Initial frequency synchronization mechanism

FIELD OF THE INVENTION

[0001] The present invention relates to frequency acquisition in general and to frequency acquisition in the presence of high power adjacent channels, in particular.

BACKGROUND OF THE INVENTION

[0002] Reference is now made to FIGS. 1A and 1B. FIG. 1A is a schematic illustration of frequency versus power, describing the initial stage of a initial frequency synchronization procedure, known in the art. The present example describes a closed loop automatic frequency control (AFC) mechanism.

[0003]FIG. 1B is a schematic illustration of frequency versus power, describing the final stage of the initial frequency synchronization procedure of FIG. 1A.

[0004] Arrow <b>14</b> represents the frequency of a mobile unit which detects and attempts to lock and synchronize with the carrier frequency <b>10</b> of a base unit transmitter having a value of F_BASE, which is located near by. In the present example the mobile unit further detects a carrier frequency <b>12</b> provided by a neighbor transmitter, having a value of F_NEIGHBOR. The value of the mobile unit F_MOBILEis located between the values of the base unit frequency F_BASEand the neighbor mobile transmitter frequency F_NEIGHBOR.

[0005] In the present example the mobile unit <b>14</b> detects the signals provided by base <b>10</b> and the neighbor <b>12</b> wherein the received power of the neighbor <b>12</b> is higher than the received power of the base unit <b>10</b>.

[0006] According to conventional initial synchronization procedures, the mobile unit frequency is synchronized with the frequency having the highest received power, which in the present example is the neighbor frequency <b>12</b>.

[0007] It will be noted that often the received frequencies are filtered so as to exclude undesired signals. Such a filter is represented by arc <b>16</b>. These techniques often fail when the power of the undesired signal is significantly high.

[0008] Accordingly the synchronization mechanism of the mobile unit sets synchronization path towards the neighbor frequency F_NEIGHBORand starts progressing its frequency <b>14</b> towards F_NEIGHBOR. Finally the synchronization mechanism allows the frequency of the mobile unit <b>14</b> to acquire and synchronize with the frequency of the neighbor unit <b>12</b>. This is shown in FIG. 1B by aligning line <b>12</b> and arrow <b>14</b>. As can be seen, at this stage the frequency <b>10</b> of the base transmitter is filtered out by the filter <b>16</b>.

[0009] A conventional synchronization mechanism provides a frequency shifts within a limited range, determined by its structure, such as VCO voltage and the like. It will be appreciated by those skilled in the art that a the F_NEIGHBORcan be located outside this range in such a case, F_MOBILE, might get stuck at the boundary frequency value which is closest to F_NEIGHBOR.

[0010] It will be appreciated by those skilled in the art that such situations, where the frequency of the mobile unit <b>14</b> is synchronized with the frequency of neighbor unit <b>12</b> instead of the frequency of the base unit <b>10</b>, is not acceptable.

SUMMARY OF THE PRESENT INVENTION

[0011] It is an object of the present invention to provide a novel method for performing accurate initial frequency acquisition in the presence of high power adjacent channels.

[0012] It is a further object of the present invention to provide a novel device for performing accurate initial frequency acquisition in the presence of high power adjacent channels.

[0013] In accordance with the present invention there is thus provided a method for acquiring frequency of a desired channel having a carrier frequency F_MAIN, for a dynamic receiver frequency F_MOBILE, from a starting frequency F_START, in the presence of high power adjacent interfering channels.

[0014] The starting frequency F_STARTis shifted from F_MAINby not more than a predetermined frequency gap ΔF. The method includes the steps of:

[0015] determining a first frequency boundary and a second frequency boundary;

[0016] detecting channels within a filtering bandwidth;

[0017] selecting a dominant channel from the detected channels;

[0018] progressing the dynamic receiver frequency F_MOBILEtowards the carrier frequency of the dominant channel;

[0019] detecting when the step of progressing has exceeded one of the first frequency boundary and the second frequency boundary;

[0020] restarting the step of detecting channels, from the other of the one of the first frequency boundary and the second frequency boundary; and

[0021] repeating from the step of detecting channels.

[0022] According to another aspect of the present invention, one of the first frequency boundary and the second frequency boundary is F_START+ΔF, while the other is F_START+ΔF.

[0023] The method of the invention can also include the step of determining a frequency advance direction. The frequency advance direction can be fixed at the beginning of each frequency acquisition cycle, wherein the frequency acquisition cycle is determined from the point where F_MOBILEshifts from F_STARTuntil the point where F_MOBILEreturns to F_START.

[0024] The step of progressing can be performed in a frequency step F_STEP. The value of the frequency step F_STEPcan be infinitesimal with comparison to the predetermined frequency gap ΔF, or adjustable. Accordingly, the method can further include the step of adjusting the frequency step F_STEPafter each step of detecting channels.

[0025] In accordance with another aspect of the present invention, there is provided a device for acquiring frequency of a desired channel having a carrier frequency F_MAIN, for a dynamic receiver frequency F_MOBILE, from a starting frequency F_START, in the presence of high power adjacent interfering channels.

[0026] The device is connected to an antenna via a receiver and to a reference frequency F_REFERENCEsource. The device includes controllable frequency generating means for generating an internal frequency F_INTERNAL, frequency shift means connected to the controllable frequency generating means, and to the receiver, for shifting received frequency F_RECEIVED, of a received channel, according to the internal frequency F_INTERNAL.

[0027] The device also includes a frequency shift detector, connected to the frequency shift means, for detecting a frequency difference between the internal frequency F_INTERNALand the received frequency F_RECEIVED, with respect to the reference frequency F_REFERENCE, thereby producing a frequency shift value F_SHIFT.

[0028] The device further includes loop filtering means, connected to the frequency shift detector, for filtering the frequency shift value F_SHIFT, thereby producing a filtered frequency shift value F_{SHIFT-FILTERED}, and controlling means, connected to the controllable frequency generating means and to the loop filtering means, for determining a frequency step F_STEPfrom the filtered frequency shift value F_{SHIFT-FILTERED}.

[0029] The controlling means provide the frequency shift value F_SHIFTto the controllable frequency generating means. The controllable frequency generating means adjust the internal frequency F_INTERNALaccording to the frequency shift value F_SHIFT, and the controlling means control the controllable frequency generating means to generate frequency in a range from a first frequency boundary F_FIRSTand a second frequency boundary F_SECOND.

[0030] According to one aspect of the invention, the controlling means set the frequency shift value F_SHIFTto be F_SECOND−F_INTERNAL, when |F_INTERNAL−F_START|≧|F_INTERNAL−F_FIRST|, while the controlling means set the frequency shift value F_SHIFTto be F_FIRST−F_INTERNAL, when |F_INTERNAL−F_START|≧|F_INTERNAL−F_SECOND|.

[0031] According to another aspect of the invention, the device further includes frequency filtering means, connected between the frequency shift detector and frequency shift means.

[0032] The controlling means reset the loop filtering means when setting the frequency shift value F_SHIFTto be F_SECOND−F_INTERNALor F_FIRST−F_INTERNAL.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which:

[0034]FIG. 1A is a schematic illustration of frequency versus power, describing the initial stage of a conventional initial frequency synchronization procedure;

[0035]FIG. 1B is a schematic illustration of frequency power, describing the final stage of the initial frequency synchronization procedure of FIG. 1A;

[0036]FIG. 2A is a schematic illustration of frequency versus power, describing the initial stage of a frequency synchronization procedure, operative in accordance with the present invention;

[0037]FIG. 2B is a schematic illustration of frequency versus power, describing the secondary stage of a frequency synchronization procedure, operative in accordance with the present invention;

[0038]FIG. 2C is a schematic illustration of frequency versus power, describing the third stage of a frequency synchronization procedure, operative in accordance with the present invention;

[0039]FIG. 2D is a schematic illustration of frequency versus power, describing the final stage of a frequency synchronization procedure, operative in accordance with the present invention;

[0040]FIG. 2E is a schematic illustration of frequency versus power, describing the third stage of a frequency synchronization procedure, operative in accordance with another aspect of the present invention;

[0041]FIG. 2F is a schematic illustration of frequency versus power, describing the final stage of a frequency synchronization procedure, operative in accordance with another aspect of the present invention;

[0042]FIG. 3 is a schematic illustration of a device for synchronizing frequencies, constructed and operative in accordance with another preferred embodiment of the invention;

[0043]FIG. 4 is a schematic illustration of a method for operating the device of FIG. 3, operative in accordance with a further embodiment of the invention;

[0044]FIG. 5A is a schematic illustration of a method for operating the device of FIG. 3, operative in accordance with yet another embodiment of the invention; and

[0045]FIG. 5B is a schematic illustration in detail of a step of the method of FIG. 5A.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0046] The present invention overcomes the disadvantages of the prior art by providing a frequency detect and fold mechanism. Accordingly, when the frequency shift exceeds a boundary value, then a predetermined frequency shift is enforced.

[0047] Reference is now made to FIGS. 2A, 2B, <b>2</b>C and <b>2</b>D. FIG. 2A is a schematic illustration of frequency versus power, describing the initial stage of a frequency synchronization procedure, operative in accordance with the present invention. FIG. 2B is a schematic illustration of frequency versus power, describing the secondary stage of a frequency synchronization procedure, operative in accordance with the present invention. FIG. 2C is a schematic illustration of frequency versus power, describing the third stage of a frequency synchronization procedure, operative in accordance with the present invention. FIG. 2D is a schematic illustration of frequency versus power, describing the final stage of a frequency synchronization procedure, operative in accordance with the present invention.

[0048] The schematic illustration provided by FIG. 2A describes the frequency <b>100</b> of a base station, having a value F_BASE, a frequency <b>104</b> of a mobile unit, having an initial value F⁰_MOBILEand a frequency <b>102</b> of a neighbor transmitter, having the value of F_NEIGHBOR, wherein

[0049] F_BASE&lt;F⁰_MOBILE&lt;F_NEIGHBOR.

[0050] In conventional communication standards, such as AMPS, NAMPS, JTACS, NTACS, USDC-TDMA and the like, the initial value of F⁰_MOBILEof the mobile unit frequency <b>104</b> can be shifted from the value F_BASEof the base station frequency <b>100</b>, by no more than a predetermined frequency gap ΔF. Another condition set by these standards is that any neighbor transmitter will transmit in a frequency F_NEIGHBOR, which is considerably shifted from F_BASE. Accordingly |F_BASE−F_NEIGHBOR|&gt;2ΔF.

[0051] The method of the present invention generally searches the received spectrum within a frequency range of [F⁰_MOBILE−ΔF, F⁰_MOBILE+ΔF], for stabilized frequency values.

[0052] According to the invention, at the initial stage (i.e., at frequency F⁰_MOBILE) the mobile unit detects all of the signals of transmitters in its vicinity and detects the frequency of the signal with the highest received power, which in the present example is the neighbor transmitted frequency <b>102</b>. Accordingly, the mobile unit commences shifting its frequency towards frequency <b>104</b> from the value of F⁰_MOBILE, towards the value F_NEIGHBORof neighbor transmitter frequency <b>102</b>.

[0053] The present invention makes use of the above limitations, of conventional communication standards, which outline that the initial value F⁰_MOBILEof the mobile unit frequency <b>104</b> has to be within a frequency gap of ΔF from the value F_BASE, of the base transmitter frequency <b>100</b>.

[0054] Accordingly, any shift from the initial stage F⁰_MOBILE, cannot exceed the value of ΔF. After the frequency <b>104</b> of the mobile unit has progressed towards the neighbor transmitter frequency <b>102</b> value F_NEIGHBOR, by a frequency shift <b>110</b>, having a value of ΔF, to the value F¹_MOBILE, then, according to the invention, any further progress in this direction would result in a detection error and hence, should not be pursued.

[0055] At this stage, the present invention determines a reversed path <b>112</b> for frequency <b>104</b> (FIG. 2C) for shifting frequency <b>104</b> from the value of F¹_MOBILEto the value of F²_MOBILEwherein the shift value of this reverse path <b>112</b>, is a frequency gap which is twice the value of ΔF.

[0056] At the final stage (FIG. 2D) the spectrum is searched, thereby detecting the base frequency <b>100</b> as the dominant signal. Accordingly, the mobile unit <b>104</b> commences shifting its frequency towards base frequency <b>100</b>, from the value of F²_MOBILEto F_BASE. This shift is shown by path <b>114</b>. According to the present example, no direction is enforced for path <b>114</b>.

[0057] It will be noted that applying a filter, such as filter <b>106</b>, improves the performance of an initial synchronization process, according to the invention. As illustrated in FIG. 2C, as long as the filter size is less than |F_BASE-F_NEIGHBOR|×2, (provided that the filter is generally symmetrical), wherein F_NEIGHBORis not a high power signal, then, F_NEIGHBORwould not be detected as a major signal by the receiver of the mobile unit, in the original direction of progress.

[0058] Reference is now FIG. 3 which is a schematic illustration of a device for synchronizing frequencies, generally referenced <b>200</b>, constructed and operative in accordance with another preferred embodiment of the invention.

[0059] Device <b>200</b> includes a frequency shift unit <b>202</b>, an inter-mediate frequency (I.F.) filter <b>204</b> connected to the frequency shift unit <b>202</b>, a frequency shift detector <b>206</b> connected to the I.F. filter <b>204</b>, a loop filter <b>208</b> connected to the frequency shift detector <b>206</b>, a non-linear controller <b>210</b>, connected to the loop filter <b>208</b> and a voltage control oscillator (VCO) <b>212</b>, connected to the non-linear controller <b>210</b> and to the frequency shift unit <b>202</b>. It will be noted that VCO <b>212</b> can be replaced with any type of controlled oscillator.

[0060] The frequency shift unit <b>202</b> is further connected to an antenna <b>220</b>. The frequency shift detector <b>206</b> is further connected to a host <b>222</b>. The host <b>222</b> provides a reference frequency value to the frequency shift detector <b>206</b>.

[0061] The antenna <b>220</b> detects frequency signals of neighbor transmitters wherein one of these detected frequency signals is transmitted by a base station. The antenna <b>220</b> provides these received frequency signals to the frequency shift unit <b>202</b>. The VCO <b>212</b> generates a signal having a frequency and provides it to frequency shift unit <b>202</b>.

[0062] Frequency shift unit <b>202</b> shifts frequencies, received from antenna <b>220</b>, according to the frequency provided by the VCO and provides the results to the I.F. filter <b>204</b>. The I.F. filter <b>204</b> filters some of these frequencies and provides the remaining ones to the frequency shift detector <b>206</b>. The frequency shift detector <b>206</b> attempts to detect the frequency shift of each of these shifted frequencies from the reference frequency value, provided by the host <b>222</b>.

[0063] Accordingly, the frequency shift detector <b>206</b> determines a frequency shift value and provides it to the loop filter <b>208</b>. The loop filter <b>208</b> includes the history of the frequency shifts performed by device <b>200</b> and accordingly determines a frequency shift direction and provides it with the frequency shift value to the non-linear controller <b>210</b>.

[0064] The non-linear controller <b>210</b> detects if the overall shift, up until this stage has exceeded the value of ΔF. If so, then the non-linear controller <b>210</b> provides VCO <b>212</b> with the command to generate a reversed frequency shift such as the one according to path <b>112</b> (FIG. 2C). If not, then the non-linear control <b>210</b> provides the VCO <b>212</b> with a frequency shift value and a frequency shift direction for further shifting the frequency towards the most dominant received frequency. Then, the VCO <b>212</b> provides a new shift frequency to the frequency shift unit <b>202</b> and the process is repeated from the beginning.

[0065] It will be noted that when using a slow loop filter, such a software implemented loop filter, it would be difficult for such a loop filter to process a considerable shift such as the one defined by path <b>112</b>, since such shifts are compared to frequency behavior history contained therein.

[0066] According to a further aspect of the invention, when the non-linear controller <b>210</b> determines a 2ΔF shift, it also sends a clear command back to the loop filter <b>208</b>, thereby erasing the frequency history contained in the memory of loop filter <b>208</b>. This operation enables the loop filter <b>208</b> to further process considerable frequency shifts.

[0067] It will be noted that the terms base, mobile and neighbor are presented as a matter of convenience only. The present invention is applicable for any type of initial frequency acquisition in the presence of a high power adjacent channels, wherein the base of the above example is assigned to a main transmitter, the mobile of the above example is assigned to a receiver and the neighbor of the above example is assigned to an adjacent interfering transmitter.

[0068] It will be noted that each of the main transmitter, the adjacent transmitter and the receiver may be implemented for a mobile unit, a base unit and the like.

[0069] Reference is now made to FIG. 4 which is a schematic illustration of a method for operating the device <b>200</b> of FIG. 3, operative in accordance with a further embodiment of the invention.

[0070] In step <b>300</b>, the device <b>200</b> stores the value F⁰of the internal initial frequency F. F⁰is used to determine, later on, the total amount of shift from the initial frequency. It will be noted that for this purpose, the device <b>200</b> can store and accumulate the values of the later frequency shifts, instead.

[0071] In step <b>302</b>, the device <b>200</b> detects incoming frequency signals.

[0072] In step <b>304</b>, the device <b>200</b> filters the incoming frequency signals, thereby obtaining selected frequencies.

[0073] In step <b>306</b>, the device <b>200</b> determines a target frequency value F_TARGET, from the selected frequencies. In the present example (FIG. 2A), the device <b>200</b> (FIG. 3) selects the right side signal <b>102</b> (F_NEIGHBOR), as the target frequency F_TARGET.

[0074] In step <b>308</b>, the device <b>200</b> progresses the internal frequency F towards the target frequency F_TARGETby a predetermined frequency step F_STEP. It will be noted that F_STEPcan be determined using a range of consideration, such as speed, accuracy and the like. In general, F_STEPis determined to be significantly smaller than ΔF, thereby yielding higher accuracy. It will further be noted that when F_STEPcan be infinitesimal thereby yielding an analog like behavior.

[0075] In step <b>310</b>, the device <b>200</b> detects if the internal frequency F was shifted beyond a gap of ΔF. If so, then the device <b>200</b> proceeds to step <b>312</b>. Otherwise, the device <b>200</b> proceeds to step <b>314</b>.

[0076] In step <b>312</b>, the device <b>200</b> reverses F by 2ΔF. In the present example (FIG. 2C), reverse path <b>112</b>, describes such a reverse shift, from the value of F¹_MOBILEto the value of F²_MOBILE. Then, the device <b>200</b> repeats the steps of the above method, from step <b>302</b>.

[0077] It will be noted that at this stage, signal <b>102</b> appears to be outside of the filtering bandwidth of filter <b>106</b>, thereby leaving the base station frequency signal <b>100</b>, the strongest, at the output of filter <b>106</b>. Accordingly, the device <b>200</b> determines F_BASEas F_TARGET.

[0078] In step <b>314</b>, the device <b>200</b> detects if the internal frequency F is synchronized with the target frequency F_TARGET. If so, then the device <b>200</b> has completed the initial frequency acquisition procedure and accordingly, locks the frequency F (step <b>316</b>). Otherwise, the device <b>200</b> repeats the steps of the above method, from step <b>300</b>.

[0079] The method of FIG. 4 overcomes a situation where there exists a interfering neighbor frequencies such as F_NEIGHBOR(reference numeral <b>102</b>) on one side of the spectrum.

[0080] In a situation where there exist interfering neighbor frequencies on both sides of the base frequency F_BASE, the present invention provides a slightly different solution, as will be disclosed hereinbelow.

[0081] Reference is now made to FIGS. 2E and 2F. FIG. 2E is a schematic illustration of frequency versus power, describing a stage of a frequency synchronization procedure, operative in accordance with another aspect of the present invention. FIG. 2F is a schematic illustration of frequency versus power, describing a final stage of a frequency synchronization procedure, operative in accordance with another aspect of the present invention.

[0082] According to the present example, there exists an additional neighbor frequency <b>120</b> having a value of F*_NEIGHBOR, on the left side of the base frequency <b>100</b> F_BASE. When the mobile frequency completes the 2ΔF frequency shift <b>112</b>, additional neighbor frequency <b>120</b> falls within the filtering bandwidth of filter <b>106</b>, together with base frequency <b>100</b>.

[0083] It will be noted that if, at the output of filter <b>106</b>, the signal of the additional neighbor frequency <b>120</b> appears to be stronger than the signal of the base frequency <b>100</b>, then, according to the method of FIG. 3, the mobile frequency <b>104</b> would be drawn towards the additional neighbor frequency <b>120</b>.

[0084] According to another aspect of the present invention, the initial direction set forth in the second stage (i.e., the direction of frequency shift <b>110</b>, (FIG. 2B)), is stored. In the present example, this direction is from left to right.

[0085] Then, after the mobile frequency completes the 2ΔF frequency shift <b>112</b>, the acquisition mechanism continues searching in that initial direction, only. It will be noted that such forced search direction provides an accurate acquisition of the desired base frequency, in one or less search cycle.

[0086] In a more detailed form, at the final stage (FIG. 2F) the spectrum is searched again in the direction set forth in the initial stage (i.e., the direction of shift <b>110</b>), thereby detecting the base frequency <b>100</b> as the dominant signal. Accordingly, a path <b>122</b> is set towards base frequency <b>100</b>, for shifting mobile frequency <b>104</b> from the value of F²_MOBILEto F_BASE.

[0087] It will be noted that the present invention provides a search shift step which can be calibrated at each search stage. For example, one the one hand, in the presence of a powerful additional neighbor <b>120</b>, frequency shift <b>122</b> may include a large number of infinitesimal frequency shift steps. Otherwise, frequency shift <b>122</b> may include a small number of larger frequency shift steps.

[0088] Reference is now made to FIGS. 5A and 5B. FIG. 5A is a schematic illustration of a method for operating the device <b>200</b> of FIG. 3, operative in accordance with yet another embodiment of the invention. FIG. 5B is a schematic illustration in detail of step <b>406</b> of the method of FIG. 5A.

[0089] In step <b>400</b>, the device <b>200</b> stores the value F⁰of the internal initial frequency F.

[0090] In step <b>402</b>, the device <b>200</b> detects incoming frequency signals.

[0091] In step <b>404</b>, the device <b>200</b> filters the incoming frequency signals, thereby obtaining selected frequencies.

[0092] In step <b>406</b>, the device <b>200</b> determines frequency step F_STEPand a frequency advance direction, in a way which is described in detail in FIG. 5B.

[0093] In step <b>418</b>, if the detection performed according to step <b>402</b> is the first detection in the current acquisition cycle, then, the device <b>200</b> proceeds to step <b>420</b>. Otherwise, the device <b>200</b> proceeds to step <b>408</b>.

[0094] In step <b>420</b>, the device <b>200</b> determines an initial advance direction which will be constant during the present acquisition cycle, and proceeds to step <b>408</b>.

[0095] In step <b>408</b>, the device <b>200</b> progresses the internal frequency F by frequency step F_STEP, in the advance direction.

[0096] In step <b>410</b>, the device <b>200</b> detects if the internal frequency F was shifted beyond a gap of ΔF. If so, then the device <b>200</b> proceeds to step <b>412</b>. Otherwise, the device <b>200</b> proceeds to step <b>414</b>.

[0097] In step <b>412</b>, the device <b>200</b> reverses F by 2ΔF. In the present example (FIG. 2E), reverse path <b>112</b>, describes such a reverse shift, from the value of F¹_MOBILEto F²_MOBILE. Then, the device <b>200</b> repeats the steps of the above method, from step <b>402</b>.

[0098] It will be noted that at this stage, additional neighbor frequency signal <b>120</b> falls within the filtering bandwidth of filter <b>106</b>, which poses a problem if additional neighbor frequency signal <b>120</b> appears stronger than the base station signal <b>100</b>, at the output of filter <b>106</b>.

[0099] Referring now to FIG. 5B, the device <b>200</b> determines a target frequency value F_TARGETfrom the selected frequencies (step <b>430</b>). In the present example, when the mobile frequency is at a value of F⁰_MOBILE(FIG. 2A), the device <b>200</b> (FIG. 3) selects the right side signal <b>102</b> (F_NEIGHBOR), as the target frequency F_TARGET. Alternatively, when the mobile frequency is at a value of F²_MOBILE(FIG. 2E), the device <b>200</b> (FIG. 3) selects the left side signal <b>120</b> (F*_NEIGHBOR), as the target frequency F_TARGET.

[0100] In step <b>432</b>, if the detection performed according to step <b>402</b> is the first detection in the current acquisition cycle, then, the device <b>200</b> proceeds to step <b>440</b>. Otherwise, the device <b>200</b> proceeds to step <b>434</b>.

[0101] In step <b>434</b>, the device <b>200</b> determines an advance direction from the mobile frequency value F and the target frequency value F_TARGET.

[0102] In step <b>436</b>, if the advance direction determined in step <b>434</b> is equal to the initial advance direction, determined in step <b>420</b>, then, the device <b>200</b> proceeds to step <b>440</b>. Otherwise, the device <b>200</b> proceeds to step <b>438</b>. It will be noted that a situation where these directions are not equal occurs, for example, when a neighbor signal, such as the one of additional neighbor frequency <b>120</b>, appears to be stronger than the signal of the base frequency <b>100</b>, at the output of the filter <b>106</b>.

[0103] In step <b>440</b>, the device <b>200</b> determines the frequency step F_SETPaccording to the position of F and F_TARGET. In the present example, F_SETP≦|F−F_TARGET|.

[0104] In step <b>438</b>, the device <b>200</b> determining the advance direction to be the initial advance direction.

[0105] In step <b>442</b>, the device <b>200</b> determining the frequency step F_SETPrelatively small. It will be noted that, according to the present example, the size of F_SETPis smaller, compared to the size of ΔF.

[0106] Referring back to FIG. 5A, wherein if the device <b>200</b> detects if the internal frequency F is synchronized with the target frequency F_TARGET(step <b>414</b>) then, the device <b>200</b> proceeds to step <b>416</b> and locks F. Otherwise, the device <b>200</b> repeats the steps of the above method, from step <b>402</b>.

[0107] Hence, the method of FIGS. 5A and 5B overcomes a situation where there exist interfering neighbor frequencies such as F_NEIGHBOR(reference numeral <b>102</b>) and F*_NEIGHBOR(reference numeral <b>120</b>) on either side of the F_BASE.

[0108] It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather the scope of the present invention is defined by the claims which follow.