Extreme Bandwidth Radio Frequency Spectrum Analyzer

with Direction Finding K. D. Merkel1, W. R. Babbitt1,2, Z. W. Barber2, S.H. Bekker1, C. R. Stiffler1,

R. K. Mohan2, C.H. Harrington2, A. Traxinger1, A. Woidtke1, M. Chase1, and P. B. Sellin1

1S2 Corporation, Bozeman, Montana 59715 2Spectrum Lab, Montana State University, Bozeman, MT 59717-3510

(406) 922-0334 babbitt@s2corporation.com

Abstract: A novel radio frequency sensor technology is presented for continuous monitoring of the radio frequency spectrum with large instantaneous bandwidth that can perform spectral analysis and direction finding simultaneously. The photonic receiver technology relies on a spatial spectral (S2) holographic crystal recording device that is an extreme bandwidth analyzer and correlator (EBAC). The S2 EBAC technology presently operates on instantaneous bandwidths (IBW) over >16 GHz that is analyzed directly anywhere from 0-40 GHz, with 10 kHz-10 MHz resolution bandwidth (RBW) and frame rate (FR) from 2-500 kHz. The IBW, RBW and FR are fully reconfigurable on the fly, with the net result of a digital data output stream of ~100 million frequency measurements per second. Typical SA performance results show >50 dB two-tone spur free dynamic range (SFDR) over the full IBW, and >62 dB SFDR has been measured in recent experimental work. Direction finding (DF) for each of the ~100 million frequencies per second is an advanced capability described and demonstrated here. The combined SA and DF results to date show similar performance to SA in IBW, RBW and FR, with <1 picosecond arrival time-difference-of-arrival measurements at each and every resolvable frequency. Status and improvements to the EBAC system in both performance and packaging are discussed.

1. INTRODUCTION Continued advancements in electronics are leading to a proliferation of radar, electronic warfare, and communication systems operating over wider and wider regions and bandwidths of the electromagnetic spectrum (EMS). A significant challenge for present and future military and commercial systems is to capture and analyze the spectrum of signals across a broad bandwidth in real time without any prior knowledge of the signals, carrier frequency, or modulation format. Intelligence, surveillance, and reconnaissance (ISR) functions, software defined radio systems, and electronic support (ES) measurement systems require the capability to continuously monitor all critical portions of the spectrum and to rapidly classify and locate emitter platforms. Systems that can handle extreme bandwidths are needed as transmissions are constantly increasing in both their quantity, band in use, and their spectral extent.

Radar, tracking, and communication system currently operate over the full spectrum from 1 MHz to over 100 GHz. Spectral monitoring in the form of power analysis per frequency, as well as identification of emitters requires a system that is capable of simultaneously handling signals throughout the 100 GHz spectrum with resolution and performance commensurate with the typical spectral structure of radar and communication signals. The system must have a high spur free dynamic range (SFDR), so that the small signals of interest (SOI) are not mistaken for the false signals (spurs) that are generated by large signals. The system must have fast update rates and sufficient bandwidth for tracking and response, where typically 10s to 100s of microsecond latency is required.

Figure 1 Overview of the S2 EBAC solutions to many problems of the present receiver systems

In the communication realm, adaptive radio systems, which operate in unused spectra within the full communication band, require spectral monitors to find and track holes in the microwave spectrum. Fast update rates are required to enable uninterrupted communication, as well as to switch carriers on the fly.

In addition to monitoring spectral energy and signal identification through fine spectral resolution, it is desired to have a system that can locate the direction of the emitter. Such an inteferometric direction finding (DF) system would use two or more antennas to determine the angle of the incoming radiation.

Figure 2 System diagram of the S2 EBAC approach and applications

A DF system must have high SNR in order to determine the phase difference (delay) of the multiple signals impinging on the antenna array from different directions and be able to handle these signals simultaneously.

The EBAC is an RF receiver offering a means to sort through, monitor, and track RF signals of interest (SOIs) over a wide bandwidth, in real time, with a full digital data output representing the result of analog signal pre-processing. The EBAC provides actionable data with a low latency that can be further analyzed digitally, without a human-in-the-loop. The term spatial spectral holography (S2) describes the underlying technology of using a cryogenically cooled crystal as the core analog pre-processor to perform physical Fourier transforms and spectrum analysis on modulated laser light representing the RF SOIs. EBAC attributes include:

--The ability to operate on SOIs over extreme bandwidth with superior performance in terms of spur free dynamic range as compared to current state-of-the-art wideband digitizers.

--The ability to monitor broad bandwidths over 16 GHz today and ability to be greater than 100 GHz in near term) continuously with 100% probability of intercept. Unlike scanning systems or system that cannot operate continuously, the EBAC captures

and stores all RF activity across all bands simultaneously

-- Good narrow resolution bandwidth variable from 10 kHz to 10 MHz

-- Updates rates variable from 2-500 kHz.

--The ability to remote the RF pick-up/antenna with a fiber optical cable and only a simple, small RF-to-light phase-only modulator at the remote location, reducing electromagnetic interference (EMI) and benefitting applications where size, weight and power (SWaP) is constrained at the antenna itself. This eliminates the need for high speed digitization at the antenna. Also, the EBAC remote link does not require high-bandwidth photo-detectors nor high bandwidth analog-to-digital-converter (ADC), as do other photonic links.

--Better overall system SWaP and cost over comparable digital system architectures

--The ability to conveniently scale up to processing many (~10) antennas in one hardware solution, where the multiple fiber optical feeds would come to a single cryocooler and large benefits are observed of using this single piece of hardware and other single photonic systems to save on size, weight, power and cost of the system.

--A simple adaptation for RF inteferometric direction finding. By coupling the outputs of a two (or more) antenna, the EBAC yields the direction (with <1 degree accuracy/resolution) of every resolvable RF emission. This enables the ability for continuous, simultaneous monitoring and geo-location of a sea of emitters with diverse bandwidths, formats, and repetition rates.

2. GENERAL APPROACH AND

OVERVIEW OF S2 MATERIALS An electronic surveillance system monitoring a complex environment of transient radio frequency and microwave signals should be able to accurately map and simultaneously locate sources of radiation [1]. Spectral energy measurements in frequency domain can be used to make these measurements. This spectral energy measurement technology represents a unique approach to the problem with a technique that can perform instantaneous capture and processing of multiple signals with complex modulation format over extreme bandwidths, with a fast response and low latency. Several analog signal processing capabilities based on S2 have been demonstrated, including spectrum analysis [2], direction finding [3], range Doppler processing [4], analog-to-digital conversion [5], and true-time delay generation [6].

A typical S2 block diagram hardware system for performing SA is shown in Figure 3.

Figure 3 Hardware overview of general S2 device of single antenna SA operation

The RF signals of interest (SOIs) are collected by one or more wideband antennas, and the voltages are applied to an electro-optical waveguide phase-only modulator. Alternatively amplitude modulators can be used. In doing so, the RF signals are modulated as sidebands onto a laser carrier, ideally low phase noise and frequency stable, that is resonant or nearly resonant with the absorbing optical transition of the crystal. The modulated laser light -- representing the RF SOIs -- then illuminates and is absorbed by the cryogenically cooled crystal. The approach at its core relies on rare-earth-ion-doped crystal capable of recording broadband optical power spectra with fine spectral resolution. To achieve high frequency resolution, the small crystalline sample (~1 cm3) is maintained at 4K, which is typically done in a commercial rugged cryo-refrigerator that uses no liquid helium, has a long lifetime, and runs on conventional wall power of ~1 kW.

RF Frequency

(a)

(b)

S2 Crystal

Laser Freq. (~300 THz)

(c)EO Modulator

CW Laser

(b) Optical Spectrum

(c) Absorption Band of S2 Crystal

(a) RF Spectrum

RF spectrum is up‐converted to optical band and 

recorded in S2 material

Fiber Optic Link

Fiber Optic LinkSpectral up‐conversion 

with passive off‐the‐shelf electro‐optical modulators.

1MHz 100 GHz

Optical Frequency

Figure 4 Recording of RF information in S2 crystal

The basis for the operation of the S2H receiver is performing power spectrum analysis in the optical domain via frequency selective absorption. S2 materials typically consist of narrow absorption linewidth rare-earth ions that are doped into a crystalline host lattice. Figure 5 shows an overview of the material, where the crystal growth processes cause local host inhomogeneities which in turn cause center frequency shifts of the individual absorption lines. This results in a S2 crystal material with a wideband absorption response and fine spectral resolution at a temperature of 4K. Our typical material has 16 GHz of bandwidth with resolution bandwidth of 10 kHz at 4K. A different material has 230 GHz bandwidth with resolution of 50 kHz. The effect of a narrow band laser on the S2 material is to burn a hole in its absorption spectrum, which is called spectral hole burning. The depth of the spectral hole in the absorption depends on the integrated power of the laser at the narrow frequency. In this same way, the effect of a broadband signal being absorbed by the S2 material is to burn the signal’s power spectrum into in the S2 material. S2 materials have large

instantaneous bandwidths, fine spectral resolution, and high dynamic range.

Figure 5 S2 material overviewS2

Once recorded, the absorption spectrum is typically stored in the S2 material for up to 1 msec, which is sufficient time for it to be readout with a frequency-swept laser. This process is shown in Figure 6.

Digital Spectral

Recovery

Post-Processing

time

Readout

( )t Recording

freqChirped readout process maps high BW power

spectrum to a low BW temporal oscillationOutput voltage has BW that is dependent on the spectral

feature size and the scan rateTypical scan is 20 GHz in 0.5 ms (Rate=40 MHz/s)

~250 times faster than conventional limit for 0.4 MHz spectral features

Digitized Power Spectrum

Voltage on photodetector is a temporal map of spectrum, but distorted

Wideband“fast" optical chirped pulse

Input Signal modulated on optical carrier

Modified Absorption is RF Signal’s Power Spectrum

time

freq

RF signal of interest (SOI)

1

2

3

CW Laser

Chirp Laser

time

Figure 6 Readout of information from S2 EBAC

For SA measurements, the energy per frequency is detected by the atoms in the crystal over a time interval. The recorded information represents a physical spectral transform. Before the information decays away, a frequency scanned readout laser is then used to measure the absorbed energy per frequency.

For DF measurements, the same basic principles apply, but now radio frequency signals received at two (or more) wideband antennas are interfered in a Mach Zehnder Interferometer (MZI), and this mixed signal is mapped in spectrum, enabling a phase sensitive measurement of the time difference(s) of arrival of the signals. Thus, for SA, energy measurements per frequency are made, while for DF the similar set of measurements are made and additionally an angle of arrival is assigned to every unique frequency measurement.

3. COMPARISON OF THE S2 SA APPROACH

TO DIGITAL APPROACHES The attributes of S2 for SA are:

(1) Typical electronic solutions to spectral monitoring require bulky electronics at the antenna to handle broad bands simultaneously or lossy RF links to connect the antenna to a base receiver. With EBAC, the bulky electronics at an antenna and lossy links are replaced with a simple, small RF-to-light phase-only modulator, with a fiber optical cable used to transport the modulated light to the receiver hardware, reducing electromagnetic interference (EMI) and benefitting applications where size, weight and power (SWaP) is constrained at the antenna itself.

(2) In the S2 photonic link, there is no high-bandwidth photodetector nor high bandwidth analog-to-digital-converter (ADC) in the system. Rather the S2 crystal absorbs and spectrally processes the optically modulated SOIs directly.

(3) The S2 system operates on SOIs over extreme bandwidth with superior performance compared to using a wideband ADC. The S2 supplants the required digital computations required for calculating power spectra with analog processing of the ions in the S2 crystal.

(4) Benefits in overall system SWaP and cost over digital that scales upward with processing many (~10) antennas in parallel.

As an example, consider a 50 Gs/s sampling rate ADC, which has a two tone SFDR of ~35 dB for a 20 GHz bandwidth capture bandwidth. The S2 EBAC device shows greater than 50 dB SFDR over the full IBW. Further, S2 has recently been demonstrated at >60 dB over 5 GHz. Similar progress in ADC performance at high sample rates is not anticipated based on historical progress [7]. The all-digital solution for real time continuous spectrum analysis requires substantial computations on the 50 GBytes/s of digital to produce a desired ~100,000,000 frequencies per second continuously, in real time, on the order of ~4.5 x 1012 floating point operations per second (4.5 TeraFLOPs). With S2, the same computational load is accomplished ~0.01 cm3 of S2 crystal material in a single “channel”. The computational load scales linearly for digital systems, while for S2 multiple spots in one crystal can be used, spreading the power load of 1 kW of the cryocooler over multiple processing functions.

4. SA EXPERIMENTAL RESULTS SA typical experimental laboratory results demonstrate the measurement of 50,000 frequencies per scan over a 20 GHz bandwidth at 2 kHz update rate. In Figure 7 this kind of spectrum is __________________________________________

shown with high performance and rapidly changing spectral content. The full 20 GHz spectrum, recorded every 0.5 msec, is displayed in color coded amplitude in the spectrogram over a 3 second time period, while the RF inputs are varied. Thus the image shown in Figure 7 has 50,000 pixels across the x-axis and 3,000 pixels across the y-axis.

S2Rx

E8257D Signal Generator

+

E8257D Signal Generator

Tektronix AWG710B

LOIF

RF

Random hopping narrowband tone

Random hopping narrowband tone

Random BW & Modulation Frequency Hopping waveform

20 GHzSpectrum:

100% Capture No missed

signals

Figure 7 Experimental demonstration of continuous spectrum analysis over 20 GHz bandwidth on a dynamic RF spectrum.

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For SFDR measurements, the input signal value of two frequency tones are varied together and any spurs including the third order intermodulation distortion is tracked and plotted. The overview of this process is shown in Figure 8, where in the S2 EBAC the primary tone compresses due to limited absorption in the crystal, but the intermodulation is not caused by this compression, but rather from the electro-optical phase modulator itself.

Figure 8 Overview of the SFDR measurement and S2 EBAC response

Figure 9 shows the result of 50 dB SFDR in the mature, rugged S2 EBAC hardware system that was measured over 10 GHz of bandwidth. Figure 10 shows the result of a 62 dB SFDR measurement in a

laboratory demonstrator. The projected performance is SFDR>60 dB over 40 GHz bandwidth.

-55 -50 -45 -40 -35 -30 -25 -20 -15 -10 -5 0 5 10

X: -10Y: 35.43

SFDR=50 dB

SNR=36 dB

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X: -10Y: 35.43

RF Signal In (dBm)-60

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Figure 9 SFDR measurement in an S2 EBAC system.

Figure 10 SFDR measurement in an S2 EBAC laboratory

demonstrator.

5. DIRECTION FINDING THEORY Consider an optical carrier E() that is split and modulated by two electro-optical modulators (EOM) that are driven with identical and time delayed RF signals of interest received by a pair of antenna elements. (R) represents these signals normalized to the half wave voltage of the EOM. The two paths are recoupled to form a interferometer system. Using the theory in reference [3] it can be shown that each output port of the interferometer contains the relative phase and power spectral information of the RF signals. For a single input tone at R, the ratio of the difference to the sum of the sideband powers normalizes the phase spectrum and enables the extraction of the desired time delay independent of the RF power at R, provided the contrast and phase of the MZI are known. The terms in Ref. [3] yield the ratio

1 1

1 2

1 1

2 sin( ) sin( )

1 2 cos( ) cos( )

R MZ

R MZ

P PS

P P

For an ideal interferometer (i.e. =1) that is configured to operate at quadrature the ratio can be evaluated to be 1

sin RS . In

general, multiple frequencies are impinging of the two antennas with potentially independent delays, ( ) , at

each frequency . The ratio of the difference to the sum of sidebands is thus 1

( ) sin ( )S , which

can be used to estimate the time delay ( ) between

the two antenna outputs for each resolvable RF frequency component independently. Thus, multiple non-spectrally overlapping RF signals from multiple directions can be located simultaneously. This interferometric technique is different from conventional range-Doppler processing methods [4] where the power spectral density of the spectral grating is used to recover the time delay, which typically requires that the spectrum be broad enough so that several spectral periods are recorded, and the time resolution and precision depend on the bandwidth of the signal.

Figure 11 shows an illustrative case of two antennas receiving RF SOIs over a wide bandwidth at multiple delays, and how the signals feed into an S2 based receiver for direction finding. In this case, signals arrive on two time delays, illustrated as the spectrum at the first delay (1 spectrum) and the spectrum at the second delay (2 spectrum). Each antenna receives both signals at different times. The signal functions performed by the S2 are on optical signals, so the key step is that the RF received signals at the antenna are converted to voltages by the antenna

and subsequent low noise amplifier, as applicable, and fed directly into an EOM, one per antenna.

Figure 11 S2 operation for DF using an interferometer. (1) RF wavefronts, any bandwidth and any RF carrier, with multiple

delays, received by wideband antenna pair. (2) Antenna voltages each modulated each onto optical carrier with electro-optical phase modulator, each in on arm of the optical interferometer, which is quadrature biased. (3) Both output of the interferometer are each processed in a unique S2 crystal volume. The spectrum of each

port is used in calculation of delay (angle) at each resolvable frequency.

Each EOM is driven optically by a coherent laser source that is resonant with the S2T crystal, the core optical signal processing engine. For a pair of antennas, the two corresponding EOMs are in an optical MZI configuration, with two ports being accessed as the outputs. For an MZI biased at quadrature, this means that for no modulation, the optical carrier power is split 50-50% between the ports, as shown for the ports. The ports are labeled at Port A and Port B, and when maintained at quadrature bias produce anti-symmetrical results around the optical carrier.

Figure 12 Operations of the S2 spectrum analyzer acting on the

optical MZI outputs Port A and Port B as processed and read out from the S2 device. Subsequent operations done digitally after

readout are shown, leading to delay estimates per frequency.

Figure 12 shows the operations that occur for each optical spectrum at the two delays. The graph x-axis is labeled in frequency [GHz] around the optical carrier, so that 0 GHz is actually the optical carrier of the laser beam, which is typically around 378 THz for the S2T crystals that are used. The RF spectra for 1 and for 2 are treated independently here. Each has been modulated onto an optical carrier via the EOPM, and the example shows each of the spectra being symmetrical around the optical carrier. For each potential delay, there is a “delay modulation function” that applies for all frequencies, regardless of the frequency specific energy content of the received signals, when the signal is recorded in by the S2T. This is the dashed line for each port. The delay modulation function is multiplied by each spectrum for each port. This delay modulation function is specific to the port of the MZI, and is asymmetrical around the laser carrier between Port A and Port B when biased at quadrature. The spectrum of the received signals, after the delay modulation function has been applied for each delay, is then in this figure summed across

all delays, for each of the two ports. This leads to the outputs labeled Port A and Port B. These graphs represent both the mathematical constructs for this example, but also are the physical output signals that are realized from having the two ports of the MZI each irradiate a small spatial volume in an S2T crystal, which records the information, and subsequently reading out that information.

Figure 13 shows the operation of processing and readout, and the relationship between the upper and lower sidebands (USB and LSB, respectively) from Port A and Port B. The ~20 GHz width is the current state of the art for the current S2 device configuration that exhibits the combination of this bandwidth with high sensitivity, good spectral resolution and a rapid update rates simultaneously. Because of this case, the approach of using both ports and process only the USB from each output port of the interferometer is preferred. This is shown as case 2 in Figure 13.

Figure 13 Different configurations for processing and readout, Case 1 is single port, Case 2 is dual port.

6. DF EXPERIMENTAL RESULTS Figure 14 shows the results of a two delay experiment and the display of 4-8 GHz being of the power spectral density. This is a single frame of data that shows producing a direction versus frequency plot for every update of the system. Figure 15 shows spectrograms streaming down monitoring the incoming power versus frequency as a function of time and a DF history streaming, showing the tracking of the two RF bands of interest. The narrowband signal is fixed in delay and the wideband signal is swept in delay. The relative movement of the two signals is shown to cross in the DF history

display. This display has 10,000 frequencies per scan over a 4 GHz bandwidth with a 2 kHz update rate.

Figure 14 Single frame of a spectrum and direction finding display over 4-8 GHz on two signals, one narrowband at 4.5 GHz

(stationary) and one wideband between 5-6 GHz (moving).

Figure 15 Direction finding display over 4-8 GHz on two signals, one stationary and one moving, showing the time history of the

emitters.

Figure 16 shows a measurement of programmed time delays as introduced by a mechanical phase shifter over a 1.3 cm stretch. The data shown here are obtained from single event measurements. The figure illustrates the time delay estimation for spectral content at 5.6 GHz, where an RMS error of the delay was measured to be ~ 1 ps over a total demonstrated unambiguous delay span of ~90 ps.

7. SUMMARY A new type of electromagnetic spectral monitoring

technology has been developed that can both track broad bandwidths up to 100 GHz instantaneous bandwidth in near future with narrow frequency resolution and with high update rates and with high dynamic range > 50dB SFDR. The system has also been adapted to show that direction finding can be implemented in addition to spectral monitoring, thereby determining the direction of incoming radiation for every resolvable frequency component measured.

Figure 16 Single shot time delay estimations with accuracy better than 1 ps and 90 ps delay range on a 5.6 GHz carrier.

8. ACKNOWLEDGEMENTS This work was supported by the Office of Naval

Research on grant N00014-07-1-1224.

9. REFERENCES [1] Electronic Warfare and radar engineering

systems handbook, Naval Air Systems Command (1999)

[2] R. K. Mohan, T. Chang, M. Tian, S. Bekker, A. Olson, C. Ostrander, A. Khallaayoun, C. Dollinger, W. R. Babbitt, Z. Cole, R.R. Reibel, K. D. Merkel, Y. Sun, R. Cone, F. Schlottau, K. H. Wagner, “Ultra-wideband spectral analysis using S2 technology, J. Lum., 127, 116 (2007)

[3] Z.W. Barber, C. Harrington, C.W. Thiel, W.R. Babbitt and R. Krishna Mohan, “Angle of arrival estimation using spectral interferometry, Journal of Luminescence, Volume 130, Issue 9, September 2010, Pages 1614-1618

[4] T. L. Harris, K. D. Merkel, R. K. Mohan, T. Chang, Z. Cole, A. Olson, and W. R. Babbitt, "Multigigahertz range-Doppler correlative signal processing in optical memory crystals," Applied Optics, 45(2), 343-352, 2006

[5] R. R Reibel, C. Harrington, J. Dahl, C. Ostrander P. Roos, T. Berg, R. Krishna Mohan,

M. Neifeld, W.R. Babbitt, “Demonstrations of analog-to-digital conversion using a frequency domain stretched processor”, Optics Express, Vol. 17 Issue 14, pp.11281-11286 (2009)

[6] R. Reibel, Z.W. Barber, J. Fischer, M. Tian, W.R. Babbitt, “Broadband Demonstrations of True-Time Delay Using Linear Sideband

Chirped Programming of Optical Coherent Transients”, J. of Lumin. 107, 103-113 (2004)

[7] R. H. Walden, “Analog-to-digital conversion in the early 21st century,” presented at the International Microwave Symposium, Honolulu, Hawaii, 3-8 June 2007 and references therein.