WO1997015988A1

WO1997015988A1 - An apparatus and method for filtering a received signal

Info

Publication number: WO1997015988A1
Application number: PCT/US1996/013485
Authority: WO
Inventors: Richard A. Dickens; Daniel Joseph Terlep
Original assignee: Motorola Inc.
Priority date: 1995-10-27
Filing date: 1996-08-20
Publication date: 1997-05-01
Also published as: FI972747A; SE521527C2; SE9701539D0; FI115681B; FI972747A0; JPH10512424A; SE9701539L

Abstract

An apparatus (10) including a compressive receiver (22) detecting the received signal and a frequency agile filter (60) responsive to the compressive receiver (22). The frequency agile filter (60) filters the received signal. A method including the steps of detecting the received signal (204), downconverting the received signal to produce a downconverted signal (206), performing a dispersive delay line calculation on the downconverted signal to produce a pulsed signal having a plurality of pulses (208), comparing at least some of the pulses within the pulsed signal to a threshold (210), measuring the time delay of the pulses that exceed the threshold over a predetermined time interval (212), determining a filter frequency based on the time delay of the pulses (214), and filtering the received signal at the filter frequency (216).

Description

AN APPARATUS AND METHOD FOR FILTERING A RECEIVED SIGNAL

Field of the Invention

This invention relates generally to receiving signals, such as radio frequency (RF) signals, and more particularly to filtering the received signals.

Background of the Invention

Conventional radio frequency receivers for use in wireless communication systems, such as frequency division multiple access, time division multiple access, and code division multiple access systems, typically operate in a multiple signal environment. In such an environment it is desirable for the receiver to have as large a dynamic range as possible without compromising the quality of the received signal. In addition, increased dynamic range is especially important in applications receiving a wideband signal. Accordingly, there is a need for an apparatus and method for filtering a received signal providing improved dynamic range while maintaining the quality of the received signal.

Summary of the Invention

In order to address this need, the present invention provides an apparatus and a method for filtering a received signal. The apparatus includes a compressive receiver and a frequency agile filter responsive to the compressive receiver. The frequency agile filter filters the received signal. According to another aspect of the present invention, the apparatus includes a frequency agile filter, a first analog to digital converter, an attenuator, and a second analog to digital converter. The first analog to digital converter is responsive to the frequency agile filter and the second analog to digital converter is responsive to the attenuator. The attenuator is responsive to the received signal and produces an attenuated signal.

The method includes the steps of: detecting the received signal; downconverting the received signal to produce a downconverted signal; performing a dispersive delay line calculation on the downconverted signal to produce a pulsed signal having a plurality of pulses; comparing amplitudes of at least some of the pulses within the pulsed signal to a threshold; measuring a delay of the pulses that have amplitudes exceeding the threshold over a predetermined time interval; determining a filter frequency based on the time delay; and filtering the received signal at the filter frequency.

The invention itself, together with its intended advantages will best be understood by reference to the following detailed description, taken in conjunction with the accompanying drawings.

Brief Description of the Drawings

FIG. 1 is a block diagram of a preferred embodiment of an apparatus for filtering a received signal.

FIG. 2 is a block diagram of another preferred embodiment of an apparatus for filtering a received signal.

FIG. 3 is a flow chart of a preferred embodiment of a method of filtering a received signal. Detailed Description of the Preferred Embodiments

Referring to FIG. 1 , an apparatus 10 of detecting and filtering a received signal is illustrated. The apparatus 10 includes an antenna 12, an input bandpass filter 14, a splitter 16, a compressive receiver 22, and a frequency agile filter 60. The input filter 14 is coupled to the antenna 12 and the splitter 16. The apparatus 10 also includes a signal path coupled to the splitter 16 including an amplifier 40, a filter 42, a mixer 44 with a local oscillator 46, a second amplifier 48, a second filter 50, a third amplifier 52, a third filter 54 and a delay circuit 57.

The compressive receiver 22 is coupled to the splitter 16 by a mixer 18 with a local oscillator (LO) 1 1 and an attenuator pad 20 and is coupled to the frequency agile filter 60 by digital components including a threshold comparator 32, a counter 34, a digital control unit 36, and a digital to analog converter 38. Timing for the compressive receiver 22 is controlled by a time base 30. The compressive receiver 22 includes a mixer 24, a sweeping local oscillator 26, a dispersive delay line 28, and a weighting filter 29.

During operation, a radio frequency (RF) input signal 13 is received by the antenna 12, filtered by filter 14, and divided by splitter 16. A first branch of the divided signal from the splitter 16 is mixed by mixer 18 with LO 1 1 ; attenuated by attenuator 20; and fed into the compressive receiver 22. A second branch of the divided signal from the splitter 16 is fed into the first amplifier 40, filtered by filter 42, mixed by mixer 44 with an output from the local oscillator 46, amplified by amplifiers 48 and 52, and filtered by filters 50 and 54 to create a resulting signal. Although it is contemplated that many alternative configurations of filters and amplifiers may be used, such as a single filter and a single amplifier, the serial arrangement illustrated in FIG. 1 provides a desired sensitivity and dynamic range for the receiver 10. The resulting signal is delayed by delay circuit 57 and fed into the frequency agile filter 60. The frequency agile filter 60 is controlled by digital to analog converter 38 that responds to the compressive receiver 22. An output from the frequency agile filter 60 is converted into a digital signal by analog to digital converter (ADC) 62 and sent as output signal 64. Further processing is then performed on the output signal 64 such as digitally inverse filtering the output signal 64 by using a digital filter that is responsive to the digital unit 36. Other further processing of the output signal 64 includes digital down conversion, filtering, and demodulation.

After the received signal 13 is detected by antenna 12, downconverted in frequency by mixer 18, and attenuated by attenuator 20, the compressive receiver 22 performs a dispersive delay line calculation on the downconverted signal to produce a pulsed signal having a plurality of pulses. Each of the pulses has a height that correspond to a signal strength and a location that corresponds to the frequency of the received signal 13. The compressive receiver 22 performs a real-time Fourier transform on the downconverted signal and provides the output pulsed signal. The compressive receiver 22 mixes the downconverted signal with a fast sweeping local oscillator signal from sweeping oscillator 26 to produce a frequency modulated (FM) waveform. The dispersive delay line 28 compresses each carrier in the downconverted signal into a narrow time domain pulse. The weighting filter 29 applies a weighting filter operation to the pulsed signal to better define and delineate pulses in the pulsed signal.

A time delay for each output pulse referenced to the start of the local oscillator sweep is related to the frequency of a carrier in the downconverted signal. Since the output pulsed signal from the weighting filter 29 is proportional to the input power of the detected radio frequency (RF) input signal 13, the threshold comparator 32 may be used to select an input frequency to be filtered based on the amplitude of pulses in the pulsed signal. The threshold comparator 32 compares at least some of the pulse amplitudes within the pulsed signal to a threshold. The counter 34, preferably a multi-event counter, determines where the pulses occur in time related to a start time of the sweeping LO 26 using time base 30.

The digital control unit 36 then deteimines a filter frequency by transforming the counter output into a digital word. The digital word is converted into an analog control signal by digital to analog converter 38, and the frequency agile filter 60 filters the received signal at the filter frequency set by the digital control unit 36. Preferably, the digital control unit 36 determines the filter frequency by using a look up table to map the measured time delay to the respective filter frequency. The counter 34 is started at the beginning of a sweep by LO 26, and the counter output is read by the digital unit 36 when the threshold is exceeded. The counter 34 is then reset when the sweep is finished. Preferably, the digital control unit 36 comprises a controller, such as an HCl 6 type controller from Motorola Inc., including a processor and a memory .

Due to the time involved between receiving a signal at the compressive receiver 22 and sending a filter frequency to the frequency agile filter 60, the second divided signal from the splitter 16 is preferably delayed in time by delay 57 so that the second divided signal reaches the frequency agile filter 60 at substantially the same time as the control signal from the digital to analog converter 38. In this manner, the frequency agile filter 60 filters the received signal at the frequency calculated by the digital unit 36. In the particular embodiment illustrated in FIG. 1 , the circuit 10 may be constructed with the following components: The filter 14 is a Filtronix CB285 filter, the splitter 16 is a Mini Circuits ZFSC-2-2 type splitter, the mixer 18 is a Mini Circuits ZFM - 150 mixer, and LOU is preferably a Panasonic ENF-VA7 series type oscillator tuned to about 880MMZ. The compressive receiver 22 is an Andersen Laboratories CR-72- 12- 15 compressive receiver, and the threshold comparator 32 is an AD9696 type of comparator from Analog Devices. The time base 30 is a US-685-120 type time base from US Crystal Corp. The counter 34 is an ICM 7216B 1 P1 type counter from Harris Semiconductor, the digital unit 36 is a Motorola MC68HC16Z1 , and the digital to analog converter 38 is a Burr Brown DAC600. The amplifier 40 is a pair of Anzac AMC155 type amplifier in series, the filter 42 is a Filtronix CB286 filter, the mixer 44 is a

Watkins Johnson WJ-4020 mixer, and the local oscillator 46 is an ENF-VA7 Series type oscillator tuned to about 930 MHz from Panasonic. The amplifier 48 is a Cougar AC379 amplifier, the filter 50 is a Networks International D318 filter, and the amplifier 52 is a pair of series Cougar API 48 amplifiers. The filter 54 is a Networks International D319 filter. The analog to digital converter 62 is an Analog Devices AD9042 converter. The frequency agile filter 60 is a notch filter such as a filter that may be constructed as described in a Milcom article "Novel Active RF Tracking Notch Filters for Interference Suppression in HF, VHF, and UHF Frequency Hopping Receivers", Conference Record 1991 IEEE Military Communications Conference Vol. 3 of 3. p. 956, by Masood Ghadaksak ( 1991 ).

The preferred method and apparatus of detecting and filtering a received RF signal has many benefits. For example, the preferred embodiment has improved dynamic range by protecting hardware components, such as the analog to digital converter 62 from large saturating signals. In addition, the preferred embodiment may be used in multiple access systems such as time division multiple access (TDMA), code division multiple access (CDMA), and frequency division multiple access (FDMA) systems. The preferred embodiment may be used over a wide range of RF frequency bands since the input signal is mixed to a proper compressive receiver input frequency.

However, although the preferred embodiment is particularly well suited for use with cellular frequencies, it should be understood that the present invention is not limited to the RF cellular frequency band.

Further, many conventional methods of limiting the signal level into a receiver detector, such as an ADC, degrade the signal-to-noise (S/N) performance of the receiver and thereby reduce receiver sensitivity. In contrast, the preferred embodiment allows for processing RF signals over the entire dynamic range for the cellular band without compromising S/N performance. In addition, the preferred embodiment provides automatic frequency control of the frequency agile filter 60. Since the frequency agile filter 60 is capable of being tuned across the desired receive band, fixed notch filters are not needed and the frequencies to which the filter 60 is tuned to can be normally processed as soon as the signal level is increased above the threshold comparator level.

Another embodiment of the present invention is illustrated in FIG. 2. The same reference numbers are used for similar components in each of the figures. As shown in FIG. 2, the apparatus 100 is similar to the circuit of FIG. 1 , but also includes an attenuator 70 and a second analog to digital converter 72 coupled to the attenuator 70. The attenuator 70 is responsive to the received signal 13 and is coupled to the delay 57. The value of a threshold for the attenuator 70 is selected so that the largest signal received by the antenna 12 is processed by the analog-to-digital converter 72 without saturating the converter 72. The frequency agile filter 60 protects the first analog-to-digital converter 62 from saturation. In this configuration, the first analog-to-digital converter 62 processes lower level signals and the second analog-to-digital converter 72 processes higher level signals. Thus, the second analog-to-digital converter 72 is able to process signals having frequencies that correspond to those frequencies filtered by the frequency agile filter 60.

In a wideband multi-carrier environment where an ADC processes multiple RF inputs, multiple notch filters placed in front of the receiver ADC 62 protects it from saturation by the largest signals. This allows lower level signals to be processed by ADC 62. The value of the threshold setting attenuator 70 is preferably chosen so that the largest signals into the antenna port 12 can be processed by ADC 72 without saturating it.

Referring to FIG. 2, an RF input is applied to two separate paths: one to the compressive receiver 22 and a second to the low level receiver branch beginning with amp stage 40. A third path is provided after the delay 57 and fed into the threshold attenuator 70.

The three paths may be coupled to an antenna input, a multiplexed antenna input, or may be replicated in a plurality of receiver branches. The compressive receiver 22 performs a real-time Fourier transform representation on its input signals and provides an output pulse for each input signal. The compressive receiver 22 input signal is mixed with a fast sweeping LO 26 thereby producing a FM waveform at the mixer output 24. The dispersive delay line (DDL) 28 is used to compress each input signal into a narrow pulse in the time domain. The weighting filter 29 shapes the output pulse from the DDL 28 in order to extend the dynamic range of the compressive receiver 22. The weighting filter 29 may be incorporated into the DDL 28 structure. The time delay of the output pulse referenced to the start of the LO sweep is related to the frequency of the compressive receiver 22 input signal. Since the output pulse amplitude of the weighting filter 29 is proportional to the power of the detected input signal, a threshold comparator 32 is used to select which input frequency is to be filtered based on its amplitude. The combination of amplitude and time delay information is then converted into a tuning voltage for frequency-agile filter 60.

A method for performing the conversion is to use a multi-event counter 34 to correlate the input signal to the detected frequency. This is accomplished by starting the counter 34 at the beginning of the sweep, latching out a count value when the amplitude threshold is exceeded, and resetting the counter 34 when the sweep is finished. The counter output is read by the digital control unit 36 and converted into either an analog voltage or a digital word depending on the filter control requirements of filter 60.

By using multiple tunable notch or bandstop filters, ADC 62 is substantially protected from multiple large RF input signals. The delay block 57 allows the compressive receiver 22 to detect and control the filter 60 before the ADC 62 becomes saturated. RF signals below a predefined threshold power level are processed by ADC 62. Large RF signals above this threshold level are processed by ADC 72 which is protected by the input threshold setting attenuator 70. Thus, in the preferred embodiment using multiple analog to digital converters 62 and

72, dynamic range of the entire receiver 10 is improved.

A preferred embodiment of a method of filtering a received radio frequency signal is illustrated in FIG. 3. As shown in FIG. 3, the method 200 begins at 202 and includes the steps of detecting the received signal, step 204; downconverting the received signal to produce a downconverted signal, step 206; performing a dispersive delay line calculation on the downconverted signal to produce a pulsed signal having a plurality of pulses, step 208; comparing amplitudes of at least some of the pulses within the pulsed signal to a threshold, step 210; measuring a delay of the pulses that have amplitudes exceeding the threshold over a predetermined time interval, step 212: determining a filter frequency based on the time delay, step 214; and filtering the received signal at the filter frequency, step 216. The step of determining the filter frequency, step 216, may use a look up table to map the number of counted pulses to the filter frequency. In addition, the method may further comprise the additional steps of applying a weighting filter to the pulsed signal or delaying the received RF signal prior to filtering the received signal at the filter frequency.

It should be understood that for purposes of this application, a second component is responsive to or in communication with a first component regardless of whether the first and second components are directly coupled or indirectly coupled, such as via intermediate components, including switches that operatively couple the components for only a segment of time, as long as a signal path can be found that directly or indirectly establishes a relationship between the components. For example, the frequency agile filter 60 is responsive to the compressive receiver 22, as defined herein, even though intermediate components, such as the comparator 32, the counter 34, the controller 36, and the D/A 38, are disposed between the compressive receiver 22 and the filter 60.

Further advantages and modifications of the above described apparatus and method will readily occur to those skilled in the art. For example, although the digital control unit 36 produces a digital word in the preferred embodiment, the digital control unit 36 may alternatively produce an analog voltage to control the filter 60 depending upon specific filter requirements. In addition, although operation of the preferred embodiment was described using two analog to digital converters, the present invention is not limited by the number of analog to digital converters used.

The invention, in its broader aspects, is therefore not limited to the specific details, representative apparatus, and illustrative examples shown and described above. Various modifications and variations can be made to the above specification without departing from the scope or spirit of the present invention, and it is intended that the present invention cover all such modifications and variations provided they come within the scope of the following claims and their equivalents.

Claims

ClaimsWhat is claimed is:

1 . An apparatus for filtering a received signal comprising: a compressive receiver; and a frequency agile filter responsive to the compressive receiver, the frequency agile filter filtering the received signal .

2. The apparatus of claim 1 , wherein the compressive receiver comprises a mixer, a dispersive delay line module, and a weighting filter; and the mixer is responsive to the received signal, the dispersive delay line module is responsive to the mixer, and the weighting filter is responsive to the dispersive delay line module.

3. The apparatus of claim 2, wherein the compressive receiver further comprises a sweeping local oscillator unit coupled to the mixer.

4. The apparatus of claim 1 , further comprising an analog- to-digital converter responsive to the frequency agile filter.

5. The apparatus of claim 4, further comprising a bandpass filter in communication with the frequency agile filter and the compressive receiver.

6. An apparatus for filtering a received radio frequency, RF, signal comprising: a frequency agile filter responsive to the received RF signal and producing a filtered signal: a first analog to digital converter responsive to the frequency agile filter; an attenuator responsive to the received RF signal and producing an attenuated signal; and a second analog to digital converter responsive to the attenuator.

7. The apparatus of claim 6, further comprising a controller in communication with the frequency agile filter.

8. The apparatus of claim 7, further comprising a compressive receiver in communication with the controller.

9. A method of detecting and filtering a received signal comprising the steps of: detecting the received signal; downconverting the received signal to produce a downconverted signal; performing a dispersive delay line calculation on the downconverted signal to produce a pulsed signal having a plurality of pulses; comparing amplitudes of at least some of the pulses within the pulsed signal to a threshold: measuring a time delay of pulses having an amplitude that exceeds the threshold over a predetermined time interval: determining a filter frequency based on the time delay: an d filtering the received signal at the filter frequency.

10. The method of claim 9, further comprising the step of applying a weighting filter to the pulsed signal.