The Technology Behind Precision Receivers
HDRR™ Technology
Analog to digital converters have increased in speed and bit depth over the years to satisfy commercial and defense requirements for SFDR, SNR, Bandwidth, and ENOB. Quantization errors, timing errors, and nonlinearities still prevent ADCs from satisfying some system application requirements. Several types of compensation have been developed to address errors among other things dithering, calibration, and commutating ADCs at lower rates with modest improvements to spurious-free dynamic range (SFDR).
Our group is introducing a novel spur removal technique that has demonstrated consistent improvement of SFDR by 9dB and more in our lab. This process is ADC-type, clock-frequency, and CW frequency agnostic, requires no calibration or learning time period, and minimizes the amount of post-processing or errant signal analysis in the end-user application. Further, the processing required for spur removal may be executed in either a low latency mode for EW or in a real-time mode for ISR, radar, or communication.
The approach, called HDRR, employs non-uniform sampling and an advanced clocking and sampling approach that mitigates the spurs and the resulting intermodulation distortion. It also preserves the original phase and amplitude of the signal as measured at the antenna. Non-uniform sampling allows a receiver system to determine the signal’s Nyquist zone location and equips the receiver to extract additional information from it.
Nyquist Tuner Technology
PRI technology has the additional capability to tune in any Nyquist zones presented to the input or to simultaneously tune in all Nyquist zones at the same time. The technology can reduce analog antialiasing filter requirements that colorize data.
Direct sampling close to the input antenna sensor serves to eliminate amplitude nonlinearities, group delays, NF, and phase distortion resulting in more accurate data. With the SFDR improvements, our technology provides, especially in the higher Nyquist zones, designers can utilize direct sampling in more applications and at higher frequencies.
Precision Receivers inc. has studied a novel sampling technique which results in a form of non-uniform sampling. In non-uniform sampling, a data set is collected which is comprised of data points sampled at non-constant sample periods. Collecting samples with a non-uniform clock allows for more things to be known about the signal such as which Nyquist zone the signal is in. Nyquist theory assumes you only know one thing about a signal. The knowing of more than one thing about a signal allows HDRR™ technology to know which Nyquist zone a signal is in. With HDRR™ tuning in the different Nyquist zones is as simple as tweaking algorithm coefficients. Figure 1 is a simplified block diagram utilizing HDRR™ technology.
Figure 1: Direct sampled receiver block diagram utilizing HDRR™ showing no anti-aliasing filter. Tuning of the Nyquist zones would be accomplished digitally.
One of the main benefits of the architecture is the removal of the mixing stage which removes all of the mixing and non-linearities associated with it. The other benefit to the system architect is the relieving of specifications on the anti-aliasing filter. The relaxation of the antialiasing filter specification to the point of deeming it unnecessary in sparse signal environments allows for the removal of another analog component in the system. This anti-aliasing filter plagued the system with group delay distortions near the edges of the filter or filters in the case of preselectors. Filter distortions complicated narrow band systems from being stitched together reliably to create a wideband staring system.
Modulated Signal with additional CW
Modulated 1030 MHz FM @ -2dBFS Nyquist Zone 1 Low End Signal of Interest
and 340 MHz @ 10 dB below 2nd Harmonic of Modulated 1030 MHz
Wideband Staring Technology
Utilizing HDRR™ technology a wide band staring receiver can be built which, in sparse signal environments can take a wide band snapshot of the entire spectrum with high dynamic range. In high signal environments a preselector can be switched in to narrow the BW thereby improving the spurious performance of the system. Figure 2 is the block diagram of a staring receiver.
Figure 2: Direct sampled receiver block diagram utilizing HDRR™ showing no anti-aliasing filter. Tuning of the Nyquist zones would be accomplished digitally.
Figure 3 shows lab results for both the staring receiver and Nyquist tuning. The setup consists of a 2.4 Gsps ADC with a 4.666 GHz CW signal presented to the input. 1 M samples
Figure 3: Lab results for a 2.4 Gsps ADC with a 4.666 GHz signal, 1 M samples 0 – 5 GHz
Two tone, IIP3 Test
IIP3
F1 = 1030 MHz @ -9 dBfs
F2 = 1025 MHz @ -9dBfs
Nyquist Zone 1 High End
Anti-Jam/Overload Protection
Another unique feature of HDRR technology Is its unique ability to soften the compression of an ADC. Another way to look at this feature is the full range of the ADC can be used because the ADC can be loaded to 0 dBfs. Typical high-speed ADC’s achieve their best spurious performance at -9dBfs which throws away 1.5 bits of the ADC’s performance. By being able to load the ADC higher, the lost 1.5 bits of performance can be preserved instead of throwing the bits away for the sake of headroom. Jamming scenarios sometimes try to overload the ADC as well as other elements of the system. PRI technology will help mitigate some of these overloaded conditions. Figure 4 and 5 shows the before and after plots, respectively of an ADC in an overloaded condition. For a full set of data click the link “Anti-Jam/Overload Protection results”.
Typical ADC in Overload
Figure 4 Typical Lab results for a 2.4 Gsps ADC with a 413 MHz signal, 96 K samples, +0 dBfs Overload
PRI ADC in Overload
Figure 5 PRI Lab results for a 2.4 Gsps ADC with a 413 MHz signal, 96 K samples, +0 dBfs Overload