Feature: RF design
Figure 3: Noise power ratio measurement
Noise power ratio measurements Noise power ratio (NPR) measures the inter-modulation and noise floor of RF transponders and components in wireless systems. NPR is defined as the relationship between the total power density in a channel and the noise power density, and is commonly used to describe the distortion caused by in-band multi-carrier intermodulation in a wideband radio channel. Te NPR concept has been around since the early days of
frequency division multiplexed (FDM) transmission systems; it is simply used to measure the “quietness” of an unused channel in a multi-channel system when there’s random activity on the others. Noise and inter-modulation distortion products fall into the unused channel causing less than ideal performance. Originally used to check 4kHz-wide voice channels in FDM links, the same concept is useful today in characterising multi-channel wideband communication systems; however, there are some important differences in modern measurement techniques. Te requirements for 5G data speeds push the limits of video
amplifier range from 10MHz to 500MHz and beyond. To deliver these data rates, amplifiers must be tested to see how they reproduce complex modulated signals with high peak-to-average power ratios. Tus, amplifier performance is important in the system because nonlinearity will reduce the dynamic range of the channel by limiting the lowest level of signal strength that can be received without error. NPR is a convenient way to test for nonlinearity, either at
component or system level. Traditionally, two-tone testing is used to measure intermodulation distortion of amplifiers, but the method has limitations since the signals used do not emulate the higher peak-to-average power ratios the amplifier needs to handle. Te peak-to-average ratio of multiple Orthogonal Frequency Division Multiplexing (OFDM) signals is much greater than that experienced during the two-tone test. Te large peaks stress the amplifier or device under test to a greater degree, making the two- tone test less useful and making NPR measurement a better figure of merit for system performance. NPR testing is done with a broadband noise source that represents
any or all the carriers in the specified operating bandwidth of an amplifier. Using a noise source has the added benefit of being very economical when compared to the high-end signal generators required to create tones in the millimeter wave range. Figure 2 shows Rohde & Schwarz FSW-K19 noise generator
that creates white noise to simulate a number of carriers or channels operating concurrently. A deep notch is created in the band of noise, typically at the centre of the band, and the test signal is then applied to the device or system under test. Te amount the notch fills in gives an indication of the non-linearity generated in the R&S FPC1500 spectrum analyser device- or system-under-test and is used to determine the NPR of an active component in the system or the system as a whole.
20 November/December 2020
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As mentioned, NPR testing was historically developed to evaluate
FDM communication systems. A typical system might consist of 4kHz voice channels, stacked up for transmission into a higher-bandwidth signal. At the receiving end, the FDM data was de-multiplexed and converted back to 4kHz voice channels. Noise and distortion was added to the signal as it passed through amplifiers, repeaters, channel banks, etc. Tus, because analogue-to-digital converters (ADCs) also process broadband signals, and because their specific applications are varied, their figure of merit finds a modern-day application in evaluating ADC performance. Equipped with the FSW-K19 option, the R&S FSW signal and
spectrum analyser offers a convenient and straightforward way to measure the NPR over a maximum of 25 notches. Te NPR at rest is defined as how quiet one unused channel in a wideband system remains when the other channels cause noise in it due to inter-modulation. Tus, this type of test signals are usually generated at the intermediate frequency (IF) and up-converted to the RF band of interest. Te NPR test is the ratio of the sum of the power inside the notched bins (PNi the sum of the power in an equal number of bins outside the notch (PNo
) to )
presented by the following relation: NRP = 10log10
PNi / PNo In fact, care must be taken to insure that this process does not
degrade the notch depth. In wideband NPR testing, amplifier noise figure can oſten limit the maximum NPR measured. Figure 3 shows a common method of NPR measurement. At this point, either a Gaussian or uniformly-distributed signal may be used for the noise source, but for ADC devices, the Gaussian source is more useful. A low-pass filter is used to prevent noise aliasing, which would lead to a higher, inaccurate NPR measurement. In this process it will be necessary to introduce the Y-factor, the ratio
between hot and cold power of the DUT. Te Y-factor indicates the quality of measurement tolerances and uncertainties. To derive it, the application measures DUT power with the noise source on (hot power) and then off (cold power) presented as:
Y-factor = Non
where Non Noff
/ Noff = Noise power (in dB) with noise source off; and = Noise power (in dB) with noise source on.
Noise figure measurements Noise figure is a key performance parameter for any RF or millimeter- wave component or system, which gives the designer an indication of the degradation of signal-to-noise in a system due to noise generated or present in the system. Te smaller the noise figure, the better the system performance and the lower the signal-to-noise degradation. As data rates and operating frequencies increase for 5G applications, systems are even more sensitive to signal-to-noise degradation on
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