This application claims priority to Chinese Application number 201010602472.6 entitled “A GFSK MODULATOR AND A METHOD FOR REDUCING RESIDUAL FREQUENCY MODULATION AND A DIGITAL ENHANCED CORDLESS TELECOMMUNICATION TRANSMITTER INCLUDING THE GFSK MODULATOR”, filed on Dec. 23, 2010, which is incorporated herein by reference.
The present application relates to signal modulation, particularly to Gaussian frequency switch keying (GFSK) modulation.
GFSK modulation is widely used in transmitters such as digital enhanced cordless telecommunication transmitters. A conventional GFSK modulator is based on an open-loop phase lock loop (PLL). However, open-loop PLLs usually have a frequency shift which can cause an increase in residual frequency modulation and worsen stability of a receiving system.
Accordingly, a GFSK modulator which is capable of controlling residual frequency modulation and ensuring stability of the receiving system is desirable.
According to an embodiment of the invention, a GFSK modulator comprises: a first compensation module, configured to receive a GFSK pulse signal, apply a first amplitude compensation and a first delay compensation to the GFSK pulse signal, so as to generate a first compensated control signal; a second compensation module, configured to receive the GFSK pulse signal, apply a second amplitude compensation and a second delay compensation to the GFSK pulse signal, so as to generate a second compensated control signal; a closed-loop PLL module configured to receive and use the first and the second compensated control signals to generate a modulated signal.
According to an embodiment of the invention, a method for GFSK modulation comprises: receiving a GFSK pulse signal with a GFSK modulator, the GFSK modulator comprises: a first compensation module configured to receive a GFSK pulse signal, apply a first amplitude compensation and a first delay compensation to the GFSK pulse signal, so as to generate a first compensated control signal; a second compensation module, configured to receive the GFSK pulse signal, apply a second amplitude compensation and a second delay compensation to the GFSK pulse signal, so as to generate a second compensated control signal; a closed-loop PLL module configured to receive and use the first and the second compensated control signals to generate a modulated signal; and performing GFSK modulation on the received GFSK pulse signal with the GFSK modulator.
According to an embodiment of the invention, a transmitter comprises: comprising a GFSK modulator, the GFSK modulator comprises: a first compensation module, configured to receive a GFSK pulse signal, apply a first amplitude compensation and a first delay compensation to the GFSK pulse signal, so as to generate a first compensated control signal; a second compensation module, configured to receive the GFSK pulse signal, apply a second amplitude compensation and a second delay compensation to the GFSK pulse signal, so as to generate a second compensated control signal; a closed-loop PLL module configured to receive and use the first and the second compensated control signals to generate a modulated signal.
The GFSK modulator according to embodiments of the invention can limit residual frequency modulation well with substantially no frequency drift. In addition, the GFSK modulator has satisfying stability and can be beneficial to lower power consumption.
Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified.
Various aspects and examples of the invention will now be described. The following description provides specific details for a thorough understanding and enabling description of these examples. Those skilled in the art will understand, however, that the invention may be practiced without many of these details. Additionally, some well-know structures or functions may not be shown or described in detail, so as to avoid unnecessarily obscuring the relevant description.
The terminology used in the description presented below is intended to be interpreted in its broadest reasonable manner, even though it is being used in conjunction with a detailed description of certain specific examples of the invention. Certain terms may even be emphasized below, however, any terminology intended to be interpreted in any restricted manner will be overtly and specifically defined as such in this Detailed Description section.
Generally, a residual frequency modulation includes two parts: FnPLL caused by phase noise of a PLL and FnMOD caused by GFSK modulation itself. In the description below, it will be discussed how to limit FnMOD.
Still as illustrated in
As illustrated in
Specifically, the DAC 1061 receives and converts the first compensated control signal 110 into an analog signal (also referred to as an analog GFSK signal) which is then forwarded to the first LPF 1062. The first LPF 1062 filters quantized noise and overlap interference from the analog signal so as to generate a first control signal. Further reference will be made to the first control signal in detail below.
The SDM 1063 is configured to receive and use the second compensated control signal 112 and a reference signal (not shown) to generate a second control signal. Further reference will be made to the second control signal in detail below.
The closed-loop PLL 1064 is configured to receive the first control signal and the second control signal and generate the modulated signal 114 controlled by the first and the second control signal. Further reference will be made to the closed-loop PLL 1064 below by referring to
The filtered signal is then forwarded by the second LPF 10642 to a voltage control oscillator (VCO) 10643 to control an output of the VCO 10643.
The VCO 10643 also receives the first control signal 110 as mentioned above, and uses the filtered signal and the first control signal to generate an oscillation signal. The VCO 10643 then forwards the generated oscillation signal to a first divider 10644 which divides a frequency of the oscillation signal by 2 so as to generate the modulated signal 114. Besides, the first divider 10644 also forwards the modulated signal 114 to a second divider 10645.
The second divider 10645 also receives the second control signal 112 as discussed above and uses the second control signal 112 and the modulated signal 10644 to generate the feedback signal.
As can be seen from the description above, a closed-loop PLL is hence formed with substantially no frequency drift.
As mentioned above, G1, D1, G2 and D2 can lower residual frequency modulation for the GFSK modulator 100 so as to improve the modulation quality. The higher the change precision of the four parameters (G1, D1, G2, D2) is, the lower the residual frequency modulation will be. Noting a bit-width of the DAC 1061 and an area of the GFSK modulator 100 increase with the change precision, it will be discussed how to determine G1, D1, G2 and D2 according to a desirable residual frequency modulation of the GFSK modulator 100.
Reference will be made to a procedure for determining at least one parameter for the GFSK modulator 100 by referring to
Refer to
A system function for a forward path:
A system function for a feedback path:
where s=j*2πf, the second divider 10645 divides a frequency of the modulated signal 114 by N.
According to a Mason's gain formula of the GFSK modulator 100 illustrated in
In practical system design, it is possible that G1·K mod≠2·G2·Fref, and D1≠D2. With the aid of G1, D1, G2 and D2, FnMOD can be controlled.
Let 2·G2·Fref=(1+ΔG)·(G1·K mod), D2=D1−ΔD, where ΔG is referred to as an amplitude mismatch, ΔD is referred to as a delay mismatch.
According to a Meson's gain formula of the GFSK modulator 100 in
Where “exp” is an exponential function, “abs” means to calculate an absolute value, “std” is a standard deviation, and “f” takes a value in
Brf is a bandwidth of the RF signal.
A change precision of each of G1, D1, G2 and D2 can be determined through a procedure as discussed below with reference to
Block 602
In a transmission system such as a digital enhanced cordless telecommunication system according to an embodiment of the invention, parameters of the closed-loop PLL 1064 may be as follows:
Blocks 604, 606
By applying the parameters above and ΔFMOD of GFSK with 288 KHz to equations (1), (2) and (3), a relationship between the amplitude mismatch and FnMOD,ΔG when ΔD=0, a relationship between the delay mismatch and FnMOD,ΔD when ΔG=0 can be also determined as illustrated in
Consider a phase noise, an actual residual frequency modulation of the GFSK modulator 100 is:
Fn=√{square root over (FnPLL2+FnMOD,ΔG2+FnMOD,ΔD2)} (4)
where FnPLL is determined by the PLL 1064 and can be obtained by simulation (block 608). In a digital enhanced cordless telecommunication, Fn is generally required to be no more than 15 KHz. By simulation, the FnPLL is about 10 KHz.
Block 610
By applying Fn and FnPLL into equation (4), and according to
Block 612
Reference signal 116 has a typical frequency of 13.824 MHz and can be easily procured and used as a sampling clock signal. The sampling clock signal has a sampling interval of 72 ns which is longer than last two combinations in Table 1, i.e., (8, 50) and (8.5, 25). Accordingly, according to an embodiment of the invention, the first combination, i.e., (7, 100) is chosen.
In view of system redundancy, an exemplary change precision of each of G1 and G2 shall be less than the chosen ΔG, an exemplary change precision of each of D1 and D2 shall be shorter than the chosen ΔD. According to an embodiment of the invention, the change precision of each of G1 and G2 is determined as 3.13%, the change precision of each of D1 and D2 is determined as 72 ns.
Accordingly, a bit-width of the DAC can be 6 bits and the reference signal can be used directly as the sampling clock signal of the system. This may be propitious to the following: (a) limit the area of the DAC and associated digital circuit; (b) simplify the system; and (c) meet the requirements regarding residual modulation.
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.
Number | Date | Country | Kind |
---|---|---|---|
2010 1 0602472 | Dec 2010 | CN | national |
Number | Name | Date | Kind |
---|---|---|---|
7224237 | Hirano et al. | May 2007 | B2 |
7343138 | Bengtson et al. | Mar 2008 | B2 |
7394862 | Jensen et al. | Jul 2008 | B2 |
7397300 | Quinlan et al. | Jul 2008 | B2 |
8081936 | Wang et al. | Dec 2011 | B2 |
20040041638 | Vilcocq et al. | Mar 2004 | A1 |
20040100588 | Hartson et al. | May 2004 | A1 |
20100171535 | Shanan | Jul 2010 | A1 |
20120082151 | Liu et al. | Apr 2012 | A1 |
Number | Date | Country |
---|---|---|
101272142 | Sep 2008 | CN |
Entry |
---|
Shih-An Yu, et al., “A 0.65-V 2.5-GHz Fractional-N Synthesizer With Two-Point 2-Mb/s GFSK Data Modulation,” Sep. 30, 2009 IEEE. vol. 44, No. 9, 2411 . . . 2424. |
Number | Date | Country | |
---|---|---|---|
20120163506 A1 | Jun 2012 | US |