Active Oldest Votes. In a software-defined radio, the analog signal level fed into the analog-to-digital converter must be adjusted either manually or by an AGC to make best use of the available dynamic range: If the analog signal is too strong, then the logically corresponding digital values are not representable in the numeric range of the samples, and the ADC will substitute the maximum or minimum possible output; this is known as clipping , which from a signal processing perspective is an extreme form of nonlinearity.
Why dynamic range matters for SDRs Given the above, it might sound like you can have an AGC that keeps the input signal at just the right level, and not need very much dynamic range.
Direct answers Why do I care about dynamic range? Signals you want to receive disappear into the quantization noise floor. As a designer, additional sample bits: require additional computation significant for embedded systems; probably moot if you're using a general-purpose machine for your DSP since the samples will likely be expanded into bit integers or floats for computation , require a higher-performance expensive ADC, and may be worthless due to noise received by the antenna if the electromagnetic noise floor is higher than the quantization noise floor when the gain is set optimally or noise getting into the system between the gain-control stage and the ADC the low bits are always noise.
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Accept all cookies Customize settings. Which of the following conditions does not affect SNR? In spectrum analysis, SNR is influenced by high-fidelity instrumentation.
The resolution falls in the range of 30 dB— 60 dB. Frequency division multiplexing uses few MHz IF-bandwidth. This imposes the design rule that the total noise power should be less than half of the least significant bit of ADC.
The ratio of spectral density outside the notch filter to the maximum spectral density inside the notch filter gives the Noise Power Ratio. The point at which the output noise power of the fundamental signal shifts away from the ideal output power by 1-dB is called 1-dB expansion point.
This is referred as IP3. The point at which the output noise power of the fundamental signal shifts away from the ideal output power by 1-dB is called 1-dB compression point. E-Systems now Raytheon in Garland, Texas, is believed to have built the first software-defined baseband receiver in Modern examples include satellite and terrestrial radio, military joint tactical radio system JTRS , and almost any cellular or land mobile radio terminal or base station. For digital conversion and signal processing to work, theory says we should have a linear, time invariant system.
Practice tells us that putting the analog pieces together is a series of compromises. By careful component selection and gain distribution, however, you can maximize the dynamic range of your SDR while still maintaining sensitivity. In some standards-based communications systems, that is cellular systems, SDRs operate in a controlled environment; the standard spells out the requirements for the receiver and transmitter, and the carrier adds margin to the standard.
In other systems, such as military, amateur, and land mobile radio, the environment is uncontrolled; the nearest emitter can be next door and the furthest can be less than a whisper over the horizon. Choosing an ADC is a discussion in and of itself. The dynamic range of the ADC determines the systems architecture, and vice versa.
Start by looking at the signal bandwidth and sampling frequencies the exact sampling frequency is usually determined by such digital signal processing requirements as clock or frame rate, or both. For the system to be a linear time invariant system, the ADC must provide enough dynamic range to accommodate the desired signal plus interferers, as well as add margin to allow for signal fading and AGC response time.
So how much dynamic range is enough? The highest performance software-defined radios and RF lab instruments typically use bit to bit, high speed ADCs to sample at as high a frequency as possible for signals as wide as MHz.
For testing the widest signals in standards such as the alphabet soup that is Some applications need less dynamic range and will typically use a bit GSPS ADC such as the AD to grab a MHz wide chunk of spectrum and bring it down to baseband using an integrated digital downconverter.
The dynamic range of the ADC is the fundamental trade-off between analog and digital filtering. More analog filtering reduces the amplitudes of the interferers and the required range of the ADC, which must digitize both the desired signal and interferers to maintain a linear system.
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