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Posted on 02 July 2019

A Practical Method to Extract and Visualize Real Time Frequency of a Switching Signal on an Oscilloscope

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Application to the observation of the clock dithering feature of TPS62674

In the domain of switching converters, frequency is often of a concern in terms of, for example, efficiency, stability or induced noise in the application. Being able to know the frequency characteristic of a given output signal gives often a lot of information about the functionalities and performances of the converter itself.

By Michael Couleur; Texas Instruments

 

One can always observe the frequency in the spectral domain using expensive spectrum analyzers. However the possibility to see the frequency behavior of a given switching signal in the time domain is not often provided by modern oscilloscopes. The following article exposes a simple method to observe real time switching signal frequency on any oscilloscope as well as an application of this method to the observation of the spread spectrum functionality of TPS62674, Texas Instruments latest 6MHz step down converter.

Method

This method can be applied to any signal switching between two logic rails. By using one of the edges of the switching signal, which frequency we want to observe, we can externally trigger a pulse generator. The pulse generator produces a single pulse of constant width each time it is externally triggered. At the output of the pulse generator a switching signal which average voltage is proportional to the observed switching signal frequency will be seen. Where average voltage and frequency are linked by the following equation:

Equation 1

where Vavg is the voltage average of the output of the pulse generator, f  is the frequency of the observed signal, Tpulse is the duration of the triggered pulse, Vpulse the voltage height of the triggered pulse.

Schematic of the described frequency to voltage real time conversion method

In order to extract the average voltage of the output signal of the pulse generator, a simple RC circuit with a judiciously chosen time constant is used. The averaged signal can then be fed as an input into any oscilloscope which will display a voltage being a real time image of the observed switching signal frequency.

The positive edges of the light blue switching signal trigger pulses of constant width

In order to correctly choose the averaging time constant, one must know roughly which type of frequency characteristic the observed signal exhibits. The averaging time constant must be big compared to the carrier period of the observed signal and small compared to the period of the events which want to be observed on the signal. For example if one wants to observe the frequency characteristic of a switching converter running at 1MHz (clocked every 1us) during a line transient event occurring very ms, a 100us averaging time constant seems to be a good choice in order to filter out the 1MHz component without averaging out the response of the line transient occurring every ms.

3V to 4V line transient observation of a 6MHz step down converter with spread spectrum feature

As active oscilloscope voltage probes often exhibit an input current leakage, it might be relevant to use a small resistor and a big capacitor in order to achieve the targeted value for the RC time constant. If the resistor is kept small, the voltage drop across it can be kept negligible and the error associated with the probe leakage current can be neglected.

By correctly choosing the value for Vpulse and Tpulse coming out of the pulse generator one can conveniently realize a direct conversion from MHz to Volts. For example, if in equation (1) Tpulse = 40ns and Vpulse = 4V, then according to equation (1), if the frequency ƒ gets 1MHz bigger, the average voltage Vavg will get 1V bigger, which provides a very direct and practical frequency to voltage conversion to the user.

Application to the observation of TPS62674 clock dithering

TPS62674, Texas Instruments’ high efficiency 6MHz step down converter offers a spread spectrum mode of operation. The main 6 MHz frequency is modulated using a 50kHz triangular signal in order to spread the clock spectrum and reduce the noise level induced by the converter in the system. The 50kHz modulating frequency is chosen to be as low as possible but far away enough from the audio bandwidth According to theory, minimizing the modulating frequency increases the noise attenuation ratio, hence the choice of this 50kHz modulating frequency. The frequency excursion is chosen to be about +/-8% of 6MHz carrier frequency. +/-8% is a good value in order to achieve a good noise reduction, without compromising the maximum output voltage ripple which gets degraded when the frequency gets too low. The triangular modulation technique was chosen because it provides a flatter spread spectrum compared to other modulation techniques. In TPS62674, the implementation allows an impressive 6dB attenuation of the noise associated with the converter output ripple around the 6MHz carrier frequency. It should be noticed that the efficiency is not affected by the spread spectrum technique, because the average operating frequency stays unchanged at 6MHz.

Picture of the output voltage spectrum of TPS62674 with and without clock dithering a 6dB attenuation

TPS62674 is a hysteretic converter naturally operating at variable frequencies and locked by an internal frequency lock loop at 6MHz. The implementation of a triangular frequency dithering around the 6MHz target frequency presented some challenges that Texas Instruments engineers successfully addressed.

By connecting the SW output of the step down converter to the above described construction one can observe on an oscilloscope the dynamic behavior of TPS62674 switching frequency.

TPS62674 inductor current (green trace) and operating frequency (pink trace) Vpulse and Tpulse are chosen in order to get 2V

It can be observed that despite its hysteretic mode of operation which does not naturally guarantee a constant frequency of operation, TPS62674 exhibits a constant +/-8% deep triangular frequency excursion around the carrier frequency over temperature and supply voltage. The depth of this excursion is internally regulated by a control system and is invariant with temperature and supply voltage.

Conclusion

By using the method described in this article, one can better understand the way functionalities such as spread spectrum work and check if the induced frequency excursion does not violate any frequency requirement of the application. This method is a good practice in order to characterize and successfully design in TPS62674, Texas instruments’ new leading edge 6MHz step down converter.

 

 

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