ElectroScience Lab

November 2002

ElectroScience Laboratory, The Ohio State University1320 Kinnear Road, Columbus OH 43221 USA

{ hampson.8, ellingson.1 }@osu.edu

Modular Digital Backendsfor Microwave Radiometry

Grant HampsonSteve Ellingson

ElectroScience Lab

Overview

l OSU/ESL has a NASA contract to develop a wideband coherentdigitizing radiometer with built-in RFI mitigation for earth remotesensing at L-band

l We have chosen a development strategy which emphasizes modularimplementation of functions using FPGAs on interconnecting PCBs

l Advantages:– System can be reconfigured by rearranging modules– Modules are reuseable (analogous to “reuseable software”)– Facilitiates a “test-as-you-go” approach (risk mitigation)– System-level testing possible in early stages of design– Less waste (replace only parts of system that don’t work, or need

updating)l These designs / design methods maybe useful in the development of

future radio astronomy backendsl Design details from this effort are freely available

ElectroScience Lab

System Block Diagram

Data Recording/Control

Low-noisefront end

AnalogDownconverter

Digital IF(DIF)

ADC

Asynchronous Pulse Blanker (APB)

1K FFTFrequency domain

blanker

Integration(SDP)

200 MSPS10 bits

80 MHz BW@ 150 MHz

50 MHz BW@ +25 MHz100 MSPS16+16 bits

FPGA

FPGA FPGA

FPGA

(In Progress)

CaptureBoard

FIFO

(In Progress)50 MHz BW@ -25 MHz100 MSPS16+16 bits

ΣΣ

Low Rate Data to PC

PC

ElectroScience Lab

RFI-Robust Total-Power Radiometer Backend

l Functional blocks on separate boards for rapid development/evaluation

l Each block controlled by a “Rabbit” µC via TCP/IP

ADC DIF APB FFT CaptureSDP

ElectroScience Lab

Digital IF (DIF) Module

ElectroScience Lab

Asynchronous Pulse Blanker (APB)

l Maintains a running estimate of mean & variance of incoming time series

l Sample mag. > β standard deviations above the mean triggers blanker

l Blanking operates on down-stream data exiting a FIFO

l Signals before and after trigger are blanked

ElectroScience Lab

FFT ModuleFFT’s implementedusing Altera FFT libraries,includes windowing

Many tradeoffs possible

Currently:(1) 1 Complex FFT (1 FPGA)1K points, 14 bits, 19% duty cycle(2) 2 Complex FFTs (1 FPGA)1K points, 10 bits, 33% duty cycle

Soon:8 Complex FFTs (2 FPGAs)1K points, 14 bits, 100% duty cycle(shown)

ElectroScience Lab

Field Demo: Time Blanking of ATC Radar

l Observations using a low gain discone antenna on ESL roof

– 1306 +/- 50 MHz using DIF (1 sideband) + APB + FFT + SDP

– Spectra integrated over 42 ms (4096 FFTs)

ATC radar at1331 MHz clearlyvisible

APB off

APB on

ElectroScience Lab

Field Demo: Time Blanking of ATC Radar

l Time domain results:

l Effect of varying APB threshold in frequency domain:

Direct path Multipath

APB “Blanking”decision

“Max held” spectra

Averaged spectra

ElectroScience Lab

Why Low-Gain Receivers with Digital Filters are Nice:

l No calibration, temperature control, or Dicke switching of any kindl Variance vs. integration time follows theoretical curve through

6 orders of magnitude (3 orders of magnitude sensitivity improvement)l Actual elapsed time for this observation was 14 minutes

(100 seconds of integrated data.)

ElectroScience Lab

For More Information

l http://esl.eng.ohio-state.edu/~swe/iip/docserv.html

(E-mail hampson.8@osu.edu for a username & password)