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Most people believe that sensitivity is most important characteristic for one radio receiver, but it is not quite true. Actually the very sensitive receiver is not problem to design and produce. Much more difficult task for radio engineers is to design a high dynamic range receiver, which will be able to receive very weak and very strong signals at the same time, without overloading.

Overloaded receiver front end means that it is not linear any more, and produces many signals by itself, increasing its noise level.
Very strong signals at the receiver front end makes "Desensitization" of the receiver, so it could not receive weak signals as long as strong signal is present.
We should not forget that the receiver front end "looks" all signals from the wide frequency range even if we want to receive only one signal at the time. The more wideband the receiver is, the higher dynamic range it has to be, for not been overloaded.

The SDR receiver quality depends of many factors. One of the most important factors that directly affect the SDR receiver performance is dynamic range of the analog-to-digital converter utilized.

What is the SDR Dynamic Range?

Generally speaking, dynamic range is the ratio between the strongest and weakest signal that can be received.

In a digital signal, the dynamic range is determined by the number of bits per sample: the strongest signal is one which uses the full range of the sample values, and the weakest signal is one which uses only two adjacent values (one bit change).

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.

If you have clipping, it can be observed as the appearance of spurious signals which are copies of actual signals in the received band at different frequencies.

If the analog signal is too weak, then its presence will not manage to make even one bit of difference in the digital output. More practically, well before the one-bit point you will be losing information due to the digital sample values being too coarse-grained to accurately represent the signal. This can be thought of as a source of noise, called quantization noise.

The dynamic range of the analogue-digital converter is the relation between the strongest signal and weakest signal (noise floor) that can be digitized without distortion

The strongest signal represent maximum input signal, which is full scale signal level. Usually the 0 dBFS (dBFS means decibels relative to full scale) is asigned to maximum possible digital level, while the weaker signal is represent in negative numbers. For example the noise floor can be -60 dBFS, -80 dBFS, -100 dBFS etc. depending of SDR system quality and signal to noise ratio (SNR) of the system. You can see this on the follow picture:


click on image to see it in high resolution


Software-Defined Radio receiver maximum Dynamic Range depend by ADC number of bits:


where n is the number of bits, DR is dynamic range

According to this formula, we can calculate that the dynamic range of the analog-digital converter with 8 bits has about 50 dB dynamic range, 12-bit analogue-to-digital converter has about 74 dB dynamic range and 16-bit analog-to-digital converter has about 98 dB dynamic range.

Why dynamic range matters for Software Defined Radios?

One can think that you could have an AGC (automatic gain control) that keeps the input signal at just the right level, and not need very much dynamic range. This would be true, except that in a software-defined radio of the kind we're interested in, the signal into the ADC is much wider bandwidth than the individual signals we want to demodulate! This additional bandwidth is what allows you to get the “waterfall” or “panadapter” display characteristic of SDRs (because you're actually digitizing a wide band of RF), but it means that the analog signal coming into our ADC contains lots of RF power which is not any one signal we want to demodulate — it includes a large amount of noise, and possibly many intentional signals, some of which may be very strong compared to the desired signal.

The best analog gain setting is determined by the total RF power coming into the ADC (from the tuner and analog filter, if present). This fixes the “strong” end of the scale of possible digital signal levels, leaving the “weak” end determined by dynamic range. Therefore, the more dynamic range we have, the less quantization noise is present in narrow-band signals (which are more or less weak compared to the overall signal), allowing them to be received more clearly.

You can also improve reception with the same dynamic range by using a narrower analog filter, thus reducing the input power and allowing more gain to be applied to it without clipping — if your SDR hardware has a usefully adjustable filter.

The extreme case of using a narrower filter is using one just as wide as a single signal you want to receive. The disadvantage is you don't have any spectrum view while you're doing that — you can only receive that one signal.

Dynamic range limitation

Besides the analog-to-digital converter number of bits, the dynamic range is given by two independent parameters: limitation by noise and limitation by spurious signals.

The simplest acquisition system is without input preamplifier. In that case system dynamic range is limited by dynamic range of ADC. If the preamplifier is used, the dynamic range may be limited by ADC or by preamplifier.

The noise limits the smallest signal that can be digitized.

The system dynamic range should be higher than signal-to-noise ratio (SNR) of the input signal. If system dynamic range is lower, the acquired data are distorted by the acquisition system. The bottleneck of the system is determined by used gain.

The spurious signals are mostly more dangerous than the noise. They have a higher amplitude than noise and their amplitude does not depend on the used passband. Spuriosus signals are given by all non linearity and their main source is analog-to-digital converter (ADC). The Spurious Free Dynamic Range (SFDR) for the ADC is defined as the ration between rms amplitude of the single tone and the rms amplitude of the worst spur. SFDR can be seen on this picture:



Using decimation to improve the dynamic range increasing the processing gain

There's often confusion about dynamic range from wideband ADCs. The confusion generally works like this: someone will lookup a data converter that runs at 20 MHz and see that it has a dynamic range of 74 dB and assume that it could never beat a radio with an 85 dB dynamic range. The problem is that this is an apples and oranges comparison. You cannot talk about instantaneous dynamic range (which is maximum input level minus the noise and distortion) without talking about detection bandwidth. For ham radio, this is the width of the actual receiver. It could be 2400Hz for SSB or 500 Hz for CW, for example.

What really happens is that in some better SDRs the process called decimation is used.

The decimation process takes the data collected at an oversampled rate (20MHz for example) and then systematically reduce the sampling rate down to the bandwidth of interest. In this process dynamic range is increased in what is called "processing gain". Processing gain occurs when noise outside the band of interest is digitally removed, which results in improved in-band SNR.

For example, a 12 bit analog-to-digital converter in one SDR receiver which operate at 10 Msps, has 10 MHz frequency bandwidth and not less than 8 MHz alias-free bandwidth. Using a decimation, we will increase the dynamic range by 3 dB (decreasing the spectrum bandwidth by half) for each doubling the decimation:

- With 0 decimation alias-free spectrum bandwidth is 8 MHz and dynamic range is about 74 dB.

- With 2x decimation alias-free spectrum bandwidth will be 4 MHz, processing gain will increase by 3 dB and dynamic range will be 77 dB.

- With 4x decimation alias-free spectrum bandwidth will be 2 MHz, processing gain will increase by 6 dB and dynamic range will be 80 dB.

- WIth 8x decimation alias-free spectrum bandwidth will be 1 MHz, processing gain will increase by 9 dB and dynamic range will be 83 dB.

- With 16x decimation alias-free spectrum bandwidth will be 500 kHz, processing gain will increase by 12 dB and dynamic range will be 86 dB.

- With 32x decimation alias-free spectrum bandwidth will be 250 kHz, processing gain will increase by 15 dB and dynamic range will be 89 dB.

- With 64x decimation alias-free spectrum bandwidth will be 125 kHz, processing gain will increase by 18 dB and dynamic range will be 92 dB.

Conclusion according to analog receivers dynamic range

In reality, it is impossible for any receiver to have blocking dynamic range or IMD dynamic range greater than its phase noise dynamic range (PNDR) otherwise known as reciprocal mixing dynamic range (RMDR). In all cases and no matter the architecture, if RMDR is less than BDR or IMD DR for a given tone spacing, the phase noise will cover the signal of interest before blocking or IMD will be a factor. In fact there is not a single transceiver from any manufacturer on the market that would not have its blocking dynamic range limited by its internal phase noise much less first by the noise from the transmitted signal.

Most of the old technology superheterodyne transceivers on the market have horrible RMDR numbers. When a strong signal is heard by them, their oscillators spread the signal all around the band as noise covering up signals you are trying to hear. Here's the simple test: Take two of your favorite legacy radios and transmit in one while listening in the other and watch what happens to the noise floor at 2, 10, 20, 50 and 100kHz from that signal. You will see that these receivers show significant noise floor increases that prevent operation near each other. This is the practical concern - there's no reason to talk about a number of mythical strong signals of all the same power that might correlate to cause an overload in a new type of receiver... the real problem is the superheterodyne receiver that folds under a single strong signal in the vicinity of small signals you are trying to copy. Most contesters have experienced this first hand when two radios are being used. If you have to tell your operating buddy in the same band to stay so many kHz away from you, you know the problem well. This is also a classic Field Day problem.

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