HK1055028A - Wireless transceiver with subtractive filter compensating both transmit and receive artifacts - Google Patents

Description

Wireless transceiver with subtractive filter compensating for transmitted and received material

Technical Field

The present invention relates to wireless data transceivers, and more particularly, to wireless data and voice transceivers having full duplex functionality provided by frequency division multiplexing.

Background

Cordless telephones and mobile telephones are used to transmit clear modulated voice to calling telephones that are located within a distance range of from several hundred feet to several miles. The telephone provided is capable of performing the above-described functions, but also requires low manufacturing costs, long battery life and compliance with the general requirements of wireless transmission. One of the simplest structures to meet the above requirements is to have a single local oscillator using direct transmission and a single variable frequency reception.

In the prior art, U.S. patents US5,493,583, 5,553,056 to criptps, and US5,550, 865 to criptps et al, U.S. patent US5,444,737, describe a configuration for direct transmission and single frequency conversion reception in a wireless data transceiver. Such wireless data transceivers are used to transmit and receive frequency modulated signals. The transceiver is provided with a single oscillator that serves as both a radio frequency source for the transmitter and a local oscillator signal source for the receiver. During signal transmission, the output of the oscillator is a modulation frequency to provide a frequency modulated transmission signal to the transmit antenna. The frequency of the oscillator output is modulated by modulating an error feedback signal that serves as a control voltage for a phase-locked loop voltage-controlled oscillator to produce a frequency-modulated transmit signal. During reception of the signal, the receiving antenna receives the output of the oscillator, i.e. the transmitted frequency-modulated signal, and down-converts the frequency-modulated received signal for mixing with the transmitted frequency-modulated signal. The binary transmitted data is subtracted as part of the down-converted FM received signal demodulation.

A problem arises with the use of directly modulated local oscillators for transmission and reception. When the modulated local oscillator produces a transmit signal, the same local oscillator is also used to down-convert the received signal to an intermediate frequency suitable for filtering and amplification. Thus, the modulated signal for transmission has a received signal within it and can be detected. In an analog fm voice system, a user hears one hundred percent of his voice to generate an echo, i.e., the volume of the echo is as great as the signal detected remotely. Such large volume wireless echoes are unacceptable to the user.

It is an object of the present invention to provide an improved transceiver architecture which is most suitable for use in cordless telephones, mobile telephones and other systems in which a transmit local oscillator is phased or frequency modulated to produce a transmit signal and the local oscillator is used to down-convert the receive signal and also to cancel echo.

Disclosure of Invention

The above objects have been achieved in a wireless transceiver that more accurately subtracts a transmitted echo signal by canceling a filter affecting the transmitted echo signal with a compensating filter. The compensator is a filter and optionally an amplitude compensator that approximately doubles the filtering effect of the phase locked loop on the modulating voltage controlled oscillator on the transmit side of the transceiver and the effect of the intermediate frequency filtering on the receive side, which delays and distorts the received signal. Therefore, a compensation signal is added or subtracted to the received signal in the baseband audio section according to the change in phase so that the received signal has no echo to transmit. Without the compensation filter, the transmitted echo is not easily subtracted, and in some cases is hardly subtracted.

The transceiver of the present invention uses nearly minimal Radio Frequency (RF) circuitry and thus meets government (FCC) requirements for most cordless telephones with good sound quality.

Drawings

Fig. 1 is a block diagram of a wireless transceiver of the present invention.

Fig. 2 is a schematic diagram of the frequency response of the circuit on the transmit and receive sides of the transceiver of fig. 1.

Fig. 3 is a circuit schematic of a compensation filter circuit for use in the transceiver of fig. 1.

Fig. 4 is a graph of gain versus frequency for the compensation filter circuit of fig. 3.

Detailed Description

Referring to fig. 1, it can be seen that the radio transceiver of the present invention has a receiving side (also referred to as a receiving circuit) 13 and a transmitting side (also referred to as a transmitting circuit) 15. The antenna 11 is used both for transmitting modulated Radio Frequency (RF) signals and for receiving modulated RF signals. When the transceiver receives a modulated rf signal, the received signal 70 is passed to the receiving side 13, which consists of an rf amplifier that directs the baseband receive rf filter 21 to a down-conversion mixer 23, an Intermediate Frequency (IF) amplifier/filter 25, a frequency modulation filter 27, a subtractor 29, and a receive audio processor 20.

The output signal transmitted via the antenna 11 is generated by the transmit side 15, which is a direct frequency or phase modulated transmitter, consisting of a transmit audio processor 30, a phase locked loop including a voltage controlled oscillator (VOC)35, a phase locked loop synthesizer 37 and a reference oscillator 39, a transmit buffer amplifier 33, and a transmit harmonic filter 31. The compensation filter 17 is connected between the transmission circuit 15 and the reception circuit 13. The compensation filter 17 is used to compensate for frequency distortion of the demodulator of the receive circuit 13 and transmit modulation distortion as may occur at the voltage controlled oscillator 35 and the phase locked loop synthesizer 37 as will be described in more detail below. Further, the amplitude compensator 80 is connected between the compensation filter 17 and the receiving circuit 13. The amplitude compensator 80 is used to compensate for amplitude distortion of the demodulator of the receiving circuit 13. The microcontroller 40 interfaces with the transceiver's audio processor, keyboard, indicator lights and other peripherals to provide process control functions.

The rf filter 21 is a wideband filter that acts on the received signal 70 to reduce input signal noise, spurious signals, and to reduce the power of the transmitted signal reaching the receiver. The filtered signal 71 is passed to a down-conversion mixer 23 where the output signal of the voltage controlled oscillator 35 is mixed with the radio frequency signal of the radio frequency filter 21 to produce an Intermediate Frequency (IF) signal. The intermediate frequency signal 72 is amplified by the intermediate frequency amplifier/filter 25 and the amplified signal 73 is passed to the frequency modulation detector 27 or, in the case of phase modulation, to a phase demodulator which demodulates the intermediate frequency signal. The intermediate frequency filter is typically a narrow band filter that has a significant effect on the intermediate frequency signal. Therefore, the imitation of an intermediate frequency filter is to consider the compensation filter of the invention, without imitating and considering a radio frequency filter having a small influence on distortion. The demodulator is provided with a characteristic filtering function which can be imitated in view of frequency and amplitude distortion.

The demodulated signal 74, which is now in the audio baseband section, is fed to the subtractor 29, where the demodulated signal 74 is differentially added to the signal 75 of the compensation filter 17. The purpose of subtractor 29 is to remove the transmit modulation from the received signal so that care must be taken with respect to phase and frequency response. For example, if the frequency of the local oscillator 35 increases with increasing transmit modulation voltage, the local oscillator 35 of the down-conversion mixer 23 will have a lower frequency than the received signal, whereas the mixer 23 reverses the frequency of the local oscillator 35 at its output end. Thus, as the transmit modulation increases, the intermediate frequencies 72 and 73 decrease. The phase of the frequency modulation detector 27 must be taken into account. For ease of explanation, we assume that the output of the frequency modulation detector 27 increases with increasing intermediate frequency 73. The resulting subtractor receive input 74 decreases as the reflection modulation increases. In this particular case, the subtractor needs to add both signals 74 and 75 simultaneously to eliminate the above effect, so that the compensation filter 17 is not inverted. To avoid confusion we will refer to the compensation block 29 as a subtractor because it can subtract the signal 75 with the signal 74, however, we can see from the above that each structure determines whether the subtractor needs to add or subtract a signal to cancel the effect. The subsequent frequency compensation is an amplitude compensation 80 that offsets the amplitude distortion emulated in the demodulation circuit.

The difference signal 76 is then passed to the receive audio processor 20. In the case of emulating voice modulation, the audio processor typically includes a companding circuit that improves the signal-to-noise ratio by compressing the volumetric range of the transmitter voice signal with a compressor, while restoring the normal range at the receiving end with an expander. In addition, speech signals are typically filtered to prevent unwanted frequency modulation range during transmission, and are limited to prevent excessive modulation levels during transmission. The received audio is also typically filtered in order to remove noise outside the audio range. This process also includes gain or volume control. There is typically no audio processor for the data, or only a much simpler processor than for speech. If the speech signal has been digitized by a vocoder, the audio processor typically consists only of a pulse shaping filter on the transmit side and a noise/bandwidth control filter on the receive side.

The baseband receiving rf filter 21, down-conversion mixer 23, if amplifier/filter 25, fm detector 27 and subtractor 29 are conventional devices known in the art. Receive audio processor 20 may employ any audio processor suitable for analog or digital signals.

For the signal to be transmitted, the analog voice or digital data signal 68 passes to the transmit audio processor 30 which converts the transmit modulation into a transmit baseband signal 67, and the transmit baseband signal 67 may be input to the compensation filter 17 and also to the voltage controlled oscillator 35. The phase locked loop synthesizer 37 has an input 66 from the stable reference oscillator 39 and a feedback input 65 from the voltage controlled oscillator 35. If the system has no input signal, the error voltage is the value required for phase locking. Therefore, the operating frequency of the voltage-controlled oscillator 35 is the carrier frequency fo. When the transmit modulation signal 67 is applied, the voltage controlled oscillator frequency control output signal 62 may be modulated, but filtered by the phase locked loop 37. In other words, when the modulated signal 67 swings at the center frequency position of the voltage controlled oscillator, the phase locked loop attenuates the swing with filtering characteristics that can be modeled with known procedures, but still modulates the voltage controlled oscillator outside the loop bandwidth. This modulation is reduced within the phase-locked loop bandwidth. The radio frequency signal 62 is transmitted through a transmitting antenna. The frequency modulated radio frequency signal 62 passes through the output buffer amplifier 33 and the harmonic filter 31 to the antenna for transmission. As with the receiving circuit, the component blocks of the transmitting circuit 15, such as the voltage-controlled oscillator 35, the phase-locked loop synthesizer 37, the reference oscillator 39, the buffer amplifier 33, and the harmonic filter 31 may use general devices known in the art. The transmit audio processor 30 may be the same type of processor as the receive audio processor described above, and the transmit audio processor 30 typically consists of a compressor, a limiter, and a filter for analog speech.

As discussed above, the compensation filter 17 is connected between the transmit audio processor 30 of the transmit circuit 15 and the subtractor 29 of the receive circuit 13. The compensation filter 17 is used to alter the spectrum and phase spectrum of the transmit modulation 67 to more closely match the transmit modulation 67 to the transmit modulated receive signal 74 components, thereby accurately removing the signal to yield a pure receive signal 76. To illustrate the compensating filter circuit, the bandwidth of a typical phase-locked loop is set to about 150Hz, and the zero lead of a phase-locked loop is set to 10 Hz. These values should include a well attenuated phase-locked loop. These assumptions imply that the phase locked loop will affect the modulation signal with a single high pass effect below 150Hz and a double high pass effect below 10 Hz. It follows that the compensation filter should also have the same function so that the subtractor completely cancels all the transmitted audio signal from the received signal.

The receiver also has some filtering functions, mainly for the intermediate frequency amplification/filter, and this also has an effect on the audio signal. For ease of explanation, it is assumed that the intermediate frequency filtering of the receiving circuit has a low pass effect that causes the audio signal to have a bipolar response at 10 Hz. Therefore, the compensation filter needs to compensate the modulated signal before it reaches the subtractor, so that the subtractor outputs an accurate signal.

Referring to fig. 2, the filter 17 must compensate for the phase-locked loop effects in the transmit circuit 15 and the filtering effects in the receive circuit 25. In the usual case, the filtering effect of the rf filter 13 on the baseband receiving side is not important, since the bandwidth of the rf filter is much wider compared to the baseband signal, and as discussed in the above embodiments, a phase locked loop below 150Hz may have a single high pass effect and below 10Hz may have a double high pass effect. This high pass effect is represented in fig. 2 by a pair of high pass filters 83, 84 of the transmit circuit 15. Also, as discussed above, the intermediate frequency filter in the receive circuit may have a low pass effect that causes the audio signal to have a bipolar response at 10 Hz. This low-pass effect is represented in fig. 2 by a pair of low-pass filters 81, 82 of the receiving circuit 13.

In order to obtain a pure receive signal 76 at the output of the subtractor, the compensation filter 17 has to compensate for the high-pass effect of the transmit circuit 15 and the low-pass effect of the receive circuit 25. The compensation filter circuit 17 shown in fig. 2 includes two high-pass filters 85 and 86 and two low-pass filters 87 and 88. The number of high-pass and low-pass effects given in fig. 2 is only given as an example, and the exact compensation filter characteristic must be determined by the effect of analog phase-locked loop filtering on the vco output signal 62 and the effect of intermediate frequency filtering on the mixer output signal 62. The compensation filter is the same as this combination. The branch circuits of each stage of the transmitting circuit and the receiving circuit have other filtering effects which can be simulated and matched by the compensation filtering circuit.

Fig. 3 shows an embodiment of the filter 17. It should be noted that the embodiment of filter 17 shown in fig. 3 is merely one structure that may be used to implement compensation filter 17. The filter may be constructed in many other ways that serve the same function of removing the transmitted signal from the received signal. In the circuit 17 of fig. 3, the input point 51 receives a transmit audio signal from the transmit audio processor of the transmit circuit. A resistor R1 and a capacitor C1 are connected in series with the input point 51. The resistor R2 is connected between one end of the capacitor C1 and ground. The capacitor C2 is connected in parallel with the resistor R2 between one end of the capacitor C1 and ground. The capacitor C3 is connected in series between the capacitor C1 and the resistor R3 after the resistor R2 and the capacitor C2. A second terminal of resistor R3 is connected to the negative input N1 of operational amplifier 60. The positive input N3 of operational amplifier 60 is connected to ground. A resistor R4 and a capacitor C4 are connected in parallel between the negative input terminal N1 and the output terminal N2 of the operational amplifier 60. The output terminal N2 is connected to the output point 52 of the compensation filter circuit. And the output point 52 of the compensation filter circuit 17 is connected to the subtractor of the receiver circuit of the transmitter.

As discussed with respect to fig. 2, the bandwidth of a typical phase-locked loop is about 150Hz, and the zero lead of the phase-locked loop is 10Hz, which means that the phase-locked loop affects the modulated signal with a single high-pass effect below 150Hz and a double high-pass effect below 10 Hz. Therefore, the compensation filter of fig. 3 should have the same function so that the subtracter completely cancels all the transmitted audio signal from the received signal. Referring to fig. 3, capacitor C1 is 22nF, and forms a high pass filter 53 with a 1M Ω resistor R2. In addition, the capacitor C3 is 22nF, and forms a second high pass filter 55 with a resistor R3 of 100K Ω.

The receiving side also has some filtering functions, mainly in the mid-band, which have an effect on the audio signal. As discussed in the above embodiments, it is assumed that the filtering of the receiving circuit has a low pass effect that makes the audio signal with a bipolar response at 10 Hz. The compensation filter needs to compensate before the modulated signal reaches the subtractor. In the circuit diagram of fig. 3, a resistor R1 of 10K Ω and a capacitor C2 of 1.5nF form the low-pass filter 54. Further, a resistor R4 of 100K Ω and a capacitor C4 of 150pF form the second low-pass filter 56. The two low pass filters have the effect of matching any effective intermediate frequency filtering effect in the receive circuitry, while the rf filters 53 and 55 have the effect of matching any effect of the phase locked loop transmitting the audio.

Fig. 4 shows a graph of the compensation filtering effect. The Y-axis 92 of the plot represents signal gain in decibels (dB) and the X-axis 91 represents frequency (Hz) on a logarithmic scale. In the portion 94 of the curve between 10 and 150Hz, the slope is 20dB per decade of high-pass. This is due to the action of the high pass filter 55. In the portion 93 of the curve below 10Hz, the slope of the high-pass is 40dB per decade. This is due to the fact that the high pass filters 53 and 55 work together. While the portion 95 of the curve above 150Hz has a slope almost equal to zero until the frequency reaches 10KHz, at which time it is necessary to compensate for the two-pole response of the intermediate frequency filtering. From this frequency, the low pass filters 54 and 56 affect the portion 96 of the curve above 10KHz with a slope of 40dB for every decade of difference.

The compensation filter and the amplitude compensator may be implemented in various ways, depending on the known mode of fabrication of the filter. These modes are applied to combine the phase-locked loop effect on the transmit side with the filtering effect of the intermediate frequency filtering on the receive side and the amplitude distortion of the detection circuit.

Claims (12)

1. A wireless transceiver, characterized by: the wireless transceiver comprises

A transmitting side provided with a voltage-controlled oscillator generating a Radio Frequency (RF) signal, the voltage-controlled oscillator having a center frequency stabilized by a reference circuit, the reference circuit applying a characteristic filtering function to the RF signal generating a modulated RF output signal in accordance with a swing of a modulation signal from a baseband modulation signal source with respect to the center frequency, and

a receiving side provided with a down-conversion mixer for mixing a radio frequency input signal with a modulated radio frequency output signal, the mixer generating an intermediate frequency signal for transmission to an intermediate frequency amplification filter having a characteristic filtering function operating with an Intermediate Frequency (IF) signal, thereby generating a filtered intermediate frequency signal, a detection circuit connected for receiving the filtered intermediate frequency signal and generating a modulated audio signal, and a subtractor connected for receiving the modulated audio signal, and

a compensation filter having an output coupled to the subtractor and a filtering function, the compensation filter having a function equal to the sum of the characteristic filtering function of the radio frequency output signal and the characteristic filtering function of the intermediate frequency amplification filter, the input of the compensation filter being adapted to receive the modulated signal and to apply a compensation signal to remove the transmit modulation from the received modulated audio signal.

2. The apparatus of claim 1, wherein: the reference circuit is a phase locked loop.

3. The apparatus of claim 1, wherein: the apparatus further includes an amplitude compensator connected to the compensation filter and having an amplitude attenuation characteristic applied to a subtractor identical to the amplitude distortion of the detection circuit.

4. The apparatus of claim 1, wherein: the input of the receiving side is connected to a radio frequency amplification filter, and the output of the amplification filter is the radio frequency input signal.

5. The apparatus of claim 1, wherein: the filtering function is a linear combination of the characteristic filtering function of the radio frequency output signal and the characteristic filtering function of the intermediate frequency amplification filter.

6. A transceiver for transmitting and receiving radio frequency signals, comprising: the transceiver comprises

An antenna for transmitting a first modulated radio frequency signal and for receiving a second modulated radio frequency signal,

a transmitting circuit coupled to the antenna and generating a first radio frequency signal, the transmitting circuit comprising a modulator for modulating the first radio frequency signal with a transmit audio signal, the transmitting circuit further comprising a phase locked voltage controlled oscillator having a characteristic filter passband and coupled to a voltage controlled oscillator output signal supplied to the antenna,

a receiving circuit coupled to the antenna and receiving the second RF signal, the receiving circuit having a down-conversion mixer and an intermediate frequency amplification filter, the input of which is the output signal of the voltage controlled oscillator, the output of which is the output of the mixer with a first frequency response, and a demodulation circuit for converting the first frequency response into a baseband signal, the demodulation circuit being connected to the subtractor, and

a compensation filter circuit coupled between the voltage controlled oscillator of the transmit circuit and the subtractor of the receive circuit and having a second frequency response identical to the first frequency response, wherein the first frequency response signal is combined with the second frequency response signal to accurately remove transmit modulation from the baseband signal of the receive circuit.

7. The transceiver of claim 6, wherein: the transmit circuit includes a phase-locked loop (PLL) synthesizer coupled to the voltage controlled oscillator, the PLL synthesizer having a third frequency response.

8. The transceiver of claim 6, wherein: the compensation filter circuit comprises at least one high-pass filter.

9. The transceiver of claim 1, wherein: the compensation filtering circuit includes at least one high pass filter and one low pass filter each.

10. The transceiver of claim 6, wherein: the compensation filter circuit comprises at least one high-pass filter connected in series with at least one low-pass filter.

11. The transceiver of claim 6, wherein: the voltage controlled oscillator of the transmit circuit is directly modulated with a baseband audio signal.

12. The transceiver of claim 6, wherein: the transceiver further includes an amplitude compensator connected to the compensation filter and having an amplitude attenuation characteristic applied to a subtractor identical to the amplitude distortion of the detection circuit.