GBT Spectrometer Modes
Sampling rates and Efficiencies
The processing of the sampled data requires a limit on the data rates. The first device in this process is the analog to digital converter (ADC). Sampling speed (maximum bandwidth available) and the spacing of the quantization levels (quantization noise and conversion efficiency) are two parameters which are compromised to reach appropriate data rates. The bandwidth requirement is more straight forward and can be fairly easily specified given the science case use of the instrument. The number of bits and level spacing effects the noise level and thus efficiency where integration times are increased by the square of noise power increase. The ability to accurately produce a spectrum with interference present, or sloped band shapes is also a consideration. The Hagen and Farley, Radio Science 8, pp775-784 analyze the increase in integration time as a function of level spacing and oversampling parameters. This paper Thompson,Emerson and Schwab
extend the analysis to odd and even multilevel sampling. A table of results list in the paper:
| # of Levels
|| Level Spacing [σ = 1,ε]
| 2 (1 bit)
| 3 (2 bit) Odd
| 4 (2 bit)
| 8 (3 bit)
| 9 (3 bit) Odd
| 16 (4 bit)
| 32 (5 bit)
| 64 (6 bit)
| 128 (7 bit)
| 256 (8 bit)
The number of levels is even if not specified, ε is in units of σ( the standard deviation of the input voltage), and
is the relative increase in integration time.
Spectrometer Resolution Requirements
With the required velocity resolution:
= 1 km/s
using the lowest frequency band:
= 19 GHz,
gives a channel spacing of
= 63 KHz.
The desired bandwidth expressed by the user community is:
The resolution determines the maximum data rate for each channel with complex Nyquist sampling thus
, which can be reduced to a manageable rate by adding spectrum determined by the desired integration time.
be the minimum number of co-added spectra, then
times 2 polarizations of data streams at a rate of
must be managed.
Since ADC technology is such that FFT algorithms shall be implemented for spectrum generation. Currently 1.8 GHz ADC's are available with 8 bit sampling with 5 GHz technology on the horizon, ie http://www.radiometer-physics.de/rpg/html/Products_Spectrometers_FFTS.html