Aerospace, Military and Defence


High IF sampling puts wideband software-defined radio within reach


By Benjamin Annino, applications director, Analog Devices


Figure 1. Dual-mixer frequency conversion used in low IF digital receivers.


Figure 2. Old spectral scanning with narrow-band superheterodyne tuning. M


ultiband radar and electronic warfare (EW) applications put a high value on wideband, high dynamic range, agile spectral


monitoring. Increasingly higher sample rate data converters are allowing architecture changes to the radio front end that shrink size, weight, power, and cost (SWaP-C), maintain performance, and evolve toward software programmable common hardware. We’ll explain the technology advancements enabling this age of wideband software- defined radio expected to transform EW and multiband radar architectures.


The discussion follows a series of frequency planning figures that show the progression of improved wideband spectral scanning methods enabled by advancing data converter technology. The example carried through is a 500 MHz to 18+ GHz EW digital receiver. The


16 March 2023


annotated figures show for a given approach why the frequency planning is necessary and what allows successive improvements to SWaP-C and flexibility while maintaining dynamic range. In the progression of improving schemes, you’ll see the receiver RF image gets easier to address, which allows software- defined flexibility. The need for tunable preselection to kill multitone IMD2 doesn’t change with the approach and will remain a critical need into the future even as direct sampling grows ever wider.


Spectral sensing in days of yore Not too long ago, industry-leading digital receivers employed digital data converters like AD9467 and covered up to a few hundred MHz instantaneous bandwidth (iBW) at a high dynamic range. They sampled at much less than 1 GSPS, and the bandwidth centered around DC (zero IF, also known as


Components in Electronics


ZIF) or centered around an IF offset (RF direct sampling). ZIF requires IQ modulators and demodulators as well as quadrature error correction (QEC) to achieve image rejection. Radar and EW applications often require wide iBW and high image rejection. It is difficult to implement QEC that achieves acceptable image rejection as iBW exceeds several hundred MHz, a modest iBW requirement by today’s EW and radar standards. This is why high performance, bandwidth-hungry multiband radar and EW prefer the latter RF direct sampling of wide iBW in the first and second Nyquist zones. To cover spectrum outside the Nyquist zones, an RF tuner uses a swept local oscillator (LO) mixer to frequency translate a sliding block of iBW into the fixed IF that matches up with the data converter direct sample zone. Figure 1 is a block diagram of a typical dual- frequency translation low IF receiver feeding a low sample rate data converter. These


receivers are capable of a high dynamic range. Figure 2 is the frequency plan employed using the low IF scheme in Figure 1. Just like the digital data converter, the RF tuner requires high RF image rejection to avoid signal ambiguation, spurs, and noise. The single-RF mixer tuner method (red x) does not meet image rejection requirements because the IF frequency is too low to allow enough spacing between the desired band (green) and image band (red). The inadequate separation makes the required RF input filter impossible (or impractical—that is, too large and/or expensive). Thus, a dual-mixer two-stage frequency translation is employed, often called the superheterodyne receiver. The input RF is frequency translated to an intermediate high IF that is several GHz higher than the final direct sample IF. The high IF is RF filtered and frequency translated again to the final IF where it is direct sampled. This method allows


www.cieonline.co.uk


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62