Impedance measurement through waveform monitoring

Embodiments of the invention provide a capability of determining an input impedance of a connected Device Under Test based on Waveform Monitoring of an output signal of a waveform generator. Using embodiments of the invention, the input impedance of DUT is determined from the waveform monitoring results. The impedance information of the DUT together with the actual waveform provided to the DUT allows systems according to embodiments of the invention capable of characterizing circuit behavior for performance optimizing and issue debugging for the DUT.

FIELD OF THE INVENTION

This disclosure is directed to a system and methods for electrical impedance measurement, and, more particularly, to a system and methods for electrical impedance measurement using waveform monitoring techniques.

BACKGROUND

An Arbitrary Waveform and Function Generator (AFG) instrument is widely utilized for generating continuous/burst user-defined/mathematical function waveform signals for electronic circuit design and testing. AFGs typically have an output impedance of 50 Ohm over their operating frequency range. It is known that the input impedance of DUT (Device Under Test) will impact the output signal created by the AFG. Measuring the DUT's impedance across a wide frequency range typically requires a Vector Network Analyzer (VNA). However, a VNA does not provide the waveform signal information applied on the DUT, which is often needed for full analysis in the time domain. Therefore, measuring the DUT's impedance while monitoring the waveform of the AFG remains unavailable to perform.

Embodiments of the invention address these and other issues in the prior art.

DETAILED DESCRIPTION

Embodiments of the invention include techniques for performing real-time waveform monitoring of the active signal on a Device Under Test (DUT) with a cable de-embedding function. Embodiments also allow the users to determine the input impedance of DUT to characterize the circuit behavior for performance optimizing and issue debugging. As described below, using embodiments of the invention, it is possible to perform both waveform monitoring and impedance measurement through one single AFG instrument.

FIG. 1is a block diagram of a test system including an AFG according to embodiments of the invention. An Arbitrary Waveform and Function Generator (AFG)100produces custom and standard waveform signals that are useful while designing and testing circuits and devices. Although description of embodiments of the invention is given with reference to an AFG, any type of device that generates waveforms could be used. The AFG100, in general, produces an output signal182at the output of a signal amplifier180. The particular desired output signal is selected by a user. Once selected, a Time/Phase to address mapper120produces addresses to store an intermediate form of the desired signal in a waveform memory140. The address mapper120includes input from a system clock110, and generates both address data and clock data to be input to the waveform memory140. A converter, such as a high-speed Digital to Analog Converter (DAC)160converts the digital signal from the waveform memory140to an analog signal. A filter, such as the waveform reconstruction filter170filters the analog signal before it is passed to the signal amplifier180for output as the output signal182.

The output signal182from the AFG100may be passed to a Device Under Test (DUT). Advantageously, by analyzing the output signal182with a waveform analyzer, such as a waveform analyzer200illustrated inFIG. 1, an input impedance of the DUT that is coupled to the output signal182may be determined.

The waveform analyzer200illustrated inFIG. 1includes an amplifier210that receives an input signal. In this case, the input signal to the waveform analyzer200is the output signal182of the AFG100. The output signal182of the AFG100is also coupled to the DUT. The input signal to the waveform analyzer200is passed through an anti-aliasing filter220before being provided to an ADC230, which converts the analog signal to digital signals, including a clock and data signals. Those data and clock signals are fed to a waveform acquisition controller240. Also provided to the waveform acquisition controller240is a trigger signal derived from a synchronizer190that is coupled between the AFG100and waveform analyzer200. The waveform acquisition controller240is coupled to waveform acquisition memory250. The waveforms may be de-embedded and supplied to a de-embedder260before being sent to a display (not separately illustrated). The display illustrates the de-embedded waveform visually for the user.

Embodiments of the invention are able to measure an input impedance of the DUT coupled to the output182of the AFG100by analyzing waveform presented to the DUT.

FIG. 2is a block diagram illustrating various example measurements used in embodiments of the invention to perform impedance determination.FIG. 3is an example flow diagram300illustrating example steps that may be used in determining the input impedance of a DUT based on waveform monitoring. Embodiments of the invention are now described with reference toFIGS. 2 and 3. A first process302in measuring the input impedance of the DUT is to measure a voltage of the signal at the output180of the AFG100. This is sometimes called the Bayonet Neill-Concelman (BNC) output because the AFG100typically connects to a DUT through a BNC connector.

More specifically, in one embodiment, the first process302measures the voltage at the output180when the AFG100is producing a sine signal of frequency f with a cable coupled to the BNC output coupled to a matching load, i.e., a load that is equal to the cable characteristic impedance, ZC, and produces the complex measurement result of nominal output Vmea_nomof the AFG100through synchronous acquisition. In other words, Vmea_nomcan be expressed in a complex format as:
Vmea_nom=Abs(Vmea_nom)ejAngle(Vmea—nom)Equation 1
where a synchronous trigger signal is used as reference phase.

Next, a process304measures the voltage at output180for the sine signal of the frequency of f with the coaxial cable terminated with an open load through synchronous acquisition. That is, the signal is fully reflected by the load, and a complex ratio koof the measured result (Vmea_open) to nominal output (Vmea_nom) may be calculated in an operation306as:

ko=Vmea⁢⁢_⁢⁢openVmea⁢⁢_⁢⁢nom=1+e-2⁢l⁡(α+j⁢⁢β)Equation⁢⁢2where α is the unknown attenuation coefficient, β is the unknown waveform number, and l is the unknown length of the coaxial cable.

The operations302,304, and306are repeated with different frequencies to scan the entire frequency range and characterize koof the coaxial cable at a specified frequency step of Δf, which is normally frequency-dependent.FIG. 3illustrates this as determining whether the entire frequency range has been scanned in an operation308. If the entire frequency spectrum of the AFG100has not been scanned, the frequency is changed in an operation310, and the operations302,304, and306are repeated using the new frequency. As mentioned above, the frequency is changed in the operation310by the frequency step of Δf.

Next, the AFG100is connected to a DUT, and an operation312measures the voltage at the output180of the AFG100for the sine signal of the frequency of f with the coaxial cable terminated at the DUT. Then, an operation314calculates the complex ratio klof measurement result (Vmea_load) to nominal output (Vmea_nom) of the AFG100through synchronous acquisition according to the following equation:

where ZDUTis the unknown input impedance of DUT.

The operations312and314are repeated at different frequencies to scan the entire frequency range and characterize klof the coaxial cable at a specified frequency step of Δf, which is normally frequency-dependent.FIG. 3illustrates this as determining whether the entire frequency range has been scanned in an operation316. If the entire frequency spectrum of the AFG100has not been scanned, the frequency is changed in an operation318, and the operations312and314are repeated using the new frequency. As mentioned above, the frequency is changed in the operation318by the frequency step of Δf.

After koand klhave been characterized using operations302-318, an operation320uses, for the sine signal of f frequency, the characterized koand klvalues to determine the complex input impedance of the DUT by using Equation 4.

By performing operation320for all of the sampled frequencies, the entire frequency range of the AFG100may be used to fully characterize the input impedance of the DUT, ZDUT(2πf), at specified frequency step of Δf, which is normally frequency-dependent.

In other embodiments, instead of a sine wave, it is feasible to use a pulse signal or other arbitrary waveform signal to accelerate the above measurement process of frequency scanning. The procedure is almost the same except for calculating the Fourier Transform of Vmea_nom, Vmea_open, Vmea_load, ko, klin frequency domain, which gives the frequency-dependent complex ZDUTas illustrated inFIG. 2and Equation 5:

Substituting a pulse waveform for the sine wave described above in characterizing the input impedance of the DUT saves measurement time, since the pulses may be analyzed quicker than the sine wave. The pulse method is less accurate than the sine wave method, however, since the pulse wave method is more sensitive to noises.

An example validation process showing impedance measurement may be emulated using the instructions set out below, using a signal generator AFG3252C and oscilloscope MS04104B, both available from TEKTRONIX, INC. of Beaverton, Oreg., USA:1) Set the output of sine wave of 10 MHz, 1 Vpp, 50 Ohm for the AFG, and use its Trigger Out for the MSO acquisition;2) Connect the cable from the AFG to the MSO and set the MSO termination to 50 Ohms, and measure the voltage at the BNC output of the AFG with a probe;3) Disconnect the cable to the input of the MSO to leave the cable as Open and measure the voltages at the BNC output of the AFG with the probe;4) Connect the cable to the DUT (for example, using paralleled multiple BNC-BNC type 50 Ohm loads including MSO input impedance) and connect the DUT to one MSO channel and set the channel to 50 Ohm termination, then use the probe with another channel to measure the voltage at the BNC output of the AFG;5) Calculate out the input impedance of the DUT using the steps described above with reference toFIGS. 2 and 3, which shows an absolute error of <11% as shown in Table 1, below.

Sources of the error of the above emulation may include a) Trigger error, since the oscilloscope is running asynchronously with the AFG; b) Measurement error, such as phase error, amplitude error, probe disturbance, etc.; and c) System error: assume ideal impedance values for load, AFG and cable.

Embodiments of the invention will mostly eliminate sources of error listed in “a” and “b” in the preceding paragraph. Also, the error “c” can be addressed further through minimizing the variations of the output impedance of the AFG100, load impedance, and cable impedance. In other words, embodiments of the invention will improve the measurement accuracy to a much higher level than is presently available.

Using the above-described techniques, the input impedance of DUT may be determined from the waveform monitoring results. The information of impedance of DUT together with the actual waveform on DUT makes AFG capable of characterizing the circuit behavior for performance optimizing and issue debugging for DUT.

The previously described versions of the disclosed subject matter have many advantages that were either described or would be apparent to a person of ordinary skill. Even so, all of these advantages or features are not required in all versions of the disclosed apparatus, systems, or methods.

Additionally, this written description makes reference to particular features. It is to be understood that the disclosure in this specification includes all possible combinations of those particular features. For example, where a particular feature is disclosed in the context of a particular aspect or embodiment, that feature can also be used, to the extent possible, in the context of other aspects and embodiments.

Although specific embodiments of the invention have been illustrated and described for purposes of illustration, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, the invention should not be limited except as by the appended claims.