Two Applications of Direct Digital Down Converters in Beam Diagnostics Page: 4 of 7
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can be programmed to a value as high as 16,384 which would provide an output
sample rate of 3.17 kHz. The minimum latency is a characteristic of the type of FIR
filter and its ability to be bypassed. The CLC5902 was implemented with a 21-tap
symmetric FIR filter followed by 63-tap symmetric FIR filter, neither of which may be
bypassed. This, coupled with the minimum decimation of 32, means that the
minimum latency is 53 s for this device. Similar devices produced by Analog
Devices and Intersil have FIR filters which can be configured as asymmetric filters or
bypassed completely. Thus the National Semiconductor parts have a latency which
must include the FIR filter response time while latency of the Analog Devices and
Intersil DDC's can be reduced to that of the CIC filter. Table 1 includes a summary of
the characteristics of a DDC devices produced by a number of different manufacturers.
TABLE 1. Characteristics of Digital Down Converters.
Mfg./Part Bit Sample Minimum Output Minimum Maximum Bypass
Number s Rate Latency BW Decimation decimation Filters
Intersi4B 14 65 MS/s 60 nS 982 kHz 4 16384 Yes
Nat'l Semi. 14 52 MS/s 53 ps 650 kHz 32 16384 No
CLC5902
Analog Dev. 14 65 MS/s 30 ns 3.6 MHs 2 16384 Yes
AD6620
Greycode 14/ 64 MS/s 1.6 ps 886 kHz 32 64000 No
GC4014 16 600 N
Intersil 16 75 MS/s 1.8 ps 645 kHz 32 131072 No
50016
BEAM POSITION MONITOR EXAMPLE
One of the more common measurement techniques for determining the beam
position is to calculate the ratio of the difference and sum of the voltage induced on a
pair of diagonal electrodes. One must be able to calibrate the system and expect that
the channel-to-channel gain difference will remain constant between calibrations.
Switched electrode electronics systems [1] overcome this differential gain problem by
using the same RF, IF and baseband electronics for the pair of electrode signals. Other
systems have been developed which use a set of matched calibration signals at the
beam frequency that are injected into the front end electronics at times when the beam
signal is not present. This concept has already been implemented for the BPM system
in the Swiss light source [2]. The design proposed below makes use of the frequency
selectivity of the DDCs and the ability of DDS devices to generate precise frequency
sources which are used as a set of calibration signals that are injected, detected, and
processed even when a beam signal is present.
Figure 3 shows a block diagram of the proposed BPM system. The 10 MHz
machine master oscillator signal is multiplied to generate a 50 MHz signal that is
synchronous with the 1497 MHz beam signal. A DDS device is used to generate a
11.539 MHz signal which is the 128" sub-harmonic of the 1477 MHz local oscillator
signal used in by the RF section. The 11.539 MHz signal is also mixed with the
1477 MHz signal in order to generate the 1488.6 MHz and 1465.4 MHz reference
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Powers, Tom; Flood, Roger; Hovater, Curt & Musson, John. Two Applications of Direct Digital Down Converters in Beam Diagnostics, article, May 1, 2000; Newport News, Virginia. (https://digital.library.unt.edu/ark:/67531/metadc722557/m1/4/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.