1. Characterizing an FPGA Based
Spectrometer Prototype for
Heterodyne Receivers at the LMT
By Tim costa

2.  Introduction
 Spectrometer Overview
 Characterization Tests
 OGP Calibration
 Linearity
 Allan Variance
 Future Work
 Summary
Overview

3. Background

4. SEQUOIA

5. The LMT

6. The Spectrometer

7.  FPGA based
 ROACH2
 Two ADC ports
 Each ADC has four cores with maximum sample rates
of 1.25 Gs/s1
The Spectrometer

8.  Offset/Gain/Phase (OGP) Calibration
 Linearity
 Allan Variance
Characterization Tests

9.  Aligning cores via OGP Calibration decreases the
effect of spurs
 Aligning cores increases signal to noise.
OGP Calibration

10. OGP Calibration

11.  Changes to input signal strength should result in a
corresponding change to output signal strength.
 Linearity is necessary for calibration.
Linearity

12. Linearity

13. Frequency
(MHz)
Noise Level (dB) Quantized Slope
100 -20 Yes 1.02 ± 0.03
200 -23 No 0.96 ± 0.01
400 -26 Yes 0.96 ± 0.07
799 -29 No 0.98 ±0.01
Linearity

14.  Spectrometers should follow the radiometer
equation
 σ α (b*t)-1/2
 Allan variance tests mimic weak signals in the
presence of high noise2
Allan Variance

15. Allan Variance

16.  Complete two more modes.
 400 Mhz
 200 Mhz
 Scale Up
 Integrate with OMAR
 Science at the LMT
Future Work

17.  OGP calibration eliminates spurs and improves S/N
 The spectrometer is linear until amplifiers in the ADCs
begin to saturate
 Allan time is at least 1000 seconds.
Summary

18.  Umass Amherst Department of Astronomy
 Five Colleges Astronomy Department
 Commonwealth Honors College
 Massachusetts Space Grant
I would especially like to thank Dr. Gopal Narayanan, my
thesis advisor, Aleks Popstefanija for his assistance with
Python, and Rurik Primiani of SAO for aspects of the
ASIAA ADC.
Acknowledgments

19.  https://upload.wikimedia.org/wikipedia/en/f/ff/FiveCollegeRadioObs.png
 http://www.astro.umass.edu/~fcrao/instrumentation/sequoia/seqhorns_
fixed_labeled.JPG
 http://astronomynow.com/wp-
content/uploads/2016/03/LMT_678x509.jpg
1.) e2v, “EV8AQ160 Quad ADC Datasheet DS0846”, 2016
2.) Schieder, R. & Kramer, C, “Optimization of Radio Astronomical
Observations Using Allan Variance Measurements”, 2001
3.) Patel, N. A. et al, “Characterizing the Performance of a High-Speed ADC
for the SMA Digital Backend”, 2014
References