SpectraBand Explained 

SpectraBand gives the A20-RX receiver and A20-TX transmitter an unprecedented tuning range: 169 MHz – 1525 MHz. The advantages of SpectraBand to sound professionals are many: 

• More operating frequencies are available in a given location, alleviating the problem of no open spectrum to operate wireless mics.  
• Money is saved by purchasing only one piece of gear, which can be used anywhere, rather than special hardware variants of kit used for different locations. 
• Worldwide travel is easier, as the A20-RX, A20-TX, and A20-Mini can use all legal frequency bands within their tuning range in any given country. 
• The SpectraBand hardware includes exceptional filtering that improves immunity from strong interfering signals (for example, a local DTV transmitter) or other transmitters close to the receiver (IFBs, 2-way radios, camera hops). 

For reference, in the USA, SpectraBand enables the A20-RX and A20-TX to tune over a super wide range of 169-1525 MHz. In the USA, the available frequency ranges are:

• The VHF band (specific frequencies within the 169-216 MHz range)
• The entire UHF TV Band (470-608 MHz) 
• The 600 MHz Guard Band (614-616 MHz) 
• The 600 MHz Duplex Gap (653-663 MHz) 
• The 900 MHz ISM Band (902-928MHz) 
• The 950 MHz STL Band (941.5-960 MHz) 
• The 1.5 GHz AFTRCC Band (1435-1525 MHz), with an appropriate license.  

In the UK, the available frequency ranges are:

• The VHF band (specific frequencies within the 173-210 MHz range)
• The core UK UHF TV Band (470-702 MHz)  
• The 800 MHz Duplex Gap (823-832 MHz)  
• The 800 MHz SRD Band (863-865 MHz) 
• The DME bands (961-1015 MHz, 1045-1075 MHz, 1105-1154 MHz), with an appropriate license.   
• The IMT band (1518-1525 MHz)  

This is not a comprehensive list of countries and their available bands. Visit this page to see frequencies available in other countries. 

Several key technologies enable SpectraBand’s incredible range. These include brick-wall, SAW-based front-end filters, a wide-range local oscillator (LO), wide-range modulator/demodulator, and a high dynamic-range low-noise amplifier (LNA). 

Filtering

In any RF system, proper filtering is necessary for excellent performance. Any time an amplifier is utilized in the signal chain, the amplifier must be preceded by a filter, or poor performance will result depending on the operating environment. This filtering prevents amplifier overload and the resulting distortion products which reduce operating range for the user. In an audio system, clipping of an amplifier is easy for a user to hear and remedy. This similar mechanism (RF amplifier overload) can be difficult to perceive in an RF system as everything can appear and sound okay, but the result (instead of distorted audio, as is the case in an audio system) is reduced operating range. Just as a audio bandpass filter helps an audio engineer deal with noise, a good RF filter helps an RF amplifier from misbehaving. The sharper the passband filter that can precede an RF amplifier without attenuating the wanted carrier, the better. 

Shown in this picture is the A20-RX’s ​brick-wall SAW ​filter compared ​against two traditionally designed and well-regarded tracking filters ​currently ​on the market. The yellow trace is the A20-RX’s SAW filter centered at 573 MHz, the blue is competitor 1 and the green is competitor 2. As can be seen, the A20-RX’s filter exhibits a brick-wall type of response, being over 45 dB down at 603 MHz, while competitor 1 is down only about 2 dB, and competitor 2 is down only about 12 dB. This plot illustrates very clearly why the A20-RX’s filter can operate in closer proximity to strong interference with no change in performance: if an interferer (DTV or other transmitter) is at 603 MHz, it is greatly attenuated by the SAW filter and does not change performance of the A20-RX. ​The A20-RX has a network of over 40 fixed SAW filters on the A and B antenna inputs. These filter ranges have been carefully selected for broad compatibility with Program Making and Special Events (PMSE) and DTV wireless spectrum worldwide. Brick-wall SAW filters allow for unsurpassed immunity to out-of-band interference.

Local Oscillator (LO) and modulator/demodulator 

In a wireless mic transmitter, digital audio signals must be up-converted to radio frequency (RF) signals with a modulator chip for transmission through the air. This modulator is driven by a local oscillator (LO). Both the modulator and LO need to be designed with extreme care in order to operate reliably from 169 MHz – 1525 MHz. Sound Devices uses the latest in low-current, low-phase-noise LO and field-programmable gate array (FPGA) design to satisfy these requirements, which were simply not available just a few years ago. 

Likewise, a wireless microphone receiver needs to down convert the received RF signal back into digital audio data. This down conversion is done with the very latest high-quality flexible demodulator and LO. The A20-RX uses a proven dual downconverter superheterodyne topology with extensive SAW filtering to ensure the best possible selectivity (see picture below). This takes more parts than other topologies, but ultimately results in excellent nearby signal rejection, yielding improved operating distance. 

Low Noise and High IP3 LNA 

Much like a mic preamp in an audio system, a low-noise RF amplifier (LNA) is the first active device in an RF signal path (see item 2 in the picture above) and can define the quality of the entire system following it. For high performance, a quiet LNA is necessary. The Noise Figure (NF) specification quantifies how quiet the LNA is. A low NF will allow for very long-range operation, much like a good, quiet mic preamp allows for recording of very low acoustic levels. However, a quiet LNA is only half of the equation. The overload of an LNA is specified by its 3^rd-order-intercept point (IP3). Again, back to the mic preamp analogy, the IP3 of an LNA is like the headroom of a mic preamp. If the IP3 of the LNA is too low, strong interfering signals, whether from a nearby 2-way radio or other strong transmitter near the receiver can overload the LNA, generating distortion, and greatly reducing the operating distance of the wireless microphone system. The Sound Devices A20-RX uses the very latest in E-pHEMT transistors, utilizing an LNA which has a NF of just 0.3 dB while simultaneously offering +39 dBm of output IP3: super quiet, with loads of headroom. 

Conclusion 

Whether you work locally or travel worldwide, the Astral System gives you a wider choice of operating frequencies. As the wireless spectrum becomes increasingly crowded, having extra flexibility in tuning range is imperative. The A20-RX is the only digital receiver on the market that can tune not only in common UHF TV bands around 470 – 608 MHz, but also far above that.