WO1990015486A1

WO1990015486A1 - Receiver with improved intermodulation performance

Info

Publication number: WO1990015486A1
Application number: PCT/US1990/002966
Authority: WO
Inventors: Joseph P. Heck
Original assignee: Motorola, Inc.
Priority date: 1989-06-05
Filing date: 1990-05-29
Publication date: 1990-12-13

Abstract

A radio frequency signal received by a receiver (10) is processed via a voltage-to-current converter (7) to provide a current signal (50) to an integrated IF filter (15). The IF filter (15) is constructed and arranged such that the filter has a capacitive element (64) across its input (62). The current signal and filter arrangement operate to alleviate the problem of the increasing intermodulation and desensitization which would otherwise degrade the performance characteristics of the receiver.

Description

RECEIVER WITH IMPROVED INTERMODULATION PERFORMANCE

Technical Field

This invention relates generally to radio frequency (RF) receivers, more specifically to RF receivers designed to control and improve their intermodulation (IM) performance.

Background Art

In an RF receiver, intermodulation distortion is caused when sufficiently strong interfering off-channel signals mix with a local oscillator (or injection) signal. This results in an undesired signal appearing in the passband of a receiver's intermediate frequency (IF) section. Generally, a desired signal is also mixed into the IF passband, and the contemporaneous presence of the undesired signal distorts the desired signal, thereby reducing the intelligibility of the desired signal. IM distortion also results from distortion products occurring near the desired carrier frequency, which may be caused by crossover effects, gain reduction at high current, device saturation, and variation of collector capacitance with collector voltage. IM typically raises the "noise floor", and reduces the dynamic range of the receiver. (The dynamic range of a receiver being the range over which the signal output of the receiver is a replica of the input signal). Receivers with integrated IF filters require significantly more gain (amplification) prior to filtering than receivers with passive ceramic or crystal IF filters because of the relatively high noise figure found in integrated filters. Unfortunately, increasing the pre-filtering gain tends to cause clipping under conditions of strong interference.

This increases intermodulation and desensitization and degrades the performance characteristics of the receiver. More particularly, when an integrated IF filter is used for selectivity, a voltage amplifier must provide a large amount of gain. Consequently, clipping will ' occur with a relatively low level interfering signal offset in frequency from the desired channel. Accordingly, a need exist for a way to employ integrated filters in a radio receiver without adversly affecting the receiver's performance.

Summary of the Invention

Accordingly, it is an object of the present invention to provide a high performance intermodulation receiver circuit that overcomes the detriments discussed above.

Briefly, according to the invention, a radio frequency signal received by a receiver is processed via a voltage-to-current converter to provide a current signal to an integrated IF filter. The IF filter is constructed and arranged such that the filter has a capacitive element across its input. The current signal and filter arrangement operate to alleviate the problem of the increasing intermodulation and desensrtization, which degrades the performance characteristics of the receiver.

Brief Description of the Drawings

Figure 1 is a schematic block diagram of a receiver in accordance with the present invention.

Figure 2 is a circuit diagram of a mixer with a voltage-to- current converter in accordance with the present invention. Figure 3 is a circuit diagram of a lowpass filter. Figure 4 is a circuit diagram of a bandpass filter. Figure 5 is a circuit diagram of OTA-C (operational transconductance amplifier-capacitor) integrated bandpass filter suitable for use with the present invention.

Figure 6 is a circuit diagram of OTA-C integrated lowpass filter suitable for use with the present invention.

Detailed Description of the Preferred Embodiment

Referring to Figure 1 , there is shown a block diagram of a receiver 10 in accordance with the present invention. Operationally, radio frequency signals received by an antenna 11 are pre-selected (12) and mixed in a mixer 13 with a local oscillator signal 9. The mixed RF and local oscillating signals are converted (processed) into a current signal 14 via a voltage-to-current converter 7. This current signal is received by the input of an integrated IF filter 15, which provides a filtered signal. By converting the received signal to a current signal 14 and filtering with a filter having a capacitor across its input, the receiver's intermodulation and desensitization characteristics are enhanced. The filtered signal is then demodulated (16) and amplified (17) before the signal is preferably provided to at least one speaker (18). Referring to Figure 2, there is shown a schematic diagram of a mixer circuit 13 and having a voltage-to-current converter 7. The voltage- to-current converter 7 preferably comprises a pair of bipolar transistors (36 & 38) driven by a differential voltage source (not shown), which preferably is an RF signal applied at inputs 32 and 34. A pair of resistors (40 & 42) disposed between the emitters of the bipolar transistors 36 & 38 are coupled to a biasing current source (44), which is coupled to ground (46). Still referring to Figure 2, complementary phased local oscillator signals are received at inputs 51 and 52 of bipolar transistor pairs 27, 28 and 29, 30 respectively. The collector of transistor 27 is coupled to the base of bipolar transistors 23 & 24, and the collector of transistor 28 is coupled to the base of bipolar transistors 21 & 22. These bipolar transistors (21 , 22, 23, 24), are preferably PNP transistors, which have their emitters coupled to a voltage source Vcc. Current from the collector of PNP bipolar transistor 22 drives a bipolar transistor 25. The bipolar transistors 21 through 26 serve as current mirrors to convey the differential current (I2-I1) to the load at port 50. The collectors of bipolar transistors 24 and 25 provide this current output signal at port 50.

Essentially, mixer circuit 13 mixes an RF voltage signal received at inputs 32 & 34 with a local oscillating voltage signal received at inputs 51 & 52, and provides a current signal (14) output at port 50.

Referring to Figure 3, there is shown a schematic diagram of a lowpass LC ladder filter circuit 60. Preferably, at least one capacitor (64) is disposed between the input 62 and ground (82). The LC ladder filter circuit 60 comprises 4 capacitors (64, 66, 68, & 70) coupled to a common ground (82). Three inductors (74, 76, & 78) are disposed as shown to provide a lowpass filter response as is known in the art. The LC ladder filter circuit 60 also has at least one resistive load (71 ) disposed between its output port 80 and ground (82). Referring to Figures 2 and 3, essentially, the input 62 receives a current output signal from the mixer / voltage-to-current converter circuit (illustrated in Figure 2), and provides a voltage signal at output port 80. The conversion of the received signal to a current signal and the preferred filter structure of the present invention improves the intermodulation and desensitization characteristics of the receiver 10 since this arrangement limits the peak voltage caused by interfering signals at node 50, which are offset in frequency from the passband of the filter 60. This attenuation occurs by virtue of the attenuating characteristics of the reactive filter elements, in particular capacitor

64 in the filter shown in Figure 3.

The resultant reduction in signal voltage swing at the input to the filter allows a much greater level of interfering signal at the receiver input before clipping or other non-linear circuit behavior occurs at the mixer output or filter input. This is particularly important when an active filter is used for receiver selectivity.

Figure 4 illustrates a conventional bandpass fi^wer 100, which is preferably used in the present invention in a superπeterodyne radio. The bandpass filter 100 preferably comprises a current source 101 , two resistors (102 & 118), three capacitors (104, 108 & 112), and two inductors (106 & 112). Every device except capacitor 108 is coupled to ground (116) on one end and to bandpass filter 100's positive terminal 114 on the other end. The ends of capacitor 108 is disposed between one end of an inductor 106 and another end of a capacitor 110.

Referring to Figure 5, there is shown an equivalent diagram of Figure 4, whereby operational transconductance amplifiers (alone or in combination) with capacitors replace the individual discreet components of Figure 4. The bandpass filter 200 is preferably an active integrated filter, which preferably is used for Superheterodyne radio receivers. Referring to Figures 4 and 5, bandpass filter 200 comprises a current source 201 , transconductance amplifiers 202 and 218 having their negative terminals shorted to their outputs (replacing resistors 102 and 118 respectively), capacitors 204, 208 and 210 (replacing capacitors 104, 108 and 110, respectively), and transconductance amplifier / capacitor combinations 206 and 212 form gyrators (replacing inductors 106 and 112 respectively). Each transconductance amplifier / capacitor combination includes a pair of transconductance amplifiers (i.e., 206a & 206b) and a capacitor (i.e., 206c). One end of a capacitor 206c is coupled to the negative terminal of the transconductance amplifier 206a and the output of the transconductance amplifier 206b. The other end of the capacitor

206c, the positive terminal of transconductance amplifier 206a, and the negative terminal of transconductance amplifier 206b are all coupled to ground (216). The output of transconductance amplifier

206a and the positive terminal of transconductance amplifier 206b are coupled to a positive output port 214. Transconductive amplifiers

212a and 212b and capacitor 212c of the transconductive amplifier/ capacitor combination 212 are similarly connected to the respective components of transconductive amplifier / capacitor combination 206.

Referring to Figure 6, there is shown a circuit diagram of a OTA-C integrated lowpass filter 300 comprised of transconductance amplifiers 304, 306, 308, 310, 312, 316, 314 and 316; capacitors 318, 320, 322, 324, 326, 328 and 330 all having one end coupled to ground (334) and another end coupled to the outputs of transconductance amplifiers 304, 308, 312, 314, 306, 310 and 316 respectively; a current source 301 ; and an output port 332. The lowpass filter 300 is desirably an active filter, which preferably is used in direct conversion type radio receivers.

The OTA-C integrated lowpass filter 300 corresponds to and is similarly connected to the lowpass filter of Figure 3. Referring to Figures 3 and 6, the operation of transconductance amplifiers 304, 306, and 308 and capacitor 326 corresponds to the operation of inductor 74 of lowpass filter 60 of Figure 3. The operation of transconductance amplifiers 308, 310 and 312, and capacitor 328 corresponds to the operation of inductor 76. Similarly, the operation of transconductance amplifiers 312, 316 and 314, and capacitor 330 corresponds to the operation of inductor 78. Capacitors 318, 320, 322, and 324 of OTA-C integrated lowpass filter 300 all correspond to capacitors 64, 66, 68, and 70 of lowpass filter 60 respectively. Finally, transconductance amplifier 314, having its output and negative terminal shorted, serves and corresponds in part to the load

71 of lowpass filter 60.

In summary, the amplification is achieved via a current to voltage conversion, with the output voltage resulting from the current flowing into the filter impedance. With this arrangement, off-channel signals are attenuated by the filter's reactive input impedance (64) and thus clipping does not occur until the interfering signal reaches a significantly higher level. In other words, there will be less gain for off channel signals than for on channel signals because of the filtering nature of the input.

What is claimed is:

Claims

1. A receiver circuit, comprising: means for receiving an information signal to provide a received signal; means for mixing said received signal with an injection signal, providing a mixed signal; means for processing said mixed signal to provide a current signal representative of said received signal after the means for mixing; filtering means for filtering said current signal to provide a filtered signal, said filtering means being constructed and arranged to have at least one capacitor disposed across the input thereto; means for processing said filtered signal to provide a recovered signal.

2. The receiver of claim 1 , wherein said means for receiving an information signal comprises at least an antenna, a pre¬ selecting device, and a mixer for mixing said received signal with a local oscillator signal.

3. The receiver of claim 1 , wherein said means for processing said received signal to provide a current signal comprises a transistor pair which is driven by said received signal, said transistor pair having a resistive load disposed between the emitters of said transistor pair.

4. The receiver of claim 1 , wherein said filtering means for filtering said current signal comprises an LC ladder circuit providing input and output terminals.

5. The receiver of claim 1 , wherein said filtering means for filtering said current signal comprises an active filter.

6. The receiver of claim 5, wherein said filtering means for filtering said current signal comprises an active lowpass filter.

7. The receiver of claim 5, wherein said filtering means for filtering said current signal comprises an active bandpass filter.

8. A receiver , comprising: means for receiving an information signal to provide a received signal; means for mixing said received signal with an injection signal, providing a mixed signal; means for processing said mixed signal to provide a current signal representative of said received signal after the means for mixing; filtering means for filtering said current signal to provide a filtered signal, said filtering means being constructed and arranged to form a lowpass filter having at least one capacitor disposed across the input thereto; means for processing said filtered signal to provide a recovered signal.

9. A receiver , comprising: means for receiving an information signal to provide a received signal; means for mixing said received signal with an injection signal, providing a mixed signal; means for processing said mixed signal to provide a

CL,.t3nt signal representative of said received signal after the means for mixing; filtering means for filtering said current signal to provide a filtered signal, said filtering means being constructed and arranged to form a bandpass filter having at least one capacitor and at least one inductive device disposed across the input thereto; means for processing said filtered signal to provide a recovered signal.

10. A method for improving the intermodulation performance of a receiver, comprising the steps of:

(a) receiving a signal to provide a received signal; (b) mixing said received signal with an injection signal, providing a mixed signal;

(c) converting said mixed signal to a current signal;

(d) filtering said current signal with a filtering means having at least one capacitor across its input terminal to provide a filtered signal.

11. The method of claim 10, which includes the step of:

(e) processing said filtered signal to provide a recovered signal.