FIELD OF THE INVENTION

The technology relates to measurement of phase frequency response of a device under test (DUT), including, but not limited to signal conversion devices, such as high speed analog to digital converters, analog and digital up-converters, down-converters and others.

BACKGROUND OF THE INVENTION

Signal conversion devices are characterized by a frequency response, consisting of amplitude and phase terms. Correction (equalization) of the frequency response is essential for a high quality converter and requires precise measurement of device properties and the phase response dependence on frequency in particular.

The conventional methods of phase frequency response measurements, for example, based on the employment of Vector Network Analyzers (VNAs) have certain limitations which prevent their use with signal conversion devices. First, they require a DUT to have input and output ports which are both of the analog type. As a consequence, these methods are not applicable to such devices as analog to digital converters (ADCs), where the input signal is analog and the output signal is digital. Second, they require that the input and the output signals of a DUT lie in the same frequency range. For these reasons, VNA-based phase frequency response measurement cannot be used for frequency converters.

A method and apparatus for group delay (phase frequency response) measurements, applicable to the signal conversion devices, were proposed in an U.S. Pat. No. 9,933,467 “Group Delay Measurement Apparatus and Method” (the “'467 patent”). The therein-proposed technology comprises means for generating a test signal using two sinusoidal signal sources at low and high frequencies, followed by amplitude limiting of a sum of those signals. The test signal, produced in this way, is injected into a DUT. An amplitude limited signal is obtained at the DUT output, and group delay is determined by simultaneous measurement of signal sideband components and low frequency fundamental phases.

The suggested approach of the '467 patent solves the problem of phase frequency response measurements in DUTs having the input and output of different character. However, application of this method presents certain difficulties. Signal conversion devices using this method, require different setups, making them adequate for different operating conditions. A front end of an analog to digital converter (ADC), for example, should provide different amplification depending on the level of the input signal. For this purpose, one amplifier of the ADC front end is substituted by another, having a different gain, or one attenuator is substituted by another with a different attenuation. However, such substitutions cause changes in frequency responses of the DUT.

The changed-over DUT possesses a number of configurations, with each configuration consisting of a corresponding set of units. Different configurations are described by different frequency responses. To achieve an adequate correction of frequency distortions, the equalizer, which is used in the DUT, should be switchable as well. Its frequency responses should be changed each time the DUT configuration is changed.

Design of a switchable equalizer requires measurements of frequency responses of every configuration in the DUT. The features of DUT configurations differ. In particular, the level of the input signal for different configurations may vary over a wide range. For example, an ADC range of input level variations may reach 30÷40 dB. This means that the generator of the test signals used for measurements of the frequency responses, should have an output level which is changeable within the corresponding limits.

Traditionally, the measurement of an amplitude frequency response is performed with plurality of test signals, with each of the test signals being a sine wave of a single frequency. A sine wave generator with wide range of output signal levels is a simple enough device, and, therefore, measurement of an amplitude frequency response for different DUT configurations does not present any difficulties. By contrast, a phase frequency response measurement method and apparatus applicable to the signal conversion devices and proposed in the '467 patent, requires generation of test signals which occupy a wide frequency range. Construction a generator of test signals occupying a wide frequency range, with a levels varying in a large dynamic range, may present a considerable technical problem. Also, measurement of phase frequency response is time consuming, and, therefore, using the method of the '467 patent for multiple settings of input attenuator, takes a long time and may be not practical. This fact considerably restricts the field of application of the described method.

The purpose of this disclosure is to propose a method of phase frequency response measurement in a DUT with a wide dynamic range of the input signal, and which presumes utilization of the method and apparatus of the the '467 patent and, at the same time, makes it possible to use a test signal generator with a constant output level. The '467 patent is incorporated herein by reference.

SUMMARY

In accordance with the present disclosure, a method of phase frequency response measurement of a DUT consists of choosing one configuration as a reference configuration and performing amplitude and phase response measurements for this reference configuration. For all other DUT configurations, only amplitude frequency responses are measured. For these DUT configurations, a minimum-phase difference between a reference and a current configuration is calculated. A desired phase frequency response is obtained by adding the calculated minimum-phase difference response to the reference phase frequency response.

The practical implementation of the proposed method was verified using a 32 GSamples/s Guzik Technical Enterprises ADP7000 Series digitizer with different input gain settings, and comparing calculated and actual phase frequency responses. The method of this disclosure reduces error of a calculated phase response from 20 degrees to less than 2 degrees in the range of 6-12 GHz, and provides sufficient accuracy for high-speed ADC calibration.

