

an input terminal for receiving signals broadcast over a plurality of CATV channels;

channel selection means developing a first control signal representing a selected one of said plurality of CATV channels and a second control signal representing a band of channels encompassing said selected channel, said band of channels having a bandwidth less than said plurality of channels;

an input stage coupled to said input terminal comprising a plurality of non-tunable circuit components including switchable filter means comprising a reactive network formed of an arrangement of inductors and capacitors, a first switching diode and a second switching diode, said second control signal being developed for selectively causing only said first diode or only said second diode to be conductive at one time such that said reactive network is selectively configured for only passing a highband or a lowband respectively of said plurality of CATV channels, at least one of said diodes being characterized by a parasitic reactance coupled across said filter means, said converter comprising compensating means coupled across said filter means for reducing the effect of said parasitic reactance; and

frequency conversion means coupled to the output of said input stage and responsive to said first control signal for processing the passed band of channels so as to develop an output signal having a predetermined frequency representing only said selected CATV channel.

9. A multichannel CATV converter according to claim 8 wherein said compensating means comprises means for inductively coupling the broadcast signals received at said input terminal to the output of said filter means in phase opposition to the received broadcast signals coupled across said filter means by said parasitic reactance.

10. A multichannel CATV converter according to claim 9 wherein the conductive one of said diodes characterizing said lowband configuration exhibits a parasitic reactance directly coupling said filter means to reference potential.

11. A multichannel CATV converter according to claim 10 wherein said network comprises;

an input node and an output node;

first and second series connected capacitors connected between said input and output nodes;

first and second series connected inductors connected between said input and output nodes;

a third capacitor connected between the common junction of said first and second inductors and a reference potential; and

a third inductor connected between the common junction of said first and second capacitors and one plate of a fourth capacitor, the other plate of said fourth capacitor being connected to reference potential;

said first and second diodes being connected between the common junctions of said first and second capacitors and said first and second inductors and reference potential respectively.

12. A multichannel CATV converter according to claim 11 wherein the anode of said first diode is connected to the common junction of said first and second capacitors and wherein the cathode of said second diode is connected to the common junction of said first and second inductors.

13. A multichannel CATV converter according to claim 12 including means for coupling said second control signal to said output node and to the common junction of said third inductor and said fourth capacitor.

14. A multichannel CATV converter having an improved intermodulation distortion characteristic comprising;

an input terminal for receiving signals broadcast over a plurality of CATV channels;

channel selection means developing a first control signal representing a selected one of said plurality of CATV channels and a second control signal representing a band of channels encompassing said selected channel, said band of channels having a band width less than said plurality of channels;

an input stage coupled to said input terminal comprising a plurality of non-tuneable circuit components including switchable filter means comprising a reactive network formed of an arrangement of inductors and capacitors, a first switching means and a second switching means, said first and second switching means being responsive to said second control signal for causing said reactive network to assume a filter characteristic for only passing the band of channels represented thereby, at least one of said switching means being characterized by a parasitic reactance coupled across said filter means;

compensating means coupled across said filter means for reducing the effect of said parasitic reactance; and

frequency conversion means coupled to the output of said input stage and responsive to said first control signal for processing said passed band of channels so as to develop an output signal having a predetermined frequency representing only said selected CATV channel.

15. A multichannel CATV converter according to claim 14 wherein said compensating means comprises means for inductively coupling the broadcast signals received at said input terminal to the output of said filter means in phase opposition to the received broadcast signals coupled across said filter means by said parasitic reactance.

16. A multichannel CATV converter according to claim 14 wherein only said first switching means is rendered conductive in response to said second control signal for causing said filter means to assume a lowpass filter characteristic, said first switching means exhibiting in its conductive state a parasitic reactance directly coupling said filter means to reference potential.

17. A switchable filter for reducing intermodulation distortion levels in a multichannel communications receiver comprising;

an input terminal for receiving signals broadcast over a plurality of communications channels;

an output terminal;

first and second series connected capacitors coupled between said input and output terminals;

first and second series inductors coupled between said input and output terminals;

a third capacitor coupled between the common junction of said first and second inductors and a reference potential;

a third inductor coupled between the common junction of said first and second capacitors and said reference potential;

first and second switching diodes for selectively shunting only the common junction of said first and second capacitors or only the common junction of said first and second inductors to reference potential, whereby to selectively establish either a lowpass or a highpass filter characteristic respectively between said input and output terminals, at least one of said switching diodes being characterized by a parasitic reactance coupled across said input and output terminals; and

compensating means for inductively coupling the broadcast signals received at said input terminal to said output terminal in phase opposition to the received broadcast signals coupled across said input and output terminals by said parasitic reactance.