Deciphering Digitizer Specs For Wireless Apps
With wireless communications proliferating as never before, several parameters are important in the selection of a digitizer for acquiring and accurately measuring signals used for the design and testing of wireless equipment.
Wireless engineering has become extremely complex in most applications, including cellphones, computer network components, satellite communications and commercial broadcast transmitters, all of which require ultra-high stability and accuracy. To help ensure that communications products meet those requirements, multi-channel analog-to-digital converters – digitizers – now are widely used in R&D prototype development, manufacturing testing and field-service testing of communication chips, sub-assemblies and finished products.
PC-based digitizer cards clearly have demonstrated significant advantages when compared to oscilloscopes in terms of such attributes as speed (sampling rate), resolution and accuracy. However, there often is cause for confusion over manufacturers’ specifications – the numbers that describe the quality and performance range of each manufacturer’s digitizers.
While manufacturers publish such critical performance metrics as speed, memory, clock accuracy and bandwidth, in actuality these specifications only should be taken at face value because they require further definition. That’s why it is important to “look behind the numbers” to see what they may or may not mean to help ensure the selection of a digitizer meets a carrier’s particular requirements. Therefore, technicians may want to request additional information from the digitizer manufacturer to make sure a digitizer model is right for their applications.
One of the important digitizer parameters that require further clarification from the manufacturer is the spectral “flatness” of the frequency response of the digitizer card. Flatness describes the frequency below which the frequency response curve remains within a ± 1 dB about 0 dB attenuation. However, achieving flatness at high frequency (>100 MHz) with no distorting oscillations in the digitizer’s pass band can be challenging.
The frequency below which the digitizer attenuates an input signal by less than 3 dB is called the “cut-off frequency” or “bandwidth” of a digitizer. Frequencies that lie below this 3 dB frequency comprise what is called the “pass band” of the digitizer.
Experts in the field note that, theoretically, they would want the frequency response curve to be flat and sustained all the way to infinite frequency, so that the signal would not be attenuated at all. However, in the real world, the acceptable attenuation generally is considered to be 3 dB.
To know the true pass band of a digitizer, techs can request that the manufacturer provide the appropriate frequency-response curve. This curve could illustrate that, while the signal has a bandwidth of 350 megahertz, the digitizer’s frequency response is flat only up to 300 megahertz. In some cases, where the attenuation may be greater than 1 dB but less than 3 dB, the response may be oscillating rapidly at some frequencies. This somewhat problematic situation would be indicated by the response curve quivering sporadically within the 0 megahertz-300 megahertz bandwidth.
When attenuation exceeds 3 dB, flatness falls off significantly, and noise occurs even more noticeably. In the absence of the full frequency-response curve, obtaining the flatness provides much more information about the frequency response than just the bandwidth.
Because most applications use a combination of frequencies, a pass band with insufficient flatness often will be detrimental to performance in wireless applications. As such, it would be helpful to technical staff if they would request from their prospective suppliers some graphical or tabular references providing sample input signals (sine waves) and corresponding frequency response curves, rather than simply a bandwidth description.
High Vertical Resolution
It’s important that a digitizer provide the best possible resolution or signal fidelity. While almost all standalone digital oscilloscopes contain embedded 8-bit digitizers, a 12-bit digitizer card enables much higher resolution – 4,096 levels across the input range compared with only 256 levels attainable from an 8-bit oscilloscope. The 16X smaller levels allow smaller signal features to be detected in the time or frequency domains.
However, the nominal resolution stated by the digitizer’s manufacturer does not indicate its performance directly. The best single measure of performance is the Effective Number Of Bits (ENOB), which is the true resolution that allows for the noise and distortion introduced by the digitizer instrument.
Generally, undesirable elements added to an input signal by a digitizer instrument can be characterized either as noise or distortion. Of these two, it’s almost always distortion that is the dominant contributor to the degradation of signal fidelity at high signal frequency. Once a signal has been distorted, there is no practical way to remove the distortion. Further, the signal-to-noise ratio (SNR) generally stays constant as a function of signal frequency, while the total harmonic distortion (THD) increases dramatically with signal frequency. Therefore, high vertical resolution and accompanying high ENOB and low THD near the intermediate frequency (IF) are important for good signal integrity.
The nominal resolution of a digitizer is theoretical, and the ENOB is the reality. For that reason, although the ENOB may not be stated in the product literature, techs need to look beyond the nominal resolution and get the ENOB from the digitizer manufacturer in order to determine if the vertical signal fidelity is sufficient for the application.
Compared with most digitizer applications, wireless apps have strict requirements for the accuracy and stability of the sampling clock signal that determines the sampling rate of the digitizer’s analog-to-digital (ADC) chips. Using a disciplined crystal oscillator, digitizers may achieve excellent sampling clock accuracy of about 1 part per million. Nonetheless, communications apps routinely require even better clocking accuracy.
Compared to standalone oscilloscopes, commercial digitizers provide much more ADC clocking flexibility. Digitizers often provide two methods of improving clock signal accuracy:
First, the user may provide an ultra-accurate external sampling clock signal that is passed directly to the ADC chips. The external clocking input (almost never available on standalone oscilloscopes) also allows the user to provide any sampling rate. For example, a user designing communications receivers with built-in 887.3 MS/s ADCs could simulate receiver operation with a commercial digitizer operating with an 887.3 MHz clocking signal being “fed” to the digitizer card from an external source.
The second method of improving clock accuracy requires a digitizer with a 10-megahertz reference signal input. The signal is used internally to control a Phase-Lock-Loop (PLL) circuit that creates the digitizer’s sampling clock. For example, the PLL would help ensure that a 500-megahertz sampling frequency is exactly 50 times the frequency of the 10-megahertz input. This is useful in many applications, providing the additional advantage of allowing for synchronization of multiple instruments, which can circumvent signal instability problems in many communications systems.
In many wireless apps, communications engineers and manufacturers want to test signals or sine waves for durations ranging from several seconds to many minutes or even hours.
Typically, engineers want to acquire uninterrupted signals at high speeds like 500 MS/s for human time-scales of seconds or minutes. In the past, this has required large amounts of dedicated onboard acquisition memory. Several gigasamples of onboard memory allow unbroken signal acquisition for several seconds at 500 MS/s.
Recently, data busses have emerged to allow continuous streaming of waveform data directly from the digitizer to fast hard drive arrays (RAIDs) having terabytes of capacity. Digitizers on such platforms may stream data at 500 MS/s for a full hour or longer. Therefore, high digitizer memory or even streaming to high-capacity storage drives allow for acquisition of communications signals for hours, if need be.
—Andrew Dawson, sales manager, GaGe Applied Technologies (www.gage-applied.com)