A digitizer is an electronic device that performs the functions of an oscilloscope but with several advantages. Measurements of input signals include voltage, frequency and time. Signals received by the digitizer are processed through an analog-to-digital converter. The resulting digital samples are then sent to a buffer, which allows them to be saved before being displayed, analyzed or otherwise processed by an attached computer system or ASICs. Digitizers can, theoretically, read and process unlimited channels simultaneously. They also allow for signal acquisition without dead time between windows.
The acquisition memory of a digitizer determines how much signal data may be stored in the buffer before it must be displayed, processed or saved. Digitizers with high acquisition memory are able to make detailed measurements that can be used for complex applications, such as signal modulation analysis and pulse height measurements. Higher memory also allows for a higher sampling rate.
Analog-to-Digital converter (ADC)
An analog-to-digital converter (ADC) transforms an analog signal captured by a digitizer into digital data that can be processed by a computer or by application-specific integrated circuits.
Application-Specific Integrated Circuits (ASICs)
Application-specific integrated circuits (ASICs) are used in a digitizer to process signal data for specific applications without having to send the data through a separate software application or a separate microprocessor. Some of the integral parts of a digitizer are ASICs, including the analog-to-digital converter.
Bandwidth (-3 dB)
Bandwidth is the range of frequencies that can be transmitted, received or processed by an electronic component. Total bandwidth is the maximum frequency minus the minimum frequency and is usually measured in hertz. However, for practical purposes, the hertz value does not include low-level frequencies of the bandwidth, which are those at or below -3 dB. This is known as -3 dB bandwidth, and it represents the frequency range starting at half the maximum value of the bandwidth. Digitizer bandwidth represents the frequency range that can go through the input without loss of signal amplitude.
The dynamic range of a digitizer determines the minimum and maximum signal voltages that can be measured in one sample set. A large dynamic range allows for the measurement of dynamic signals that have both small and large voltage components.
Flatness is a measure of the amplitude variance of signals as they enter the digitizer and make their way to the ADC. When the frequency of incoming signals rises, the amplitude falls toward the bandwidth -3 dB point. Favorable flatness values are especially important when measuring dynamic signals with components of differing frequencies. When flatness is at maximum, the measured signal is equal to the input signal.
LXI is the newest of the two form factor standards used for modular signal testing instruments, such as digitizers. The LXI standard was introduced in 2004. LXI uses an Ethernet interface to connect the component digitizer to a local area network (LAN).
Noise Spectral Density
Noise spectral density is the power of the noise in a signal per each frequency of bandwidth. Noise spectral density is a more specific form of power spectral density. It is also known as noise power density or spectral noise density.
PCI, cPCI, PCIe
PCI, compact PCI (cPCI) and PCI express (PCIe) are three types of interfaces used to connect a digitizer or other components to a computer directly through the motherboard of the computer. These three interfaces are primarily used for digitizers that are made to be housed in the chassis of a computer rather than modular or standalone digitizers.
PXI is one of two standards used for modular digitizers and other types of testing instrumentation. Of the two standards, PXI is the oldest, but it is still widely used alongside the newer standard, LXI. PXI is an acronym of PCI extensions for instrumentation, and it is used to connect digitizers to a computer or to a network through a computer.
The resolution of a digitizer indicates how many bits of digital data the analog-to-digital converter can produce from an analog signal. Resolution is important for measuring dynamic signals and small signals. The minimum practical resolution of a digitizer is 8 bits, but 16 bits to 32 bits are required for many digitizer applications.
Return loss is the power of an input signal that does not make it through to the output because of impedance or reflection. Return loss may alternately be defined as input power minus reflected power, and it is usually expressed in dB. Return loss can be measured at any point in a network after the loss occurs.
The sampling rate, or sample rate, of a digitizer is the speed at which analog signals can be converted to digital data by the analog-to-digital converter. For measurements to be effective, the sample rate of a digitizer must be at least double the speed of the highest signal frequency, but experts agree that a sample rate three to four times the highest signal frequency is preferable.
Signal processing is the conditioning or analysis of analog signals and the digital data created from the signals. Common signal processing applications include filtering, analog-to-digital conversion, spectrum analysis and storage.
A spurious response occurs when an input signal becomes mixed with frequencies generated by the receiving equipment. In a digitizer, spurious response is most often caused by the analog-to-digital converter.
A time-to-digital converter (TDC) is a type of analog-to-digital converter that measures the time of reception and the intervals between signals. Because a TDC does not measure signal strength or frequency, it is used only to determine problems based on the timing of signal pulses.