DIN EN 62761-2014 Guidelines for the measurement method of nonlinearity for surface acoustic wave (SAW) and bulk acoustic wave (BAW) devices in radio frequency (RF).
5 Nonlinearity measurement
5.1 Measurement equipment
5.1 .1 Signal generator and power amplifier
In the setups shown in Figures 4-9, SGs shall possess the following properties:
a) small nonlinearity,
b) good short term stability (small frequency fluctuation),
c) capability to synchronise with an external standard oscillation signal (usually 1 0 MHz).
Requirements b) and c) are imposed to reduce the thermal noise level in the SA read as will be discussed later.
When the use of Pas is needed, their choice is crucial. Namely, the output stage of the PA shall operate in the class A mode, and the nominal maximum output of PA shall be sufficiently larger than the value required for the measurement. For example, use of PAs with maximum output of 5 W seems appropriate for 500 mW output. Since thermal noise is also emitted from PAs, the use of PAs with too large maximum output power may result in an increase in the noise level in the SA read.
5.1.2 Spectrum analyser
In the nonlinearity measurement, various spectrum components are simultaneously incident to the SA, and some of them may be much stronger than the target frequency component. Thus the SA shall possess good linearity and wide dynamic range. Since minimum detection level is determined by the noise level, SAs with low noise level is preferable. One may think that the vector network analysers (VNAs) can be used for this purpose. Since VNAs possess smaller linearity and dynamic range than SAs in general, applicability of VNAs might be limited.
It should be noted that the thermal noise level in the SA read is inversely proportional to the resolution bandwidth (RBW), which is adjustable in conventional SAs. Namely, the noise level decreases in the form of 10 log(RBW) in dB. For the RBW reduction, fluctuation in the SG and/or SA frequencies shall be suppressed sufficiently. Or the fluctuation will result in decrease of the SA read.
A convenient technique to reduce the fluctuation is synchronisation of all SGs and the SA.
Current RF instruments generate RF signals using frequency synthesisers, and their output frequencies are given by the standard signal frequency f s (usually 1 0 MHz) times a digitally preset coefficient. Thus, provided that common f s is used in all SGs and the SA, fluctuation in f a , f b and f t caused by that of f s can be cancelled out by taking out f s from one of these instruments and supplying it to the others. Present commercial RF instruments equip functions to input and output the standard signal. This technique allows to reduce RBW to 1 Hz for the measurement in GHz range. If not, fluctuation more than 1 00 Hz might be observable on the SA display output. When the RBW is too narrow, the SA read decreases due to uncorrelated frequency fluctuation among f a , f b and f t . Attention should also be paid not to confuse RBW with another adjustable parameter of SAs called the video bandwidth (VBW). Reduction of VBW enables to smooth out the SA display output. This is sometimes effective to suppress fluctuation caused by the thermal noise, but it may also smooth out line spectra. This causes decrease of the SA read. Usually VBW is set automatically. Some SAs offer the averaging function. It stores results of multiple measurements and outputs their average.
This is also effective to suppress fluctuation caused by the thermal noise, but the output becomes inaccurate when the frequency fluctuation is not sufficiently smaller than the RBW. We shall check whether the SA read does not change when the use of the averaging function is desired.DIN EN 62761 pdf download.