CIRCUIT FOR DETERMINING THE FREQUENCY OF A SIGNAL

According to various embodiments, a circuit for determining the frequency of a signal is described, comprising an input configured to receive an analog input signal, an analog to digital converter configured to convert the analog input signal to a digital input signal, a digital mixer configured to generate a mixing result signal by mixing the digital input signal with a single bit binary signal having a reference frequency, a low pass filter configured to generate a filtered signal by filtering the mixing result signal, a measuring circuit configured to measure the period of the filtered signal and an output configured to output a value differing from the frequency of the single bit binary signal by the inverse of the measured period as the frequency to be determined.

REFERENCE TO RELATED APPLICATIONS

This application claims priority to German Patent Application 10 2022 128 332.1, filed on Oct. 26, 2022 and German Patent Application 10 2023 121 088.2, filed on Aug. 8, 2023, the contents of which are hereby incorporated by reference in their entirety.

FIELD

The present disclosure relates to a circuit for determining the frequency of a signal.

BACKGROUND

In various applications of chips it is desirable to accurately determine the frequency of a signal. For example, for function safety, it may be required to periodically check whether a clock in the chip is correct. Another example is that the temperature dependency of the frequency of a signal generated by a crystal oscillator of a chip may be used to accurately determine the temperature of a chip provided it is possible to accurately determine the frequency of the signal generated by the crystal oscillator. Another application is checking the oscillator frequency for a communication interface which needs to be very accurate for certain communication protocols. Accordingly, approaches which allow accurate determination of the frequency of a signal are desirable.

SUMMARY

According to various embodiments, a circuit for determining the frequency of a signal is described, comprising an input configured to receive an analog input signal, an analog to digital converter configured to convert the analog input signal to a digital input signal, a digital mixer configured to generate a mixing result signal by mixing the digital input signal with a single bit binary signal having a reference frequency, a low pass filter configured to generate a filtered signal by filtering the mixing result signal, a measuring circuit configured to measure the period of the filtered signal and an output configured to output a value differing from the frequency of the single bit binary signal by the inverse of the measured period as the frequency to be determined.

DETAILED DESCRIPTION

The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and aspects of this disclosure. Other aspects may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the disclosure. The various aspects of this disclosure are not necessarily mutually exclusive, as some aspects of this disclosure can be combined with one or more other aspects of this disclosure to form new aspects.

FIG.1shows chip100as an example for an application of a frequency determination.

The chip100comprises a source101of an analog signal102whose frequency should be determined. The signal source101may for example be a crystal oscillator whose frequency should be determined to measure the temperature of the chip (by making use of the fact that the frequency is temperature-dependent). Another example is that the signal source101is an oscillator which provides a timing signal for a communication interface which needs to have a certain frequency with high accuracy to avoid timing errors between the transmitter and the receiver.

Accordingly, the analog signal102is fed to a circuit103for determining the frequency of the signal102. According to various embodiments, the circuit103outputs a digital value104indicating the determined frequency. The value104may then be evaluated by an evaluation circuit105, e.g. a processing circuit which derives a temperature of the chip from the digital value104or a controller which controls the signal source101such that the frequency of the signal102is adjusted (e.g. to meet the frequency requirements of a communication interface).

FIG.2shows a frequency determining circuit200according to an embodiment which may be used as the circuit103of the chip ofFIG.1.

The frequency determining circuit200receives an analog input signal201(e.g. corresponding to the signal102). The analog input signal201has a frequency finwhich is the frequency to be determined.

The frequency determining circuit200comprises an analog to digital converter (ADC)202, in this example a delta-sigma ADC (DSADC) comprising a Delta Sigma (Δ-Σ) Analog-to-Digital conversion stage203(e.g. first order or second order) followed by a Cascaded Integrator Comb (CIC) filter204. The analog input signal201is fed to the analog to digital converter (ADC)202whose output is a digital (input) signal205which is a digitized version of the analog input signal (e.g. a digital 16 bit signal, i.e. the various levels of the analog input signal201are represented as 16 bit values).

FIG.3shows an example of a digitized input signal300generated by the ADC202.

The digital signal205is then fed to a mixer206which mixes the digital signal205with a single bit binary signal207which switches form the value 0 to 1 (or from −1 to 1) and back with a frequency fsign. The frequency fsignis predetermined and set similar to fin(i.e. what is expected for fin, e.g. within 10%, within 5%, within 1% or even with 0.1% or 0.01%, e.g. depending on the desired sensitivity of the frequency determination). The single bit binary signal may for example be generated from a reference frequency (e.g. used for analog-to-digital conversion) by a frequency divider.

FIG.4shows an example of a single bit binary signal400(in this example switching between 0 and 1 and with a 6.25 ns resolution).

It should be noted that if the signal that is fed to the mixer206switches its sign like the single bit binary signal207the mixer206acts as a rectifier (when the single bit binary signal207switches between −1 and 1). So, a rectifier component is in this example used as a mixer for a frequency determination circuit. For example, when the frequency determining circuit200is provided in a vehicle, the same type of component as used for rectifying for motor control (e.g. for resolver support) may be used for the frequency determining circuit200.

The mixing result signal208, i.e. the output of the mixer206is fed to a low pass filter209(e.g. a low power filter realized by digital hardware or a digital signal processor).

FIG.5illustrates a mixing result signal500.

As the result of the mixing of a signal with frequency finand a signal with frequency fsign, the mixing result signal has a frequency component with frequency fin+fsignand a frequency component with frequency fin−fsign.

The frequency fsignis chosen to be similar to fin. Thus, according to the frequency component with frequency fin+fsign, the mixing result signal500has about double the number of peaks than the digitized input signal300and the single bit binary signal400. Further, the mixing result signal500has a component with a relatively low frequency fin−fsignwhich can be seen by the decreasing amplitude of the peaks (which increases again but the increase is not visible in the depicted portion of the mixing result signal500). The low pass filter209filters out the high frequency component so the filter output signal210(seeFIG.2) is an envelope of the mixing result signal500.

The filter output signal210has a frequency of fin−fsign. Therefore, by measuring the time between two zero crossings TS1, TS2 the frequency determining circuit200can determine the difference between the frequency of the input signal201and the frequency of the single bit binary signal207(which is known) by Δf=fin−fsign=1/2*(TS2−TS1), i.e. the frequency determining circuit200measures the time between the two zero crossings TS1 and TS2 and outputs fsign+1/2*(TS2−TS1) as the value of fin.

It should be noted that if fsignis smaller than fin, then fin=fsign+1/2*(TS2−TS1). However, is finis smaller than fsign, then it fin=fsign−1/2*(TS2−TS1). However, for applications like a clock monitor it is not important whether to output fsign+1/2*(TS2−TS1) or fsign−1/2*(TS2−TS1) because both may be used to check whether the frequency of the input signal201is within e.g. +/−1% range of a nominal frequency. So, either of fsign+1/2*(TS2−TS1) or fsign−1/2*(TS2−TS1) may be output according to various embodiments. For an application like temperature sensing, a single bit binary signal can be provided by suitable selection of a frequency divider ratio for generating it such that it has a frequency which is sure to be smaller than fin. Then fin=fsign+1/2*(TS2−TS1) may be output (or, alternatively, fsignis set to be higher than finand fin=fsign−1/2*(TS2−TS1 is output).

It should be noted that measuring TS2−TS1 and calculating 1/2*(TS2−TS1) corresponds to measuring the period of the filter output signal210.

In the following, example values for fin, fsign, the output period (i.e. the period of the filter output signal210) and the sensitivity (i.e. the ratio of an error in the measurement of the output period to the error in the determined frequency) are given.fin=32.768 kHz, fsign=33.768 kHz=>output period=1 ms, sensitivity=3.28 us/0.1%fin=32.768 kHz, fsign=32.868 kHz=>output period=10 ms, sensitivity=328 ns/ppm,fin=32.768 kHz, fsign=32.778 kHz=>output period=100 ms, sensitivity=32.8 us/ppm (allows checking 1 ppm deviation)fin=8.192 kHz, fsign=8.192.1 Hz or 8.1916 Hz=>output period=2.5 s, sensitivity=12 ms/ppm.

The frequency determining circuit200may be used to determine the frequency of multiple signal sources101. For example, a multiplexer may be arranged at the input of the frequency determining circuit200for the analog input signal201such that it may forward any signals whose frequencies should be monitored (e.g. in a round robin scheme) such as a real-time clock signal, a communication bus timing signal (e.g. of CAN (controller area network) or LIN (local interconnect network) bus), a carrier signal (e.g. for NFC (near field communication)), an Ethernet clock etc. Further, for example at least some of the inputs of the multiplexer, a clock divider may be provided to reduce the respective frequency to be determined, e.g. such that all frequencies that are forwarded by the multiplexer are similar to fsign. Alternatively, fsign(i.e. the reference frequency) may be changed depending on the expected frequency of the forwarded signal.

In summary, according to various embodiments, a circuit is provided as illustrated inFIG.6.

FIG.6shows a circuit600for determining the frequency of a signal according to an embodiment.

The circuit600comprises an input601configured to receive an analog input signal and an analog to digital converter602configured to convert the analog input signal to a digital input signal.

Further, the circuit600comprises a digital mixer603configured to generate a mixing result signal by mixing the digital input signal with a single bit binary signal having a reference frequency (i.e. switching from one binary state to the other and back with the reference frequency) and a low pass filter604configured to generate a filtered signal by filtering the mixing result signal.

The circuit600further comprises a measuring circuit605configured to measure the period of the filtered signal and an output606configured to output a value differing from the frequency of the single bit binary signal by the inverse of the measured period as the frequency to be determined.

According to various embodiments, in other words a signal whose frequency is to be determined is mixed with a single bit binary signal, i.e. using a component which may for example be used for rectifying an input signal, as it is for example provided for resolver support.

Various Examples are described in the following:

Example 1 is a circuit for determining the frequency of a signal as described above with reference toFIG.6.

Example 2 is the circuit of Example 1, wherein the analog to digital converter comprises a sigma delta analog to digital converter.

Example 3 is the circuit of Example 2, wherein the analog to digital converter comprises a cascaded integrator comb filter configured to generate the digital input signal from an output of the sigma delta analog to digital converter.

Example 4 is the circuit of any one of Examples 1 to 3, wherein the digital input signal is a 16 bit or 32 bit digital signal.

Example 5 is the circuit of any one of Examples 1 to 4, wherein the measuring circuit is configured to measure the period of the filtered signal by doubling the time between two zero crossings of the filtered signal.

Example 6 is the circuit of any one of Examples 1 to 5, wherein the single bit binary signal is a signal alternating between 0 and 1 or is a signal alternating between −1 and 1.

Example 7 is the circuit of any one of Examples 1 to 6, wherein the low-pass filter is configured to filter out a component of the mixing result signal having the sum of the reference frequency and the frequency of the digital input signal when the frequency of the digital input signal differs from the reference frequency by less than 10 percent.

Example 8 is the circuit of Example any one of Examples 1 to 7, comprising a signal generation circuit configured to generate the single bit binary signal with the reference frequency.

Example 9 is the circuit of any one of Examples 1 to 8, comprising a multiplexer configured to forward signals to be monitored sequentially to the input.

Example 10 is the circuit of any one of Examples 1 to 9, comprising a clock divider configured to generate the analog input signal from a signal to be monitored by dividing its frequency.

Example 11 is a method for determining the frequency of a signal comprising supplying the signal to the input of the circuit of any one of Examples 1 to 10, wherein the reference frequency is set within a predetermined range of an expected frequency of the signal.

Example 12 is the method of Example 11, comprising determining the range depending on a predetermined desired sensitivity of the determination of the frequency.