Comparative Signal Strength Detection
A method for signal strength detection begins by comparing a signal strength representation of a signal with a signal strength representation of a reference signal. The method continues by adjusting, when the signal strength representation of the signal compares unfavorably with the signal strength representation of the reference signal, at least one of the signal strength representation of the signal and the signal strength representation of the reference signal until the signal strength representation of the signal compares favorably with the signal strength representation of the reference signal. The method continues by determining signal strength of the signal based on the adjusting of the signal strength representation of the signal and signal strength of the reference signal.
The present application is a divisional of U.S. patent application Ser. No. 11/388,675, titled “Comparative Signal Strength Detection,” filed on Mar. 24, 2006, which disclosure is incorporated herein by reference.
BACKGROUND OF THE INVENTION1. Technical Field of the Invention
This invention relates generally to signal processing and more particularly to determining signal strength of a signal.
2. Description of Related Art
As is known, wireless communication devices (e.g., a radio receiver) may tune to receive a selected radio frequency (RF) signal from within a frequency band of interest. For example, a frequency modulated (FM) radio receiver has a frequency band of interest from approximately 76 MHz to 108 MHz, where radio stations transmit on particular frequencies (e.g., 103.5 MHz). Thus, even though the radio receiver is tuned to a particular channel (e.g., 103.5 MHz), it has to be capable of receiving the entire frequency band of interest. This may present an issue if the signal strength detection for automatic gain control (AGC) is based on a narrow band signal (e.g., the particular station of 103.5 MHz) and not on the wide band signal (e.g., the frequency band of interest) being received.
For instance, if AGC is based solely on the narrow band signal and the wide band signal includes significant energy outside of the narrow band signal (e.g., outside of the selected channel), the low noise amplifier and/or the analog to digital converter of the radio receiver may saturate, causing an error. Accordingly, most radio receivers include a wide band signal strength detector for the automatic gain control.
As is further known, there are a variety of ways in which a wide band signal strength detector may be implemented. For example, a signal strength detector may be implemented as a peak detector or as an RMS (root mean square) detector. Issues with such detectors include complexity of circuit design, power consumption, accuracy, and bandwidth limitations of signals being detected.
Therefore, a need exists for a method and apparatus of signal detection that overcomes at least one of the above mentioned issues.
In operation, the low noise amplifier section 12 amplifies an RF signal 20 based on a gain setting 30 to produce an amplified RF signal 24. The low noise amplifier section 12 provides a representation 22 of the RF signal 20 to the signal strength detection module 18. The representation 22 of the RF signal may be the RF signal 20, an amplified version thereof, or a scaled version thereof The signal strength detection module 18 compares the representation 22 of the RF signal 20 with a reference signal 26 to produce a signal strength indication of the RF signal 28. The automatic gain control module 20 interprets the signal strength of the RF signal 28 to produce the gain setting 30. The functionality of the signal strength detection module 18 will be described in greater detail with reference to
The down conversion module 14 converts the amplified RF signal 24 into a near DC signal 32. The near DC signal 32 may have an intermediate frequency at DC or up to a few megahertz. The decoding module 16 converts the near DC signal 32 into a digitized signal 34. In one embodiment, the near DC signal 32 is a composite FM signal wherein the decoding module 16 converts the composite FM signal into a digitized audio signal.
In this embodiment, the AGC module 20 generates the gain setting 30 based on the signal strength 28 of the RF signal 20, which corresponds to a wide band signal, and a narrowband RSSI 46, which corresponds to a narrow band signal. The gain settings 30 may include gain settings for each LNA of LNA section 12, for the mix & filter module 40, for the ADC module 42, and/or for the decoding module 16. In this embodiment, the AGC module 20 may shift the gain between the wide bandwidth signal components (e.g., the LNA section 12 and the mix & filter module 40) and the narrow bandwidth signal components (e.g., the ADC module 42 and the decoding module 16) based on the signal strength of the wide band signal in relation to the signal strength of the narrow band signal. For example, if the RF signal includes significant energy from undesired channels with respect to the signal strength of the desired narrow band channel, the wide band signal strength will be noticeably greater than the narrow band signal strength. In this instance, more gain should be placed in the narrow bandwidth signal components and less in the wide bandwidth signal components to avoid saturating the wide bandwidth signal components, but still provide the desired signal levels for the narrow band signal.
To generate the signal strength 28 of the RF signal 20, the signal strength detection module 18 receives a differential representation 22 of the RF signal 20 from the first LNA of LNA section 12. The signal strength detection module 18 compares the differential representation of the RF signal 22 with the reference signal 26 to produce a signal strength indication 28 of the RF signal 20.
The mix and filter module 40 effectively mixes the amplified RF signal 24 with a local oscillation frequency and performs the equivalent of a low pass or band pass filter on the resulting mixed signal to produce an analog near DC signal. The analog-to-digital conversion module 42 converts the analog near DC signal into the near DC signal 32, which is in the digital domain. The narrow band RSSI module 44 determines the signal strength of the digital near DC signal 32. Note that if the mix and filter module 40 and/or the ADC module 42 receives a gain setting 30, the modules 40 and/or 42 perform their respective functions based on the corresponding gain setting 30.
In operation, the 1st signal metric conversion module 52 receives a signal 60, which may be RF signal 20 or 22 of
The 2nd signal metric conversion module 54 performs a signal metric conversion upon an adjusted AC voltage level 66 of a reference signal 64 to produce a signal strength representation 68 of the reference signal 64. The adjustable input module 58 is operably coupled to adjust the AC voltage level of a reference signal 64, which may be reference signal 26 of
The relational module 56 compares the signal strength representation 62 of the signal 60 with the signal strength representation 68 of the reference signal 64 to produce the relationship 70. For example, if the 1st and 2nd signal metric conversion modules 52 and 54 perform an RMS computation, the signal strength representations 62 and 68 will be RMS values of the corresponding signals 60 and 64. The relationship 70 will indicate a difference between the RMS values of the signal strength representations 62 and 68. The adjustable input module 58 adjusts the AC voltage level 66 of reference signal 64 based on the difference between the signal strength representations 62 and 68. With the AC voltage level of the reference signal adjusted, the 2nd signal metric conversion module 54 produces a signal strength representation 68 of the reference signal with an adjusted AC voltage level. The relational module 56 again produces relationship 70 based on the signal strength representation 68 of the reference signal with the adjusted AC voltage level and the signal strength representation 62 of the signal.
When the difference between the signal strength representations 62 and 68 converges to near zero, the adjustable input module 58 produces a signal strength 72 of signal 60 based on the amount of adjusting of the reference signal 64. For example, if the reference signal 64 has a known magnitude of 0 dBm and the difference is 20 dB (e.g., the adjustment was a divide by 10), then the signal strength of the signal is −20 dBm.
In operation, transistors T3 and T4 of the 1st signal metric conversion module 52 and transistor T5 of the 2nd metric conversion module 54 are biased with a voltage (V_bias) and AC coupled via capacitors C2, C3 and C4 to their respective inputs: signal 60 and adjustable AC voltage level 66 of reference signal 64. The transistors T1 and T2 of relational module 56 are biased, via filter 86 (which attenuates a linear component of the signal strength representation of the reference signal), to mirror the current of each other. As configured, the current into the drain of transistor T3 of the 1st signal metric conversion module 54 corresponds to the signal strength representation 62 of the signal 60 and the current into the drain of transistor T5 of the 2nd signal metric conversion module 54 corresponds to the signal strength representation 68 of reference signal 64.
In operation, counter 80 is initially set to zero for an up counter (or to a maximum value for a down counter). The voltage divider 82 receives the initial count form counter 80 and sets its divider ratio to divide-by-1. The number of bits used to indicate the count produced by counter 80 indicates the granularity of voltage divisions that may be performed by voltage divider 82. For example, 4 bits indicate 16 different voltage divisions that may be performed by voltage divider 82. In addition, the count produced by counter 80 provides a digital representation of the signal strength 72 of the signal 60.
With the counter 80 initialized, the voltage divider 82 divides the reference signal 64 by 1 such that the adjusted AC voltage level 66 of reference signal 64 is at the same level as the AC voltage level of reference signal 64. Note that in one embodiment, the reference signal 64 may be a clock signal that has a frequency comparable to the frequency of the signal 60. The 2nd metric conversion module 54 generates a current that represents an RMS value of the adjusted AC voltage level 66 of reference signal 64. This may be done in accordance with the equation:
where gm*Vgs is a linear term that is removed by the filter 86, 2(VGS-VT) is a DC portion that is removed by AC coupling, thus I is approximately equal to gmVgs2, which approximates a mean of the square function. Note that, in this embodiment, the mean of the square function may be used to approximate a root mean square (RMS) function.
Similarly, the 1st signal metric conversion module 52 converts the signal 60 into a current that represents an RMS value of the signal strength of signal 60. If the magnitude of the reference signal 64 is set at a maximum value at which signal 60 may be received and the signal 60 is not at the potential maximum value, the signal strength representation 62 will swing to a logic high voltage (since the T1 current will be larger than the T3 &T4 currents). The logic high voltage of the signal strength representation 62 will cause inverter 84 to present a count-down signal to the counter 80.
With the inverter 84 toggled the counter 80 counts up (or down), which causes the voltage divider 82 to divide the reference signal 64 by a new fixed value. For example, if this is the first clocked count-down signal, the divider ratio may change from a divide-by-1 to a divide-by-4. In general, the divider ratio will be set based on the range of the signal strength of the signal and the number of bits in the count. For example, if the signal strength of the signal 60 may range from 0 dBm to −100 dBm, and the count is a 4-bit count, the divider ratio may be 100 dB/16, which equals 6.25 dB per step. In practice, a 6 dB per step may be used. Note that finer granularity may be obtained by using a high bit counter.
With the counter performing a 1st count down, the voltage divider 82 divides the reference signal 64 by 4 to produce the adjusted AC voltage level 66 of reference 64. In this example, the adjusted AC voltage level 66 is one-fourth that of reference signal 64. Note that other divider levels may be used by voltage divider 82.
The 2nd signal metric conversion module 54 produces a current that represents an RMS value of the adjusted AC voltage level 66 as indicated by signal strength representation 68. The 1st signal metric conversion module 52 produces a similar current that represents the RMS value of signal strength representation 62 of signal 60. In this instance, if the signal level of the signal 60 is still below the adjusted AC voltage level 66, the inverter 84 will again provide a count-down signal to the counter 80. This causes the voltage divider 82 to divide the reference signal 64 by the next divider value to produce a new adjusted AC voltage level 66. This continues until the adjusted AC voltage level 66 of reference signal 64 is less than the AC voltage level of signal 60. Once this occurs, the current into transistors T3 & T4 will be greater than the current into transistor T5, which establishes a logic low input voltage to the inverter 84 that then provides a count-up signal to the counter. This will cause the adjusted AC voltage level 66 to be increased and will result in a toggling between the two count values where the adjusted AC voltage level 66 is higher, then lower than signal 60. When the counter is toggling between two values the RMS value is considered to be “resolved”. The signal strength 72 of the signal may be based on one or both of the toggling count values.
As an example, the divider 82 provides a 16 step, −1 dB per step, logarithmic divider where n=1 and x=2 such that R10 & R14 provide a divide by 1.12 that is gated via the least significant bit of the signal strength 72, R11 & R15 provide a divide by (1.12 2) that is gated via the second least significant bit of the signal strength 72, R12 & R12 provide a divide by (1.12 4) that is gated via the second most significant bit of the signal strength 72, and R13 & R17 provide a divide by (1.12 8) that is gated via the most significant bit of the signal strength 72. With these resistors values, the following divider ratios may be achieved with a four-bit signal strength 72. TABLE-US-00001 Signal Strength 72 Divide-by-Value Step Value in dB 0000 0 0 0001.1.12-1 0010 1.26-2 0011 1.41-3 0100 1.58-4 0101 1.78-5 0110 2.0-6 0111 2.23-7 1000 2.51-8 1001 2.82-9 1010 3.16-10 1011 3.54-11 1100 4.0-12 1101 4.47-13 1110 5.01-14 1111 5.62-15
As another example, a resistive divider 82 includes three resistive pairs where the logarithmic step is −3 dB and the control signal is a 3-bit signal. For this example, n=1 and x=21/2 such that the first resistive pair provides a divide-by-21/2, the second resistive pair provides a divide-by-2, and the third resistive pair provides a divide-by-4. Accordingly, this example of a resistive network provides a divider scale from 0 dB to −21 dB at −3 dB increments.
As one of average skill in the art will appreciate, other resistive divider ratios may be used to obtain a different logarithmic resistive divider scale and/or may include more resistor pairs and corresponding bits of a control signal (e.g., the signal strength). As one of ordinary skill in the art will appreciate, the resistive divider 82 may be part of a bigger divider circuit that includes fixed divider resistive pairs and/or adjustable divider resistive pairs. As one of average skill in the art will still further appreciate, the resistive divider may be coupled in a parallel with another resistive network for adjusting the another resistive network, or the resistors may be more generalized impedance elements with reactive components. Lastly, the impedance ratios shown above may be approximated in practice, without deviating from the scope of the present invention.
The process then proceeds to Step 102 where a determination is made as to whether the comparison was favorable. If not, the process proceeds to Step 104 where at least one of the signal strength representation of the signal and the signal strength representation of the reference signal is adjusted. In one embodiment, the adjusting may be done by amplifying the reference signal based on a 1st adjustable gain prior to generating the signal strength representation of the reference signal. The 1st adjustable gain may be set based on the comparing of the signal strength representation of the signal with the signal strength representation of the reference signal. Note that the 1st adjustable gain may be a fraction or a whole number. Alternatively, the signal may be adjusted by amplifying it based on a 2nd adjustable gain prior to generating the signal strength representation of the signal. In this embodiment, the 2nd adjustable gain is based on the comparison between the signal strength representation of the signal and the signal strength representation of the reference signal.
With one of the signal strength representations adjusted, the process repeats at Step 100. The process then proceeds again to Step 102 where a determination is made if the comparison of an adjusted signal representation with the signal strength representation is favorable. If not, the process remains in a loop of Steps 100, 102 and 104 until the comparison is favorable.
Once the comparison at Step 102 is favorable, the process proceeds to Step 106. At Step 106, the signal strength is determined based on the adjusting of at least one of the signal strength representation of the signal and the signal strength representation of the reference signal. Note that in one embodiment as depicted in
The process then proceeds to Step 114 where the signal strength representation of the signal is compared with the signal strength representation of the reference signal. The process then proceeds to Step 116 where a determination is made as to whether the comparison was favorable. If not, the process proceeds to either Step 118 or Step 120. At Step 118, the voltage-to-current factor of the reference signal is adjusted, which may be done by adjusting the transconductance of the transistors, adjusting the size of the transistors used for the conversion of Step 110, changing the biasing levels, and/or changing the magnitude of the AC voltage level. If Step 120 is used, the voltage-to-current factor of the signal is adjusted. The process then reverts to Steps 110 and 112 to produce a corresponding signal strength representation based on the adjusting of the reference signal or the signal. The process continues at Step 114 and Step 116 and remains in a loop until the comparison is favorable. Once the comparison at Step 116 is favorable, the process proceeds to Step 122 where the signal strength of the signal is determined based on the adjusting of the signal strength representation of the signal or the adjusting of the signal strength representation of the reference signal.
The method of
The method of
The method of
Step 132 of the method of
Step 132 of the method of
As one of ordinary skill in the art will appreciate, the term “substantially” or “approximately”, as may be used herein, provides an industry-accepted tolerance to its corresponding term and/or relativity between items. Such an industry-accepted tolerance ranges from less than one percent to twenty percent and corresponds to, but is not limited to, component values, integrated circuit process variations, temperature variations, rise and fall times, and/or thermal noise. Such relativity between items ranges from a difference of a few percent to magnitude differences. As one of ordinary skill in the art will further appreciate, the term “operably coupled”, as may be used herein, includes direct coupling and indirect coupling via another component, element, circuit, or module where, for indirect coupling, the intervening component, element, circuit, or module does not modify the information of a signal but may adjust its current level, voltage level, and/or power level. As one of ordinary skill in the art will also appreciate, inferred coupling (i.e., where one element is coupled to another element by inference) includes direct and indirect coupling between two elements in the same manner as “operably coupled”. As one of ordinary skill in the art will further appreciate, the term “operably associated with”, as may be used herein, includes direct and/or indirect coupling of separate components and/or one component being embedded within another component. As one of ordinary skill in the art will still further appreciate, the term “compares favorably”, as may be used herein, indicates that a comparison between two or more elements, items, signals, etc., provides a desired relationship. For example, when the desired relationship is that signal 1 has a greater magnitude than signal 2, a favorable comparison may be achieved when the magnitude of signal 1 is greater than that of signal 2 or when the magnitude of signal 2 is less than that of signal 1.
While the transistors in the above described figure(s) is/are shown as field effect transistors (FETs), as one of ordinary skill in the art will appreciate, the transistors may be implemented using any type of transistor structure including, but not limited to, bipolar, metal oxide semiconductor field effect transistors (MOSFET), N-well transistors, P-well transistors, enhancement mode, depletion mode, and zero voltage threshold (VT) transistors.
The preceding discussion has presented a plurality of method and apparatus for signal strength detection. As one of ordinary skill in the art will appreciate, other embodiments may be derived from the teaching of the present invention without deviating from the scope of the claims.
Claims
1. A impedance divider network comprises:
- a plurality of impedance pairs, wherein each of the plurality or impedance pairs has a corresponding divider ratio; and
- a plurality of switching elements operably coupled to the plurality of impedance pairs, wherein, the plurality of switching elements enable at least one of the plurality of impedance pairs to provide a divider value for the impedance divider network based on a state of a control signal, wherein the plurality of corresponding divider ratios provide a desired step change of the divider value in accordance with a corresponding state change of the control signal, wherein the desired step change contains nonlinear steps.
2. The impedance divider network of claim 1, wherein the desired step change comprises a logarithmic step change.
3. The impedance divider network of claim 1 comprises:
- a first divider ratio of the plurality of corresponding divider ratios including a divide-by-x1*n ratio.
- a second divider ratio of the plurality of corresponding divider ratios including a divide-by-x2*n ratio; and
- a third divider ratio of the plurality of corresponding divider ratios including a divide-by-x3*x ratio, where “n” is an integer and “x” is a real number.
Type: Application
Filed: Mar 10, 2010
Publication Date: Jun 24, 2010
Inventors: Matthew D. Felder (Austin, TX), Michael R. May (Austin, TX)
Application Number: 12/721,014
International Classification: H03H 7/00 (20060101);