Analog Filters: High Dynamic Range RF Systems Benefit from Digital and Analog Hardware
Analog Receivers and Analog-to-Digital Converters in Radio Frequency Engineering
Analog filters? Who needs those?
These days, everything is digital. Digital television, digital audio, digital just about everything. It seems that everything analog has fallen out of fashion and been replaced by a digital substitute.
But is that always best?
I have friends in the audio business who still prefer vinyl records. I admit that I like the sound of a good analog recording, whether vinyl or open reel tape (but do spare me from cassettes, please). The spatial presence just seems better to me. There are reasons for this, but that’s beyond the topic I want to discuss in this post.
What do we mean when we say a receiver is “digital” as opposed to “analog?” In an analog receiver, signal processing, such as amplification, filtering, gain control, demodulation and other related functions, are done by physical, analog circuits. In a digital receiver, the analog signal is converted to digital values (much like a digital voltmeter, but faster) by an analog to digital converter (A/D). Once the input signal is in the form of numerical data, all of the same analog functions can be implemented using mathematical calculations instead of analog circuits.
What RF Systems Need High Dynamic Range?
The use of digital receivers allows for features and functionality that analog receivers are unable to provide, such as the ability to see wide swaths of RF spectrum simultaneously, or provide detailed analysis of signals that are beyond the capability of conventional analog receivers.
However, there are times when, for all its positive aspects, digital is simply not as good as analog. This is especially true in the case of some (note the word “some”) RF filters.
Specifically in RF systems that demand very high dynamic range operation. In these situations, a combination of analog and digital technology gives the best results.
Analog to Digital (A/D) Converters
RFDU-300
The RFDU-300, made by Radio Design Group, is an excellent example. This unit is part of a SIGINT (signals intelligence) system that uses digital receivers to monitor broad swaths of spectrum. The input to the digital receiver starts with an analog to digital (A/D) converter, as previously mentioned.
It’s a very good A/D converter, but it has limitations, as all A/D converters do. It has a low end sensitivity that is limited by the converter’s noise figure, and a high end limit where the converter overloads. Basically, the A/D converter runs out of numbers! As long as all the signals in the monitored spectrum are within the levels the A/D converter can handle, there are no problems. But if there is a very strong signal in the band of interest, what happens?
If the A/D converter goes into an overload condition, it ceases to output accurate information. To prevent this, the receiver has an AGC (automatic gain control) circuit that attenuates the input level to keep the converter out of overload. This works well, except that as the AGC turns down the receiver sensitivity, weak signals are lost in the noise of the converter.
Why Are Weak Radio Signals Important In SIGINT Systems?
In a SIGINT environment, those weak signals could be very, very important. Perhaps the weak signal is a distant “Russian Trawler,” or perhaps another submarine. This is where the analog filters, located in front of the A/D converter, are important. When a strong signal drives the receiver into an AGC reduced gain situation, the filters can be applied to attenuate the interfering signal without affecting the other parts of the spectrum being monitored. The analog filter has a much higher dynamic range, which, in turn, gives the digital receiver additional dynamic range.
In the RFDU-300, up to two analog filters can be put into the signal path, each with a minimum attenuation of 20 dB. They are completely digitally controlled, and allow the system to provide a reduction of 20 dB for two interferers, or alternately, more than 40 dB for one interferer, should that be necessary.
So, the digital part of the system does the job that it does best, while the analog filters do the job that they do best. Each compliments the other!
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