Apparatus with tunable filter and related adjusting method

The present invention discloses a method for adjusting a tunable compensation filter within a communication device. The method includes the following steps: generating at least a detecting signal according to at least a pole of the tunable compensation filter; generating a transmitted signal according to the detecting signal; receiving the transmitted signal via a loop switch; generating a filtered signal by filtering the transmitted signal; generating a received signal by compensating the filtered signal; generating at least an indicating signal by comparing the detecting signal with the received signal; and determining whether to adjust at least a coefficient of the tunable compensation filter or not according to the indicating signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to communication systems, and more particularly, to a communication apparatus for calibrating a tunable compensation filter and method thereof.

2. Description of the Prior Art

Continuing developments of communication technologies have given rise to many communication specifications. One of the most important issues of communication technologies is to develop a solution that supports different communication specifications simultaneously. For instance, Software Defined Radio (SDR) technology supports different communication specifications. A hardware device of the SDR technology is applied to different communication channels and various signal modulations.

Utilizing conventional SDR technology, users can conveniently switch among various communication specifications (e.g., specifications of GSM system and/or Bluetooth, etc.) by using one hardware device (e.g., a mobile phone).

However, because the development of the SDR technology emphasizes both flexible hardware and software a platform, designing an excellent SDR scheme has inherent difficulties.

Usually, there are a tunable filter (e.g., a low pass filter) and a compensation filter for compensating the tunable low pass filter in a conventional SDR communication system, and a 3-dB cut-off frequency of the tunable filter is changeable; in most cases the tunable filter is a sin c like filter. A sin c function is expressed as:
sinc(x)=sin(x)/x.

Please refer toFIG. 1.FIG. 1is a diagram illustrating frequency responses of a conventional SDR filter. As shown inFIG. 1, a pass band response of the conventional SDR filter is not flat but has a gain drop. The compensation filter is therefore used for compensating the un-ideal frequency response to obtain a wanted flat gain curve of the pass band. Please refer toFIG. 2.FIG. 2is a diagram illustrating a total frequency response of combining a sin c filter and a 1/sin c filter. The 1/sin c function filter is used as a compensation filter of the sin c filter. As shown inFIG. 2, the compensation filter can be implemented by an ideal 1/sin c filter. Therefore, the required response for the SDR communication device is obtained by multiplying the frequency response of sin c filter with the frequency response of 1/sin c filter, that is, the total frequency response of the two filters will be flat.

Usually, there are a tunable filter (e.g., a low pass filter) and a compensation filter for compensating the tunable low pass filter in a conventional SDR communication system, and a 3-dB cut-off frequency of the tunable filter is changeable; in most cases the tunable filter is a sin c like filter. A sin c function is expressed as:
sinc(x)=sin(x)/x.

Please refer toFIG. 1.FIG. 1is a diagram illustrating frequency responses of a conventional SDR filter. As shown inFIG. 1, a pass band response of the conventional SDR filter is not flat but has a gain drop. The compensation filter is therefore used for compensating the un-ideal frequency response to obtain a wanted flat gain curve of the pass band. Please refer toFIG. 2.FIG. 2is a diagram illustrating a total frequency response of combining a sin c filter and a 1/sin c filter. The 1/sin c function filter is used as a compensation filter of the sin c filter. As shown inFIG. 2, the compensation filter can be implemented by an ideal 1/sin c filter. Therefore, the required response for the SDR communication device is obtained by multiplying the frequency response of sin c filter with the frequency response of 1/sin c filter, that is, the total frequency response of the two filters will be flat.

Unfortunately, in real cases the frequency response of the SDR filter is not an ideal sin c function, and the ambient temperature and process deviations also affect the frequency of the sin c filter. Conventionally, the coefficient of the compensation filter is inflexible and thereby fails to compensate the SDR filter successfully even if the frequency response of the SDR filter varies.

In actual circuit design the operation for predicting the frequency response of the SDR filter is both difficult and imprecise. Using the fixed compensation filter (e.g., using a 1/sin c filter) fails to compensate the SDR filter to make the communication device have a pass-band signal which has a flat gain response/gain curve; this leads to a bad signal processing performance at the receiver end.

According to an embodiment of the present invention, a method for adjusting a tunable compensation filter within a communication device is disclosed. The adjusting method includes the following steps: generating at least a detecting signal according to at least a pole of the tunable compensation filter; generating a transmitted signal according to the detecting signal; receiving the transmitted signal via a loop switch; generating a filtered signal by filtering the transmitted signal; generating a received signal by compensating the filtered signal; generating at least an indicating signal by comparing the detecting signal with the received signal; and determining whether or not to adjust at least a coefficient of the tunable compensation filter according to the indicating signal.

DETAILED DESCRIPTION

As mentioned above, an objective of the present invention is to provide a tunable compensation filter within a communication device, wherein coefficient(s) of the tunable compensation filter is adjustable when a frequency response of the SDR filter varies with a status of the communication device, to hence improve the whole frequency response of the SDR communication device.

Please refer toFIG. 3.FIG. 3is a block diagram illustrating a communication device100according to a first embodiment of the present invention. As shown inFIG. 3, the communication device100includes a loop switch110, a transmitter120, and a receiver130. In this embodiment, the loop switch110is on (enabled/connected) for connecting the transmitter120with the receiver130to adjust coefficients of a tunable compensation filter170while the communication device100is under a test mode.

The loop switch110is disabled (disconnected) when the communication device100is under a normal mode. At the normal mode, an output signal of the transmitter120is transmitted via a power amplifier122and an antenna124, and then received by the receiver130via an antenna128and a low noise amplifier126.

As shown inFIG. 3, in this embodiment the transmitter120includes a transmitting circuit140and an up converter150. The transmitting circuit140generates a detecting signal Sdec, which represents an original signal from the transmitter (Tx). The up converter150then up samples the detecting signal Sdecto generate a transmitted signal St. In addition, in this embodiment the transmitting circuit140includes a first processing circuit142(i.e., as the transmitting end, Tx) and a digital to analog converter146.

In this embodiment, the receiver130includes a low pass filter160as the SDR filter, a tunable compensation filter170, a receiving circuit180and a calibration unit190. When the communication device100is under the test mode, the low pass filter160receives the transmitted signal Stfrom the transmitter120, and filters and down samples the transmitted signal St to thereby generate a filtered signal Sf. Since people in this art know the operation and structure of the SDR filter well, further description is omitted here for brevity.

As shown inFIG. 3, the tunable compensation filter170compensates the filtered signal Sfto thereby generate a received signal Sres. The receiving circuit180includes an analog-to-digital converter186and a second processing circuit182(i.e., a receiving end, Rx). The receiving circuit180compares the Sresdetecting signal Sdecin a digital format with the received signal Srecin a digital format to generate at least an indicating signal Si.

Then, the calibration unit190adjusts at least a coefficient of the tunable compensation filter170according to the detecting signal Sdec. That is, the receiving circuit180adjusts the coefficient(s) of the tunable compensation filter170according to an inaccuracy degree (difference level) between the detecting signal Sdecand the received signal Srec.

In this manner, the communication device100adjusts the coefficients of the tunable compensation filter170when the frequency response of the SDR filter160varies to make the whole frequency response of the communication device100be a substantially flat curve.

It should be noted, however, that the above is not meant to be limitations of the present invention. In other embodiments of the present invention, the calibration unit190can compare a detecting signal Sdecin an analog format (i.e., signal at Tx) with a received signal in an analog format (i.e., signal at Rx) for executing the calibration.

From the above description, the calibration method tests and calibrates at least a pole of the tunable compensation filter170. That is, in one embodiment where the tunable compensation filter170has a plurality of pole pairs, the communication device100at this time will respectively compares and calibrates the signal on each of the pole pairs to hence adjust the coefficient(s) of the tunable compensation filter170when needed.

After the abovementioned operation (e.g. the calibrating operation), the gain response of the tunable compensation filter170is complementary to the gain response of the low pass filter160for compensating the low pass filter160. In addition, the communication device100can execute the calibrating operation when the communication device100is powered on, and/or when the performance is downgraded, and/or dynamically execute the calibration operation according to the design requirement or depending on the user/the system.

From the description above, the calibration method tests and calibrates at least a pole of the tunable compensation filter170. That is, in one embodiment where the tunable compensation filter170has a plurality of pole pairs, the communication device100at this time respectively compares and calibrates the signal on each of the pole pairs to hence adjust the coefficient(s) of the tunable compensation filter170when needed.

In a case where the tunable compensation filter170has N pole pairs, a calibrating loop (formed by the receiving circuit140and the receiving circuit180inFIG. 3) will have N sets of detecting signals Sdecand N sets of received signals Sreccorresponding to the N pole pairs of the tunable compensation filter170respectively.

Under the calibrating operation, each set of the detecting signals Sdecand the received signals Srecare compared for calculating the signal inaccuracy between the signals and the coefficient corresponding to a respective set in a predetermined period number is adjusted. The magnitude of the period number for calibrating each pole pair varies according to the design requirements.

Furthermore, the coefficient keeps its original value and is determined regardless of adjustment when an error value between one set of the detecting signals Sdecand the received signals Srecis smaller/equal to a predetermined threshold value. A coefficient of the tunable compensation filter170is adjusted when the error value corresponding to a pole pair of the tunable compensation filter170is larger than the predetermined threshold value.

Since the tunable compensation filter170compensates the low pass filter160, the tunable compensation filter170has a high frequency gain. In an embodiment, the tunable compensation filter170has four poles (two pole pairs) and a zero. Please refer toFIG. 5.FIG. 5is a Z-plane diagram of the tunable compensation filter170according to an embodiment of the present invention. Since the meaning of Z-plane and poles and zeros are known by people in this art, further description is omitted here for brevity.

As shown inFIG. 5, if there are a pole P1at π/4 and a pole P3at π/2 on the Z-plane, there must be a pole P2at −π/4 and a pole P4at −π/2 on the Z-plane. The pole P1and the pole P2form a first pole pair, while the pole P3and the pole P4form a second pole pair, and a zero Z0on a real axis (FIG. 5).

Supposing that a distance between the first pole pair (P1, P2) and the origin is expressed as m and a distance between the second pole pair (P3, p4) and the origin is expressed as n, and a distance between zero Z0and the origin is expressed as b0. An equation (1) of the tunable compensation filter170at this time can therefore be expressed as follows:

In this embodiment, the frequency response of the tunable compensation filter170is compensated by adjusting the magnitude of m and n. In this invention the distance between zero Z0and the origin b0can be any value except for 1. For instance, in an embodiment of the present invention, b0is set as −0.07.

When an equation of the tunable compensation filter170in the frequency domain is expressed as equation (1), a corresponding equation in time domain of the tunable compensation filter170can be expressed as:
y(n)=w(1)y(n−1)+w(2)y(n−2)+w(3)y(n−3)+w(4)y(n−4)+x(n−3)−b0×x(n−4)  (2)
Wherein
W=[w(1)w(2)w(3)w(4)]=[√{square root over (2)}m(m2+n2)√{square root over (2)}mn2m2n2]  (3)

Please refer toFIG. 6.FIG. 6is a block diagram illustrating a circuit structure of the tunable compensation filter170established according to the equation (1). In alternative designs, the circuit structure of the tunable compensation filter170differs depending on the equation (1). InFIG. 6, the circuit structure of the tunable compensation filter170includes a plurality of addition units210, a plurality of delay units220, and a plurality of multiplication units230. Since the associated meaning of m, n and b0have been detailed above; further description is omitted here for brevity.

As shown inFIG. 6, a coefficient K illustrated in the multiplication units230represents that, when necessary, a parallel shifting operation can be executed by multiplying K with the multiplication units230corresponding to x[n−3], and x[n−4] respectively.

Please refer toFIG. 7.FIG. 7is a curve diagram illustrating a frequency response and phase response of the tunable compensation filter170according toFIG. 6. As shown inFIG. 7, the magnitude of m is 0.51, and the magnitude of n is 0.73. As mentioned above, a frequency curve illustrated at f(a) has a vertical shift when multiplying K with the multiplication units230corresponding to x[n−3], and x[n−4] respectively.

Please refer toFIG. 8,FIG. 9andFIG. 10.FIG. 8is a diagram illustrating executing calibrating operation upon the communication100according to the detecting signals Sdecfrom the transmitter120and the received signals Srecfrom the receiver130.FIG. 9is a diagram illustrating adjusting the coefficient m corresponding to the first pole pair on the Z-plane.FIG. 10is a diagram illustrating adjusting the coefficient n corresponding to the second pole pair on the Z-plane.

For instance, in one embodiment of the present invention, the magnitude of the period number for calibrating the first pole pair (P1, P2) is 2, and the magnitude of the period number for calibrating the second pole pair (P3, P4) is 3. When an amplitude of the received signals Srecand an amplitude of the detecting signals Sdecare different and a corresponding error value is larger than a predetermined threshold range, the coefficient m corresponding to the first pole pair at this time will be adjusted.

Supposing that a sampling frequency of the communication device100is 40 MHz, the first processing circuit142within the transmitting circuit140at this time transmits a single tone signal having a frequency of π/4 (a separated signal, where every 8thsampled point forms a period) as the detecting signal Sdec—1, the digital detecting signals Sdec—1express continuous signals with frequency 5 MHz.

That is, the loop circuit formed by the transmitter120, the loop switch110and the receiver180transmits the detecting signals Sdec—1with frequency 5 MHz and receives the received signals Srec—1with frequency 5 MHz.

In a case where the amplitude of the received signals Srec—1is smaller than the amplitude of the detecting signals Sdec—1and the error value between the received signals Srec—1and the detecting signals Sdec—1exceeds a predetermined threshold value, the communication device100at this time determines the coefficient m of the first pole pair needs to be adjusted and the calibration unit190controls the tunable compensation filter170to adjust the coefficient m. The distance between the first pole pair and the origin is modulated by adjusting the magnitude of the coefficient m. Therefore, after the calibrating operation to adjust the magnitude of the coefficient m corresponding to the tunable compensation filter170, the amplitude of detecting signals Sdecand the amplitude of the received signals Srecwill be the same.

After calibrating the coefficient m of the first pole pair (P1, P2), the communication device then calibrates the coefficient of the second pole pair (P3, P4) of the tunable compensation filter170. Similarly, for calibrating the second pole pair (P3, P4), the first processing circuit142within the transmitting circuit140at this time transmits a single tone signal having a frequency of π/2 (a separated signal, and every 4thsampled point forms a period). The detecting signals Sdec—1 in a digital format express continuous signals with frequency of 10 MHz when the sampling rate of the communication device100is 40 MHz.

That is, when executing the calibrating operation corresponding to the second pole pairs, if the amplitude of the received signals Srecis different from the amplitude of the detecting signals Srec, the calibration unit190generates indicating signals Sito the tunable compensation filter170for adjusting the coefficient n corresponding to the second pole pairs of the tunable compensation filter170. Adjusting the magnitude of the coefficient n simultaneously means modulating the distance between the second pole pair and the origin.

Furthermore, in other embodiment, the tunable compensation filter170has N pole pairs, the communication device100will execute a calibrating operation N times to each pair of the pole pairs of the tunable compensation filter170.

To determine whether or not to adjust the corresponding coefficient of the tunable compensation filter170by comparing the corresponding voltage magnitude of the detecting signal Sdecwith the voltage of the received signals Srec. In an alternative design, any coefficients representing the signal magnitude can be used for comparing the signal unity between the transmitting end and the receiving end.

Since the operation of the communication device100has been disclosed above and is known by people in this art, further description is omitted here for brevity.

By adjusting the coefficients of the tunable compensation filter170, the tunable compensation filter170can adjust the coefficient (e.g., m, n) to thereby change the frequency of the tunable compensation filter170for compensating the low pass filter160while the frequency response of the low pass filter160deviates owing to the ambient temperature and/or environmental conditions. In this manner, the whole frequency response of the communication device100continues to meet the design requirements.

In conclusion, because of the low circuit complexity, the communication device100has an improved performance and a low cost. In addition, by changing the sampling rate of the analog to digital converter186, the communication device100can receive signals that comply with various specifications such as 802.11 1/b/g, GSM, Bluetooth, etc. The communication device100of the present invention is a flexible SDR communication device complying with various communication specifications that is able to simultaneously maintain good communication qualities.

In conclusion, because of the low circuit complexity, the communication device100has improved performance and a low cost. In addition, by changing the sampling rate of the analog to digital converter186, the communication device100can receive signals that comply with various specifications such as 802.11 1/b/g, GSM, Bluetooth, etc. The communication device100of the present invention is a flexible SDR communication device complying with various communication specifications that is able to simultaneously maintain good communication qualities.