Frans Verbeyst

Next-generation comb generators for accurate modulated measurements

When calibrating instrumentation used for measurements at RF and microwave frequencies, periodic pulse-shaped signals are used during phase calibration. However to support measurements of narrowband (a few % of the carrier frequency) signals, the repetition rate of the pulse-shaped signal has to be decreased dramatically from GHz to kHz. The signal-to-noise ratio required to measure such a signal is more than a challenge for state-of-the-art instrumentation. A new approach is presented which combines two dividers, a psseudo-random bit sequence (PRBS) generator and a logical and operation which triggers a pulse generating unit. These degrees of freedom allow to concentrate energy within a given bandwidth around a specified fundamental frequency and its harmonnics, supporting a frequency spacing in the order of kHz in combination with fundamental frequencies in the order of GHz. Another new tone selective bit sequence approach is also explained.

A phase calibration methodology for accurate modulated measurements

To address the challenge of the phase calibration of modulated RF and microwave signals or the response of a linear or nonlinear system under modulated excitation, a next-generation comb generator is characterized. The latter allows kHz-level modulation tones in addition to the GHz-level RF and microwave tones, while still exhibiting an acceptable power level for each tone. This comb generator includes a PRBS generator, two dividers and a logical “and” operation to trigger the internal pulse generating unit. The pulse generator is characterized using a calibrated sampling oscilloscope. Depending upon the mode of operation this generator could be operated using a plentitude of possible values for every setting (input frequency, divider value, PRBS length). Each setting results in a different amplitude and phase spectrum. In practice it is impossible to characterize this pulse generator and provide a spectral phase dataset for every possible setting of each mode. To address this problem it is investigated if it is feasible to mathematically obtain the complete amplitude and phase spectrum of each possible setting by just using the characterized information of a single pulse. The amplitude and phase characteristic obtained by the measured and predicted spectrum are analyzed and compared. In this paper this technique will be applied to a comb generator operating under divider or PRBS mode.

Asynchronous electro-optic sampling of all-electronically generated ultrashort voltage pulses

We measure the output of an electrical pulse generator with a repetition rate of 76u2009MHz employing a laser-based asynchronous sampling technique with an effective sampling frequency of 250u2009GHz. A best estimate of the resulting 13u2009ns long waveform is obtained from multiple waveform measurements, which are taken without any trigger event and subsequently aligned in time. This asynchronous sampling scheme can even be adopted in situations where small phase drifts between the electrical pulse generator and the laser occur, making synchronized sampling very difficult. In addition to accurate measurements, the proposed asynchronous measurement scheme allows for the construction of covariance matrices with full rank since a large number of time traces is acquired. Such matrices might reveal correlations which do not appear in low-rank matrices. We believe that the asynchronous sampling technique advocated in this paper will prove to be a valuable characterization tool covering an ultra-broadband frequency range from below 100u2009MHz to above 100u2009GHz.

Optimization of a next-generation comb generator for accurate large-signal measurements on a user-defined frequency grid

In order to accurately calibrate large-signal measurement instruments at RF and microwave frequencies, periodic pulse-shaped signals are used. However, when performing modulated measurements, the repetition rate of this pulse-shaped signal has to be kept very low. As a result, it is almost impossible to measure such a signal using state-of-the-art instrumentation due to the required signal-to-noise ratio. Recently, a next-generation comb generator using Pseudo-Random Bit Sequence (PRBS) has been proposed. It allows to generate dense modulation tones around the carrier and its harmonics and to balance the power of these modulation tones and the desired modulation bandwidth. Moving it one step further, the energy can be optimized for a desired number of discrete tones using a Tone Selective Bit Sequence (TSBS) instead of a PRBS sequence. In this paper two methods are presented to realize such a TSBS using a genetic-algorithm-based global optimization. It is shown that these methods outperform the initially proposed method which is based on random phase and clipping, especially when the number of desired tones increases.

Traceable phase calibration of a wide-bandwidth microwave Vector Signal Analyzer

Numerous wide-bandwidth and multi-channel applications benefit from accurate test and measurement equipment which minimize the distortion of the generated and analyzed signals. Different techniques are described in literature to achieve this for Vector Signal Analyzers. This paper presents a simple traceable phase calibration technique for wide-bandwidth Vector Signal Analyzers using a comb generator which is able to realize a dense frequency grid up to 40 GHz and above. The comb generator itself is calibrated using a technique which is traceable to electro-optical sampling as primary standard. The calibration procedure is explained in detail showing the characterized phase distortion for different center frequencies and different IF bandwidths of a commercial VSA.