The present invention relates generally to a receiver for receiving and tuning radio frequency (RF) signals and more specifically to an adaptive receiver for receiving RF signals and converting them into intermediate frequency (IF) signals.
RF receivers in general have been designed to convert incoming RF signals into IF signals and then convert IF signals to baseband.
However, some BPFs or LPFs may not have a frequency response as illustrated in
An adaptive receiver is disclosed, which may obviate one or more problems resulting from the limitations and disadvantages of the prior art.
Examples of the present invention may provide an adaptive receiver for receiving a radio frequency (RF) signal and converting the RF signal at an RF frequency FRF toward an adaptive intermediate frequency. The adaptive receiver may comprise (1) a two-dimensional look-up table (LUT) containing the frequency offset values, Δf, for a desired range of temperatures and process corners, (2) given the LUT, temperature and process corner values, a micro controller configured to estimate the center frequency of a BPF: FC=FIF+Δf, and (3) a local oscillator to generate sinusoidal signals at a frequency FLO equal to FRF minus FC. In the above, FC is an estimated center frequency of the BPF in the presence of temperature changes and process corner variations. The adaptive receiver uses FC as an “adaptive” intermediate frequency, also denoted as FIF,AD.
Oftentimes, one may not know either the temperature or process corner values. In that case, examples of the present invention may provide an adaptive receiver that comprises (1) a one-dimensional LUT containing the frequency offset values, Δf, for a desired range of two-tone voltage differences, wherein a measurement procedure is required to obtain the two-tone voltage differences, (2) a micro controller configured to estimate an adaptive intermediate frequency FIF,AD (=FIF+Δf) based on the measured two-tone voltage differences, and (3) a local oscillator to generate sinusoidal signals at a frequency FLO equal to FRF minus FIF,AD. In one example, the two-tone measurement procedure may be performed before an adaptive IF is estimated.
As another alternative to the above example, assuming that one can measure the temperature change accurately and hence only perform the two-tone measurement after power-on, the adaptive receiver may comprise (1) a one-dimensional LUT containing the frequency offset values, Δf, for a desired range of two-tone voltage differences, wherein a measurement procedure performed after the adaptive receiver is powered-on to obtain the two-tone voltage differences at a temperature Ton, (2) a temperature sensing device, when working together with a microprocessor, to measure the temperature difference ΔT between the operating temperature Top and Ton, (3) a micro controller configured to calculate an adaptive intermediate frequency FIF,AD (=FIF+Δf+Fslope*ΔT) based on Δf, a pre-estimated temperature coefficient, Fslope, and the temperature difference ΔT=Top−Ton, and (4) a local oscillator to generate oscillating signals at a frequency FLO equal to FRF minus FIF,AD. In this example, the two-tone measurement is performed only once after power on and the temperature sensing device, when working together with the microprocessor 13, is assumed to be capable of monitoring the temperature accurately. An adaptive intermediate frequency may thus be estimated with an accurate “temperature difference” value.
Examples of the present invention may also provide an adaptive receiver for receiving a radio frequency (RF) signal and converting the RF signal at an RF frequency FRF toward an adaptive intermediate frequency. The adaptive receiver may comprise (1) a two-dimensional look-up table (LUT) containing the frequency offset values, Δf, for a desired range of temperatures and process corners, (2) a micro controller configured to estimate the center frequency of the BPF based on the known temperature and process corner: FC=FIF+Δf, and (3) a local oscillator to generate sinusoidal signals at a frequency FLO=FRF−FIF,AD. The local oscillator further comprises a prescaler configure to operate in a first mode with a first prescaler factor N and a second mode with a second prescaler factor (N+1), N being a positive integer, and a sigma-delta modulator configured to generate a carry signal every M cycles of a reference clock to switch the prescaler from the first mode to the second mode.
Examples of the present invention may further provide an adaptive receiver for receiving a radio frequency (RF) signal and converting the RF signal at an RF frequency FRF toward an adaptive intermediate frequency. The adaptive receiver may comprise (1) a two-dimensional look-up table (LUT) containing the frequency offset values, Δf, for a desired range of temperatures and process corners, (2) a micro controller configured to estimate the center frequency of the BPF based on the known temperature and process corner: FC=FIF+Δf, and (3) a local oscillator to generate sinusoidal signals at a frequency FLO=FRF−FIF,AD. The local oscillator further comprises a crystal oscillator configured to generate a reference clock at a frequency F0, wherein F0 is a function of FD, a prescaler configured to operate in a first mode with a first prescaler factor N and a second mode with a second prescaler factor (N+1), N being a positive integer, and a sigma-delta modulator configured to generate a carry signal to switch the prescaler from the first mode to the second mode.
Additional features and advantages of the present invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The features and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one example of the present invention and together with the description, serves to explain the principles of the invention.
The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings examples which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.
In the drawings:
In this detailed description, for purposes of explanation, numerous specific details are set forth to illustrate examples of the present invention. One skilled in the art will appreciate, however, that examples of the present invention may be practiced without these specific details. Furthermore, one skilled in the art can readily appreciate that the specific sequences in which methods are presented and performed are illustrative and it is contemplated that the sequences can be varied and still remain within the spirit and scope of embodiments of the present invention.
To address the issue of temperature change and/or process corner variation, a frequency offset “Δf” may be generated by the adaptive receiver to compensate for the frequency shift in FIF due to either environmental temperature change or process variations. Accordingly, an adaptive frequency FIF,AD may be identified, where FIF,AD=FIF+Δf. A local oscillator (LO) may be configured to generate an LO signal at a frequency FLO=FRF−FIF,AD=FRF−(FIF+Δf). The frequency offset and the LO signal will be further discussed below.
Similarly, in the Q-path, a second frequency converter such as a Q-mixer 112 of the first mixer stage in the RF section 11 may down-convert the RF signal from the LNA 110 to the adaptive frequency (FIF+Δf) based on a second LO signal, i.e., a Q_LO signal, from the fractional LO generator 113 operating at the frequency FLO=FRF−(FIF+Δf). The I_LO and Q_LO signals transmit at the same frequency in quadrature phase to each other. Next, a quadrature channel selection filter such as a Q-BPF 115 with the center frequency FC=(FIF+Δf) may filter image or interference signals. The Q-BPF 115 has also two inputs, with its first input being the output of the I-mixer 111 and its second input being the output of the Q-mixer 112. The output signal of the Q-BPF 114 may be processed in a second VGA 117 and in turn a second ADC, Q-ADC 122, before sent to a down-conversion to baseband 123 in the BB section 12.
The two inputs (xI and xQ) to the down-conversion to baseband 123, when considered as complex equivalent signal xI+ixQ, where i=√{square root over (−1)}, is a signal with a center frequency at FC=(FIF+Δf). The down-conversion to baseband 123, equivalently a pair of second-stage I/Q mixers with the same center frequency at FC=(FIF+Δf), down-converts the complex equivalent signal to a pair of I/Q baseband signals, xI,BB and xQ,BB. Subsequently, a demodulator 124 may process the output signal from the down conversion to baseband 123 to retrieve the modulated data.
The fractional LO generator 113 may be configured to generate the I_LO and Q_LO signals at FLO=FRF−(FIF+Δf), based on a reference clock frequency F0, a first value ND related to the frequency shift FD, and a second value NCH related to the period a carry signal. In one example, the first value ND and the second value NCH may be related to a mean division ratio of a pre-scaler or frequency divider, which will be discussed by reference to
The first BPF 114 and the second BPF 115 in one example may be designed with a center frequency in a relatively low IF band, for example, ranging from approximately 1 MHz to 1.5 MHz. The center frequency may change as the environmental temperature changes. Table 1 below lists an example for IF center frequency variation at different temperatures, namely 95 degrees Celsius (° C.), 25° C. and −45° C. and for all process corners from TTT to FFF. Hence, it is a two-dimensional table. The FIF+Δf values shown in Table 1 are simulation results. The nominal intermediate frequency is at 1.248 MHz. In reality, one may not know the process corner of a specific receiver. Neither will a temperature sensing device 14 provide decent absolute accuracy in temperature measurement. Instead, a typical temperature sensing device 14, when working together with the microprocessor 13, can provide a decent accuracy while measuring temperature difference ΔT. Therefore, Table 1 is used to explain our concept below assuming one knows exactly both the temperature and the process corner. In reality, a calibration process is used to estimate the center frequency of the BPF accurately. This process will be detailed later.
1.248
1.485
0.902
In Table 1, the process corner may refer to a semiconductor process by which the adaptive receiver 10 may be fabricated. The characters “T”, “F” and “S” may represent typical, fast and slow, respectively. For example, TTT may mean typical split for thin oxide devices, typical split for thick oxide devices, and typical split for poly resistors. Furthermore, shown in bold characters in Table 1, a typical center frequency shift may be 1.248 MHz (TTT corner at 25° C.), the maximum center frequency shift may be 1.485 MHz (FFF corner at −45° C.) and the minimum center frequency shift may be 0.902 MHz (SSS corner at 95° C.). Moreover, given a same process corner, the maximum frequency deviation may fall within approximately 250 kilohertz (KHz). For example, the maximum frequency deviation may take place at the SST corner with a value of 245 KHz (=1.164 MHz−0.919 MHz).
The process corner information may be accessible from, for example, the manufacturer of the adaptive receiver 10. Furthermore, the temperature information may be obtained by the temperature sensing device 14, which may be configured to detect the environmental temperature during a power-on stage of the adaptive receiver 10 is turned on. Given the information on the process corner and the environmental temperature, as previously discussed with reference to
Our discussions have been focused on how the center frequency Fc of the band pass filter can vary as a function of temperature and process corner. The innovation resides in using this “varying” center frequency as the intermediately frequency, FIF,AD, for an adaptive receiver. The above discussions assume that the temperature and the process corner of the adaptive receiver are both known. Oftentimes, a temperature sensing device 14 does not provide an accurate absolute temperature, but the temperature difference can be measured with a decent accuracy. Typically, one does not know the process corner for a specific chip either. In that case, a calibration procedure can be used to estimate the FIF+Δf value.
Before describing a complete calibration procedure, a measurement procedure for estimating the FIF+Δf is first proposed. The measurement can be used even if neither the operating temperature nor the process corner is known.
Subsequently, the in-phase component of the second test tone, denoted “I-signal” in
The first voltage level VFX1 and the second voltage level VFX2 may be compared with each other, resulting in a difference value ΔVFX2-FX1 (=VFX2−VFX1) which may contain information on the center frequency shift Δf in the nominal intermediate frequency due to both the ambient temperature change and the process corner variation. Table 2 below lists examples of Δf values for different ΔVFX2-FX1 values.
Referring to Table 2, the value of Δf may increase as ΔVFX2-FX1 increases. In one example according to the present invention, Table 2 may be expanded to cover more comparison values in the same comparison range between −20 dB and 15 dB or in other desirable comparison ranges. Also referring to
As mentioned in the above, one may not know the process corner. The temperature measurement may not be absolutely accurate. However, the temperature difference can be accurately measured. Although one can simply repeat the two-tone measurements immediately before the adaptive receiver is used, it is not quite convenient in the presence of temperature changes. In that case, the following simplified calibration procedure can be used to accurately estimate the FIF,AD, and hence, FLO. It is assumed that the temperature during Steps 1 to 3 below remains unchanged.
A Simplified Calibration Procedure Based on Two-Tone Measurements:
Step 1: After power on, use the temperature sensing device 14 illustrated in
Step 2: Use the loop-back mode as described in the above using
Step 3: Determine the Δf value from the LUT 131;
Step 4: When operating at a different temperature (say Top, as measured by the temperature sensing device 14), compute
F
LO
=F
RF−(FIF+Δf+FslopeFslope×ΔT)
based on the temperature difference ΔT=Ton−Top, where Fslope is the FIF+Δf slope coefficient related to temperature difference ΔT.
End of Calibration Procedure.
When referring to
An output signal from the quadrature signal generator 515 may be fed through the prescaler 516 back to an input of the PFD 511, producing a negative feedback loop. If no error occurs, the frequency of the output signal is approximately 2.4 GHz. If the output frequency drifts, the error signal may increase, driving the frequency in the opposite direction so as to reduce the error. Thus the output is locked to the frequency at the other input, i.e., the reference clock.
The prescaler 516 may be operable in at least two modes with each mode having assigned a different prescaler factor. In the present example, the prescaler 516 may be configured to divide the frequency of the output signal from the quadrature signal generator 515 by a first prescaler factor “N” until a carry signal is provided thereto, N being a positive integer. The carry signal may be generated every M cycles of the reference clock and, when provided to the prescaler 516, may switch the division modulus from the first prescaler factor N to a second prescaler factor N+1.
The sigma-delta modulator 52 may further include an accumulator 520 and a delay element denoted as Z−1. The accumulator 520 may be configured to generate a carry signal for changing the mode of the prescaler 516. A summing device 514 such as an adder may be used to calculate the sum of a first value ND and a second value NCH. The generation of the carry signal may be done by storing a sum value of ND and NCH and processing a modulus function for updating the sum value. When the sum value reaches M, a carry signal may be generated. In the present example, ND and NCH are related to a mean division ratio Nf, which may be expressed in an equation given below.
Wherein k is the bit width of the carry signal and M, by definition, is equal to ND+NCH. The oscillating frequency FLO of the local oscillator 50 may be equal to the product of the reference frequency F0 and the division ratio Nf and thus may be written as follows.
Given the above Equation 2, the terms (FRF−FIF) and Δf in FLO may be defined in equations as given below.
Given FRF=2403.2 MHz, F0=16M and k=12, we may obtain
That is, N=150 and M (=0.2×4096=819.2) is rounded to 819, the closest integer.
of the reference frequency F0, b being the bit width of ND provided to the crystal oscillator 61. Furthermore, NCH may be related to a division ratio
of the prescaler 516. The oscillating frequency FLO of the local oscillator 60 may be equal to the product of the reference frequency F0, the trimming ratio
the division ratio
and may be written as follows.
Given the above equation, the terms (FRF−FIF) and FD in FLO may be defined as given below.
It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.
Further, in describing representative embodiments of the present invention, the specification may have presented the method and/or process of the present invention as a particular sequence of steps. However, to the extent that the method or process does not rely on the particular order of steps set forth herein, the method or process should not be limited to the particular sequence of steps described. As one of ordinary skill in the art would appreciate, other sequences of steps may be possible. Therefore, the particular order of the steps set forth in the specification should not be construed as limitations on the claims. In addition, the claims directed to the method and/or process of the present invention should not be limited to the performance of their steps in the order written, and one skilled in the art can readily appreciate that the sequences may be varied and still remain within the spirit and scope of the present invention.