WO2005057811A1

WO2005057811A1 - High accuracy synchronization of test equipment

Info

Publication number: WO2005057811A1
Application number: PCT/US2004/040896
Authority: WO
Inventors: Roland Rick
Original assignee: Qualcomm Incorporated
Priority date: 2003-12-08
Filing date: 2004-12-07
Publication date: 2005-06-23
Also published as: KR20060097063A; JP4392028B2; CN1890900B; JP2007513589A; KR100839401B1; EP1692786A1; CN1890900A

Abstract

To determine the delay between true GPS time and the arrival time of a CDMA signal, a GPS receiver generates a first reference signal that is locked to true GPS time, and applies this signal to a CDMA base station test equipment. The CDMA base station test equipment receives the CDMA signal and generates a second reference signal which has transitions occurring substantially concurrently with transitions of an internal synchronization clock used to sample the first reference signal. The CDMA base station test equipment provides the delay between the second reference signal and the CDMA signal. A frequency/time counter provides the delay between the first reference signal and the second reference signal. The sum of the delays supplied by the CDMA base station test equipment and the frequency/time counter represents the delay between true GPS time and the CDMA signal.

Description

HIGH ACCURACY SYNCHRONIZATION OF TEST EQUIPMENT

CROSS-REFERENCES TO RELATED APPLICATIONS [0001] This application claims the benefit of U.S. Provisional Application No. 60/527,989, filed December 8, 2003, entitled "HIGH ACCURACY SYNCHRONIZATION OF TEST EQUIPMENT".

FIELD

[0002] The present disclosure relates to measuring the time delay between two signals, and more particularly to measuring the delay between true GPS time and the arrival time of a CDMA signal using conventional test equipment.

BACKGROUND

[0003] The demand for locating the position of a mobile wireless device, such as a cellular phone, is on the rise. For instance, a mandate by the Federal Communications Commission (FCC) requires that the location of a caller dialing 911 on his/her cellular phone be identified with an accuracy of 400 feet when the 911 call is received. A widely known method for determining the position of a mobile wireless device uses the information obtained from the Global Positioning System (GPS).

[0004] The GPS is a satellite-based navigational system formed by a network of satellites broadcasting pseudo-random noise (PRN) codes modulated on a carrier band. The GPS satellites transmit signals from which mobile GPS receivers may estimate their locations. Each GPS satellite transmits signals using two carrier signals. The first carrier signal is modulated using two PRN codes, namely a coarse acquisition (C/A) code, and a precise (P) code. Each GPS satellite uses different PRN codes to distinguish it from the other satellites of the GPS.

[0005] To determine the location of a GPS receiver, acquisition and tracking of at least four satellite signals are required. GPS signal acquisition often involves computing the correlation between the received GPS signals and the C/A code of associated satellites at various phase offsets and Doppler-shifted frequencies. Following signal acquisition, a signal tracking process decodes the signals from the identified satellites at the phase offsets and Doppler-shifted frequencies. During the signal tracking phase, navigation data is received from the identified satellites. Embedded in the navigation data transmitted by the GPS satellites are data related to satellite positioning as well as clock timing (i.e., time stamp), commonly referred to as ephemeris data, from which the position of the GPS receiver is detected. Many techniques have been developed to acquire and track GPS signals and to read the ephemeris data so as to detect the location of the GPS receiver.

[0006] GPS-based position detection systems, however, have a number of disadvantages. One such disadvantage is that a GPS receiver must have a clear and unobstructed view of at least four GPS satellites in order to have its position detected accurately. Therefore, if a user of a GPS receiver is, for example, in a wooded or an urban area containing tall structures, the user may not have an obstructed view of the required number of satellites to be able to detect its position. The problem may further be compounded if the user is indoors.

[0007] To detect the position of a wireless receiver when fewer than four satellites are in clear view, algorithms have been developed that use the CDMA signals transmitted by the ground base stations. Communication between a wireless device and a base station is often established by a forward link via which signals are transmitted from the base station to the wireless device, and a reverse link via which the signals transmitted from the wireless device are received by the base station. One known algorithm developed for detecting the position of a mobile wireless device using ground base stations is referred to as Advanced Forward Link Trilateration (AFLT). To detect the position of a mobile wireless device using AFLT, the location of the base stations disposed in the wireless network, the transmission times of the signals from the base stations as well as the arrival times of the signals at the mobile wireless device are required.

[0008] The front-end processing delays in a base station is often different from that in a GPS satellite transmitter. Accordingly, differences between the time of arrivals of corresponding CDMA and GPS signals need to be accounted for if both the CDMA and GPS signals are used together to detect the position of the mobile wireless device. CDMA signals transmitted by base stations are typically required to be synchronized to within 10 microseconds of their counterpart GPS signals.

[0009] For accurate position determination using both CDMA and GPS signals, the timing delay between the transmissions of the CDMA and GPS signals has to be known. Such a timing delay is measured during a CDMA base station calibration phase using conventional off-the-shelf test equipment. Typical off-the-shelf test equipment often use timing synchronization clocks that have frequencies between approximately 10MHz to 20 MHz (i.e., periods between 50 nsec to 100 nsec ). Therefore, each such delay measurement made by these equipments has a resolution of approximately 50-100 nsec and thus may lead to inaccurate position determination measurements.

[0010] Accordingly, there is a need for a technique that uses conventional off-the- shelf test equipments that use approximately 10-20 MHz clocks for timing synchronization and that enable the delay between corresponding CDMA and GPS signals to be measured with a relatively higher accuracy.

SUMMARY

[0011] In accordance with one embodiment of the present disclosure, to determine the delay between true GPS time and the arrival time of a CDMA signal, a GPS receiver receives the GPS signal via an antenna and generates a first reference signal that is locked to true GPS time. The first reference signal is applied to CDMA base station test equipment which also receives the CDMA signal via an antenna. In response, the CDMA base station test equipment generates a second reference signal which has transitions occurring substantially concurrently with transitions of an internal synchronization clock used to sample the first reference signal. The CDMA base station test equipment provides the delay between the second reference signal and the CDMA signal. A frequency/time counter receives the first reference signal and the second reference signal and provides the delay between these two signals. The sum of the delays between the second reference signal and the CDMA signal—as provided by the CDMA base station test equipment— and the delay between the first reference signal and the second reference signal —as provided by the time/frequency counter—represents the delay between true GPS time and the arrival time of the CDMA signal.

[0012] In some embodiments, the GPS receiver is a Symmetricom model No. 58503B GPS receiver, available from Symmetricom Corporation located at 2300 Orchard Parkway, San Jose, California 95131. In these embodiments, the first reference signal has a period of 2 seconds and is commonly referred to as an even-second signal.

[0013] In some embodiments, the CDMA base station test equipment is an Agilent model No. E6380A CDMA base station test equipment, available from Agilent Corporation located at 395 Page Mill Road, Palo Alto, California 94303. In these embodiments, the second reference signal has a frequency of 1.2288 MHz and the internal synchronization clock has a frequency of 19.6608 MHz.

[0014] In some embodiments, the frequency/time counter is a Hewlett-Packard model No. 53131A 53132A frequency/time counter, available from Hewlett-Packard located at Hanover Street, Palo Alto, California 94304-1185. In these embodiments, the frequency/time counter includes an HP-IB interface that may be programmed to sum the measured delays and display this sum on a monitor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Fig. 1 is a simplified high-level block diagram of a GPS receiver coupled to a base station transmitter as well as the delays contributing to the forward link calibration.

[0016] Fig. 2 is a high-level block diagram of a test set-up adapted to determine the time delay between corresponding CDMA and GPS signals, in accordance with one embodiment of the present disclosure.

[0017] Figure 3 shows the relative positions of a GPS receive antenna, a CDMA receive antenna, and a base station transmit antenna during base station sector calibration phase.

[0018] Figure 4 shows a timing diagram of signals associated with the test set-up of Fig. 2 and that are used to determine the delay between arrival times of corresponding CDMA and GPS signals, in accordance with one embodiment of the present disclosure.

DETAILED DESCRIPTION

[0019] In accordance with one embodiment of the present disclosure, to determine the delay between true GPS time and the arrival time of a CDMA signal, a GPS receiver receives the GPS signal via an antenna and generates a first reference signal that is locked to true GPS time. The first reference signal is applied to CDMA base station test equipment which also receives the CDMA signal via an antenna. In response, the CDMA base station test equipment generates a second reference signal which has transitions occurring substantially concurrently with transitions of an internal synchronization clock used to sample the first reference signal. The CDMA base station test equipment provides the delay between the second reference signal and the CDMA signal. A frequency/time counter receives the first reference signal and the second reference signal and provides the delay between these two signals. The sum of the delays between the second reference signal and the CDMA signal— as provided by the CDMA base station test equipment— and the delay between the first reference signal and the second reference signal —as provided by the time/frequency counter—represents the delay between true GPS time and the arrival time of the CDMA signal.

[0020] Fig. 1 is a simplified high-level block diagram of a GPS receiver 10 coupled to a CDMA base station transmitter 20. GPS receiver 1O receives the GPS signals via GPS receive antenna 15 and CDMA base station transmitter 20 transmits CDMA signals via CDMA transmit antenna 25. In order to use a time of arrival measurement of a CDMA signal in a position computation, the time at which the CDMA signal is transmitted from the transmit antenna 25 has to be known in relation to the GPS time and with a relatively high degree of accuracy. This time is referred to as the forward link calibration value and is a measure of the timing of the transmission of the CDMA signals from base station 20 relative to the GPS time.

[0021] Fig. 1 also shows various components of time delays that contribute to the forward link calibration value. Often, during a forward link calibration, these delays, discussed further below, are measured and recorded in the base station almanac. Time delay Ti represents the cable delay associated with GPS receive antenna 15. Time delay T₂ represents the difference between the GPS time of the signal applied to the RF input terminal of GPS receiver 10 and the reference signal generated by GPS receiver 10. Time delay T₂ is often relatively small and negligible and thus may not need to be taken into account. Time delay T₃ represents the delay associated with the cable delivering the reference signal generated by GPS receiver 10 to base station transmitter 20. Time delay T₄ represents the difference between the reference input signal applied to base station transmitter 20 and the CDMA system time present at the RF output terminal of base station transmitter 20. Time delay T₅ represents the cable delay associated with CDMA receive antenna 25.

[0022] As is known to those skilled in the art, many techniques exits for determining the forward link calibration value described above. In accordance with some techniques, when access to a base station equipment is available, individual delay components that contribute to this value, as described above, may be measured. In accordance with other techniques, when access to the base station is not readily available, a single measurement directed at measuring the forward link calibration value may be made. The following is a description of the measurements made during a calibration phase of base station transmitter 20 without the need to measure the individual components of delay described above. During this calibration, the transmission time of the CDMA signals with respect to true GPS time is measured.

[0023] Fig. 2 is a high-level block diagram of a test set-up 50 that is used to measure the delay between true GPS time and the arrival time of a CDMA signal with a high resolution, e.g., 10 nsec, in accordance with one embodiment of the present disclosure, in order to calibrate the timing of the transmission of the CDMA signals relative to the GPS time. In this exemplary embodiment, test set-up 50 is shown as including a GPS receiver 60, CDMA base station test equipment 70, and a time/frequency counter 80. Each of GPS receiver 60, CDMA base station test equipment 70 and time/frequency counter 80 maybe a conventional off-the-shelf test equipment and commercially available from a number of vendors. In some embodiments, the GPS receiver 60 maybe a Symmetricom model No. 58503B GPS receiver, available from Symmetricom Corporation located at 2300 Orchard Parkway, San Jose, California 95131. CDMA base station test equipment 70 may be an Agilent model No. E6380A CDMA base station test equipment, available from Agilent Corporation located at 395 Page Mill Road, Palo Alto, California 94303. Frequency/time counter 80 may be a Hewlett-Packard model No. 53131A/53132A frequency/time counter, available from Hewlett-Packard located at Hanover Street, Palo Alto, California 94304-1185. It is understood, however, that many other commercially available test equipments may be used, in accordance with the present disclosure, to measure the delay between true GPS time and the arrival time of a CDMA signal with a relatively high resolution, e.g., 10 nsec.

[0024] GPS receiver 60 has a GPS receive antenna 64 adapted to receive the GPS signals. GPS receiver 60 generates a 10MHz timing clock signal CLK at its first output terminal OUTι and a reference signal REF, commonly referred to as an even-second signal, at its second output terminal OUT₂. A transition appears on signal REF every two seconds. Signal REF is locked to the received GPS signals and thus maintains a fixed timing relationship therewith. Both signals CLK and REF are applied to CDMA base station test equipment 70 at its respective input terminals IN_! and IN₂. CDMA base station test equipment 70 has a receive antenna 74 via which it receives the CDMA signals. CDMA base station test equipment 70 generates, in part, a reference signal REF2 at its output terminal OUTi and that is applied to input terminal LN₃ of time/frequency counter 80. Signals INT_CLK and REF are respectively applied to input terminals ESTi and IN_. of time/frequency counter 80. CDMA base station test equipment receiver 70 and time/frequency counter 80 are also coupled to one another via a GPIB interface. The 10MHz timing clock signal CLK provides a common clock for all of the test equipment.

[0025] During a calibration phase, GPS receiver 60 is optionally programmed with the known position of the user and is also programmed to have a settling time of, e.g., approximately 60 minutes. The programming of these values maybe done with a computer connected via a standard serial port and using vendor-supplied software. In these embodiments, GPS receiver 60 is also configured to also output the timing enor between signal REF and the true GPS time, in the event the user finds the 60 minutes settling time inconvenient.

[0026] To receive CDMA signals that have minimized interference from the signals transmitted by various other sectors of the base station with which CDMA base station test equipment 70 is in communications with, CDMA receive antenna 74 is optionally a directional antenna. Directional antenna 74 also enables CDMA base station test equipment 70 to receive CDMA signals that have minimized interference from signals transmitted by other base stations and signals that have minimized rr ltipath interference. CDMA antenna cable 72 optionally has a length that enables the CDMA receive antenna 74 to be moved in various positions in each sector of any given base station without the need to move the remainder of test setup 50.

[0027] Some embodiments of CDMA base station test equipment 70, such as Agilent E6380A, include software configured to control and read output results generated by time/frequency counter 80 as well as those generated by base station test equipment 70. The software from Agilent 6830A also provides a measurement of the base station calibration, including compensation for the time bias and various other cable delays. The software is further configured to receive, as inputs, the delay associated with GPS receive antenna cable 62, CDMA receive antenna cable 72, and cable 92 coupling output terminal OUT₂ of GPS receiver 60 to input terminal IN₂ of CDMA base station test equipment 70. The delay associated with cables 94 and 96 maybe ignored if these two cables are selected to have equal lengths. [0028] Figure 3 shows the relative positions of GPS receive antenna 64, CDMA receive antenna 74 and base station transmit antenna 94 during base station sector calibration phase. GPS receive antenna 64 is set up so as to have an unobstructed view of the sky from 10 to 90 degrees above the horizon and from 0 to 360 degrees from the true north. If an unobstructed view of the sky is not achieved, the default elevation mask of 10 degrees of Symmetricom 58503B is changed to an elevation that has an unobstructed view of the sky from 0 to 360 degrees from true north. The latitude, longitude, and height above the WGS-84 ellipsoid of the GPS receive antenna 64 may be optionally obtained from a differential GPS receiver. The required precision for the position of the GPS receive antenna 64 during the calibration phase is within, e.g., less than one meter of the true position. The directional CDMA receive antenna 74 is set up so as to have a line of sight to the base station transmit antenna 94 and is recommended to be within 30 degrees of the sector orientation.

[0029] The latitude, longitude, and height above the WGS-84 ellipsoid of the base station transmit antenna 94 is obtained from the GPS receive antenna 64 position and the relative azimuth, elevation, and distance of the GPS receive antenna 64 to the base station transmit antenna 94. The precision with which the position of the base station transmit antenna 94 is obtained during the calibration phase is, e.g., less than one meter of the true position. The relative azimuth, elevation, and distance of the GPS receive antenna 64 to the base station transmit antenna 94 may be obtained using a laser sight and compass. Commercially available differential GPS receivers with coupling laser sight/compass maybe used to determine the position of the base station transmit antenna 94 as described above. The precision with which the distance between the CDMA receive antenna 74 and the base station transmit antenna 94 is determined is also, e.g., one meter.

[0030] Figure 4 shows a timing diagram of signals used to determine the delay between true GPS time and the arrival time of a CDMA signal as generated by test setup 50 of Fig. 2. As described above, signal REF has a period of 2 seconds and is locked to true GPS time. As seen from Fig. 2, signal REF is generated by GPS receiver 60 and is applied to CMDA base station test equipment 70. Signal INT_CLK is a reference clock internal to the CDMA base station test equipment used to generate other clocks (not shown) and synchronize CMDA base station test equipment 70 to external timing sources. Signal REF_IN is generated internally within CDMA base station test equipment 70. As shown in Fig. 4, a transition 106 occurs on signal REF_IN when signal INT_CLK makes a low-to-high transition 104. Therefore, signal REF_EN may be generated internally by applying, for example, signal REF to a data input terminal of a flip-flop (not shown) and signal INT_CLK to a clock input terminal of that flip-flop; signal REF_IN is the output signal of such a flip-flop. Accordingly, transition 106 of signal REF_IN appears concurrently with transition 104 of signal INT_CLK regardless of the time at which transition 100 of signal REF appears between transitions 102 and 104 of signal INT_CLK.

[0031] CDMA base station test equipment 70 receives CDMA signal 108 via its CDMA receive antenna 74. As shown in Fig. 4, the delay T₈ between the arrival time of the CDMA signal-represented by transition 108 of signal CDMA— and true GPS time- represented by transition 100 of signal REF— includes time delay T₆, which is the delay between transitions 100 and 106, and time delay T₇, which is the delay between transitions 106 and 108. Conventional off-the-shelf CDMA base station test equipments, such as CDMA base station test equipment 70 only supply delay component T-y of time delay T₈. Therefore, they do not take into account delay component T₆, which changes depending on the relationship between true GPS time and the signal INT_CLK_.. Because signal INT_CLK has a period of approximately 50 nsec or 100 nsec, delay component T₆ may vary from 0 to 50 nsec or from 0 to 100 nsec. In other words, the delay between true GPS time and the arrival time of the CDMA signal as measured by CDMA base station test equipment 70 may be off by up to 50 nsec or 100 nsec depending on the frequency of signal INT_CLK, thus leading to unreliable base station calibration and or position determination.

[0032] In accordance with the present disclosure, delay component T₆ between transitions 100 of signal REF and 106 of signal REF_IN is measured, as described below, to improve the accuracy with which time delay T₈ is measured. Conventional off-the-shelf CDMA base station test equipment, such as Agilent E6380A, generates a number of clock signals internally. One such internally generated clock signal that is synchronous and has a relatively small time delay with respect to signals REF_IN and INT_CLK is a 1.2288 MHz clock signal, shown as signal REF2 in Figs. 2 and 4. In Fig. 4, transition 110 of signal REF2, transition 106 of signal REF_IN, and transition 104 of signal INT_CLK occur concurrently and at substantially the same time. Therefore, the delay T₆ between transitions 100 and 106 is substantially the same as that between transitions 100 and 110.

[0033] As seen from Fig. 2, to measure the time delay T₆ between transitions 100 and 110, in accordance with the present disclosure, signals REF and REF2 are applied to time/frequency counter 80. Time/frequency counter 80 which as described above, may be an Hewlett-Packard model no. 53131A/53132A time/frequency counter, is configured to measure time delay T₆.

[0034] To further simplify the measurement of the delay between the REF and the CDMA signal, a fourth tester with an HP-IB interface (not shown) may be coupled to both CDMA base station test equipment 70 and time/frequency counter 80. The HP interface of such equipment enables it to read and sum the time delay T₆ and T₇ and display the sum, i.e., T₈, on a monitor.

[0035] In some embodiments, CDMA base station test equipment 70 may be adapted to include time/frequency 80 as well as hardware/software modules to sum time delays T₆ and T₇, and to display the sum on a monitor. In other embodiments, signal REF2 may be a signal other than the internally generated 1.2288 MHz. In yet other embodiments, both the GPS receiver and the CDMA base station test equipment may be placed in the same housing and be made commercially available by a manufacturer or vendor as one unit.

[0036] The above embodiments of the present disclosure are illustrative and not limitative. The disclosure is not limited by the type or manufacturer of the GPS receiver, CDMA base station tester or the time/frequency counter. The disclosure is not limited to any specific clock frequency or any interface used for communication between various equipment. The disclosure is not limited by the kind of software/hardware module used to input parameters related to calibration. Other additions, subtractions, deletions, and modifications maybe made without departing from the scope of the present disclosure as set forth in the appended claims.

Claims

CLAIMS:

1. A method for measuring a delay between a first signal and a second signal, the method comprising: generating a plurality of clock signals; generating a first reference signal that is locked to the first signal; generating a second reference signal using the first reference signal and a first one of the plurality of clock signals; measuring a second delay between the second reference signal and the second signal; identifying a second one of the plurality of clock signals having transitions that occur substantially concunently with transitions of the second reference signal; and

measuring a third delay between the first signal and the identified second one of the plurality of clock signals, wherein a sum of the second and third delays represents the delay between the first and second signals.

2. The method of claim 1 wherein said first signal is a GPS signal and said second signal is a CDMA signal.

3. The method of claim 2 wherein said first reference signal has a period of 2 seconds.

4. The method of claim 3 wherein said first one of the plurality of clock signals has a frequency of 19.6608 MHz.

5. The method of claim 4 wherein said first reference signal is generated by a GPS receiver and in response to receipt of the GPS signal.

6. The method of claim 5 wherein said identified second of the plurality of clock signals has a frequency of 1.2288 MHz.

7. The method of claim 6 wherein said identified second of the plurality of clock signals and said second reference signal are generated by a CDMA base station test equipment and in response to receipt of the CDMA signal and the first reference signal.

8. The method of claim 7 wherein said second delay is measured by the CDMA base station test equipment.

9. The method of claim 8 wherein said third delay is measured by the CDMA base station test equipment.

10. The method of claim 8 wherein said third delay is measured by a time/frequency counter.

11. An apparatus configured to measure a delay between a first signal and a second signal, the apparatus comprising: means for generating a plurality of clock signals; means for generating a first reference signal that is locked to the first signal; means for generating a second reference signal using the first reference signal and a first one of the plurality of clock signals; means for measuring a second delay between the second reference signal and the second signal; and means for measuring a third delay between the first signal and a second one of the plurality of clock signals having transitions that occur substantially concurrently with transitions of the second reference signal, wherein a sum of the second and third delays represents the delay between the first and second signals.

12. The apparatus of claim 11 wherein said first signal is a GPS signal and said second signal is a CDMA signal.

13. The apparatus of claim 12 wherein said first reference signal has a period of 2 seconds.

14. The apparatus of claim 13 wherein said first one of the plurality of clock signals has a frequency of 19.6608 MHz.

15. The apparatus of claim 14 wherein said means for generating the first reference signal is disposed in a GPS receiver and wherein said means for generating the first one of plurality of clock signals is disposed in a CDMA base station test equipment.

16. The apparatus of claim 15 wherein said identified second of the plurality of clock signals is a 1.2288MHZ signal.

17. The apparatus of claim 16 wherein said means for generating said second reference and said second one of the plurality of clock signals are disposed in the CDMA base station test equipment and in response to receipt of the CDMA signal and the first reference signal.

18. The apparatus of claim 17 wherein said means for measuring the second delay is disposed in the CDMA base station test equipment.

19. The apparatus of claim 18 wherein said means for measuring the third delay is disposed in the CDMA base station test equipment.

20. The apparatus of claim 19 wherein said means for measuring the third delay is disposed in a time/frequency counter.

21. An apparatus operative to measure a time delay between a first signal and a second signal, the apparatus comprising: a first device adapted to receive the first signal and to generate, in response, a first reference signal; a second device adapted to receive the second signal and the first reference signal and to generate a second reference signal and a clock signal having transitions that occur substantially concurrently with transitions of the second reference signal, said second device further adapted to measure a time delay between said second reference signal and said second signal; and a third device adapted to receive the first reference signal and the second reference signal and to measure a delay between the first reference signal and the second reference signal.

22. The apparatus of claim 21 wherein said first signal is a GPS signal and said second signal is a CDMA signal.

23. The apparatus of claim 22 wherein said first reference signal has a period of 2 seconds.

24. The apparatus of claim 23 wherein said first device is a GPS receiver adapted to generate the first reference signal in response to receipt of the GPS signal.

25. The apparatus of claim 24 wherein said clock signal has a frequency of 1.2288 MHz.

26. The apparatus of claim 25 wherein said second device is a CDMA base station test equipment adapted to receive the CDMA signal and the first reference signal and to generate the second reference signal and the clock signal.

27. The apparatus of claim 26 wherein said third device is a time/frequency counter.