The present invention concerns the provision of a pre-characterised high frequency signal, for example for use as a phase reference standard. More particularly, but not exclusively, this invention concerns a method of providing a pre-characterised high frequency signal and a phase reference standard generating apparatus. The invention also concerns a calibration verification method, a method of measuring the response of an electronic device to a high frequency input signal and a method of producing a data file for a phase reference standard.
The provision of a pre-characterised high frequency signal for use as a phase reference standard facilitates, in the context of a high frequency measurement system, the transfer of absolute phase information to a time domain measurement system. This in turn assists with the analysing, and characterising, of the large signal behaviour of a high frequency device, commonly referred to in the art as a device under test (DUT). Such analysis and characterisation is for example useful when seeking to improve or optimize the performance of devices or circuits for use in high power, high frequency amplifiers, such as an amplifier for use in a mobile telephone network. Analysis is typically carried out with the assistances of a high frequency measurement apparatus, such as a vector network analyser (VNA). A VNA is typically capable of measuring the small signal (linear) performance of a DUT. However, to enable such a measurement apparatus to make non-linear (large signal) measurements, additional calibration steps are required, to ascertain absolute magnitude (power) and phase measurements of signals at the DUT. A VNA, once so calibrated and set-up to make non-linear measurements, is typically termed a non-linear vector network analyser (NVNA). Ascertaining the absolute magnitude of signals from measurements made by the VNA can be achieved by calibrating the VNA with reference to measurements directly made with a power meter. Ascertaining the absolute phase of signals from measurements made by the VNA can be achieved with the use of a phase reference standard, which may be defined by a pre-characterised high frequency signal.
There are proposals in the prior art relating to the provision and use of phase reference standards, such as the paper entitled “Time-Domain Calibrated Measurements of Wideband Multisines Using a Large-Signal Network Analyzer” by Mohammed El Yaagoubi, Guillaume Neveux, Denis Barataud, Tibault Reveyrand, Jean-Michel Nebus, Frans Verbeyst, Francis Gizard, and Jérôme Puech (IEEE Transactions on Microwave Theory and Techniques Vol. 56, No. 5, May 2008, pp 1180 to 1192)—the “Yaagoubi Paper”. The Yaagoubi Paper proposes a pulse/comb generator made using step recovery diodes (SRD), yielding a multitude of harmonics with a known phase relationship. The fundamental frequency is relatively low, 20 MHz. The Yaagoubi Paper discusses use of the 80th to 83rd harmonics, corresponding to RF frequencies of 1.6 GHz, 1.62 GHz, 1.64 GHz, and 1.66 GHz. The multitude of harmonics have a relatively low and ever decreasing magnitude (typically following a 20 dB drop-off per decade) that has the potential benefit of forming a relatively fine calibration grid that can be aligned with frequencies to be measured. However, this spreads the available energy from the standard over a large frequency range and a large number of harmonics. With increasing frequency, the harmonics have a lower and lower signal to noise ratio, until the amplitudes of components of the comb drop towards the noise floor of the measurement system. This limits the accuracy of the measurements particularly at higher frequencies. Coping with the next generation of VNA, which should operate at yet higher frequencies, will place greater emphasis on accuracy of a phase reference standards at high frequencies.
The present invention seeks to mitigate one or more of the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved method of providing a pre-characterised high frequency signal and/or an improved phase reference standard generating apparatus.