Tabor`s Modular & Wonder Wave Series as Signal Sources for RF Testing
October 2007
Tabor's Modular & Wonder Wave Series as Signal Sources for RF Testing
Tabor's high performance Wonder Wave Series become the ultimate modulation generators when linked to the Modular
By Joan Mercade and Ron Glazer, Tabor Electronics Ltd.
Overview Tabor's Wonder Wave Series of Arbitrary Waveform Generators (AWGs) are among the most powerful and flexible test tools available. Their unique capability to generate virtually any signal makes them the preferred instrument when generation of realistic signals is required.
AWG's basic architecture is quite simple: samples defining the waveform are stored in a waveform memory, this memory feeds a digital-to-analog converter (DAC) at a rate defined by its time base, and finally the output signal is amplified and filtered to meet the application requirements. Their performance is defined by specifications such as maximum sampling rate, DAC resolution, and waveform memory size.
Tabor's Wonder Wave Series' outstanding performance of Sampling speeds higher than 1 GHz, converter resolutions up to 16 bits, and waveform memory sizes over 16 MSamples define the current state-of-the-art technology. This increasing performance of the Wonder Wave Series helps in traditional applications such as telecom, disk drive, electro-mechanical, and biomedical. Simultaneously, it opens new application areas such as RF/wireless testing.
Applying the Wonder Wave Series to RF Tabor's Wonder Wave Series can be used to generate complex waveforms, especially baseband IF or low frequency RF signals. These signals can be used directly in many tests at both ends of the signal path. Baseband signals, I and Q components, can be applied to quadrature modulators to generate the required modulated carrier. Alternatively, IF signals may be up-converted to the desired frequency.
What Makes the Wonder Wave Series a Unique Hardware Solution for RF Testing There are many reasons that make Tabor's Wonder Wave Series of AWGs a good fit for modulation generation:
There is no need for extra dedicated hardware
They can generate any set of carriers from DC to the Nyquist frequency, using up to 1.2GS/s clock with up to 16bits resolution and 16MSamples memory
They come with standard AM, FM, ASK, FSK, PSK, Frequency and Amplitude Hopping, 3D, Sweep and (n)QAM and (n)PSK modulations
They can generate virtually any distortion, linear and non-linear
They can generate high voltage signals to 32Vpp in to open circuit
Some of the models, such as 2571A and 2572A incorporate parallel digital outputs, useful in testing software-defined radio (SDR) systems.
As stand-alone they are also the best general-purpose tools
Understanding Performance One of the most common objections to using AWGs as RF generators is the available dynamic range. Some important tests in this application area, such as adjacent channel power ratio (ACPR), depend heavily on this specification. Actual performance from the high-performance WW1281A that has sample rates as fast as 1.2 GS/s and high linearity 12-bit DACs, show ACPR values better than 75 dB for 10 MHz carriers.
On the other hand, some engineers will tell you that the most significant specification is the signal-to-quantization noise ratio (SQNR). For a sine wave running at full-scale amplitude, SQNR can be calculated as:
SNRsine (dB) = 6.02N + 1.76
Where: N = the resolution of the DAC expressed in bits
For any other signal, this expression should be corrected by the crest factor or peak to average power ratio (PAPR) and the fraction of the DAC output range used.
SNR (dB) = 6N + 1.76 - PAPRdB - 20 log10(FS/A)
Where: FS = full-scale amplitude A = peak-to-peak signal amplitude
But, is this really the number an RF engineer tries to measure using a spectrum analyzer and real-world signals? The answer is No.
What is more important is the noise power density, or in other words the noise that is located at frequency bands that are occupied by the signals under study. Noise power density must be integrated over the signal bandwidth (SBW) to obtain the in-channel SNR.
Here's a good example of a 5MHz bandwidth 3GPP CDMA downlink IF signal with a 10 dB crest factor (PAPR) being generated with the WW1281A, a 1.2GS/s, 12 bit, that shows a signal to quantization noise ratio (SQNR) of 82 dB.
Another important factor, yet difficult to express in a single spec is linearity. That's why typically spurious-free dynamic range (SFDR) is more descriptive for DACs. SFDR measures the worst-case ratio between the fundamental frequency and any other spurious or nonlinear products, such as intermodulations or harmonics. As time goes by, linearity becomes more important than resolution in many applications, and the importance of spurious signals greatly depends on their location. Tabor's WW1281A show SFDR better than 75 dB for 10MHz carriers and around 45 dB for 400MHz carriers.
RF generation requires very high sample rates. Record length becomes an issue in applications where the statistical behavior is important. There is a requirement to accommodate a complete frame of symbols or implement any channel coding. That's why record length is another important factor. Sampling rate, record length, and the available time window are interrelated. The higher the sampling rate, the shorter the time window will be for a given record length.
For example, a complete 3GPP CDMA scrambling code sequence requires the transmission of 38,400 chips at 3.84 Mchips/s, which is at least 12 MS of waveform memory. It is clear that this is an easy task for the WW1281A, which comes with 16MSample.
Wrapping Around Wireless test is much more demanding than most other areas of AWGs application, when looking at signal quality and consistency requirement, as it involves many signal domains simultaneously, such as: time, frequency, modulation, channel coding, protocol and last but not least, wrap-around discontinuities that may affect the signal at all these domains.
The wrap-around artifacts are a memory-related issue, since the only way to create continuous signals in any AWG is by repeating the same segment seamlessly. These artifacts are produced when the beginning and the end of the signal are not consistent at all the required levels, which is usually caused by the discontinuity between both ends of a given waveform being repeated in a loop.
The effect of wrap-around discontinuities can be seen in several areas. Any discontinuity between the beginning and the end of the signal will cause a frequency domain impairment known as spectral growth. At the modulation level, these discontinuities affect the processing operations such as carrier and symbol clock recovery, I and Q component demodulation, and symbol estimation applying the right base band filtering.
This is why it is important to design the signals in such a way that all the wrap-around artifacts are removed so they can be looped without any glitch just as in real transmitters when transmitting live traffic. In order to have glitch-free signals, base-band filtering must be performed seamlessly between the symbols at the end of the current waveform and the beginning of the next one.
Tabor's Modular software automatically eliminates the wrap-around artifacts for any number of carriers by applying an operation called circular convolution. By using the Tabor's Wonder Wave Series, you can rest assured that you'll overcome this issue, as its outstanding architecture links signal segments without any gap or glitch related to the sampling period, as samples are converted seamlessly regardless of their location.
Calculating the Signal Tabor's Modular software package uses algorithms that speed-up calculations without sacrificing signal quality, therefore allowing fast signal creation and a high level of test interactivity. Ideally, unimpaired waveforms are used in many tests but some of them require the simulation of real world conditions. Modular may generate signals involving linear and non linear distortions (AM/AM and AM/PM), multipath and all kind of interferences. It provides enough signal visualization features and analysis capabilities to validate the signals being synthesized. Modular can show a variety of graphs such as constellation and phase diagrams, spectrum, I&Q vs. time, CCDF and histograms. By combining the Modular with the remarkable Wonder Wave Series, the client gets an outstanding solution that allows resolution of any current or future wireless testing need.
Open Environment with Virtually Endless Possibilities Although modulation scheme support is an important characteristic of any Modulation Source, wireless technology is evolving all the time. It is very important to be able to define new modulation schemes and to import data from external simulation tools so that new or proprietary standards and technologies can be supported. Modular can import externally defined modulations, I and Q time domain waveforms, baseband filters, and data sequences. It can also export waveform data in ASCII delimited formats so the same test waveforms may be used in any simulation tool so results may be correlated with the real behaviour of the device under test.
Conclusion As Tabor's AWGs become more powerful with higher sampling rates, resolution, record length, and linearity, they will be even more useful in RF applications. Tabor's Wonder Wave Series offers all types of built-in modulations: FM, AM , SWEEP, PWM, FSK, PSK, ASK, (n)QAM, (n)PSK and even a unique 3D modulation which combines FM, AM, and PM on one signal. Now by being able to combine its new Modular software with its powerful Wonder Waveform Series, Tabor now offers one of the most powerful tools for RF/wireless applications.
