**NuMI BPM Measurements and Signal Processing Characteristics**

Peter Prieto

6/23/04

This note explains the interface to the BPM system, the available
front-end electronics set-up modes, and the beam conditions where it is
used. It also describes the Echotek setup parameters.

## Data Acquisition Specification

Every measurement taken by the BPM front-end system is based on an
**Acquisition Specification**. The user must supply sufficient
information for a program to generate a complete and unambiguous
acquisition specification. (Bracketed words in italics refer to
nomenclature used in the document "Event Driven Data Acquisition for
NuMI BPM Front-end" by Duane Voy.)

An Acquisition Specification is loaded into the NuMI front-end by ACNet
and treated as if are constants remaining active. The specified
measurements are performed every time the specified arm/trigger
conditions are met. The following elements are contained in the
specification.

1.  Set the acquisition on. \[**Enable**\].

2.  Identifies whether the specification refers to the Periodic
    Background measurement process or to a user or TCLK triggered
    measurement \[**Measurement**\].

3.  Identifies whether the measurement is to be of a proton. Identifies
    the beam structure to be measured in order to select the appropriate
    MDAT frame for timing reference \[**Beam Type**\]

4.  Identifies the setup parameters the digital down-convert hardware
    needs for signal processing mode \[**Beam Measurement**\]

5.  Identifies the TCLK event the measurement will be armed. \[**Arm
    Event**\].

6.  Identifies the BEAM SYNC event used as a trigger to collect data
    \[**Trigger Event**\].

7.  Introduce a delay before data is acquired in quantity turns
    \[**Pretrigger Enable**\].

8.  Introduce a delay in terms of turns before the data is acquired
    \[**Trigger Delay** (turns)\].

9.  Every BPM gets delayed by a known amount of buckets \[**Global
    Delay**\].

10. Identifies an "intensity threshold" below which data will be flagged
    as suspect. \[**Intensity Threshold**\].

11. Set the time between the arm and trigger event the front-end will
    wait if this time is not met the measurement is aborted
    \[**Timeout**\].

## Data Readout Specification

In normal operation, data from any completed data acquisition is
available for readout until over-written. There are separate data
readout devices for Flash, and Turn-by-Turn data, each with an
associated Readout Specification. The user must supply sufficient
information for a program to generate a complete and unambiguous readout
specification.

A Readout Specification:

1.  Identifies the desired data buffer (one of sixteen, each associated
    with an Acquisition Specification) \[**EventIndex**\]

2.  Identifies the beam structure for which data is desired
    \[**DataType**\]

3.  Identifies the starting turn number and the turn count for
    turn-by-turn and closed orbit data \[**beginTurn** and
    **numTurns**\]

4.  Identifies BPM location for which turn-by-turn data is to be readout
    \[**Channel**\]

## Signal Processing Mode

The NuMI BPM system allows selection of one signal-processing mode and
it is specified in the Acquisition Specification \[**Beam
Measurement**\] section.

# 6 Batch 53 MHz -- '6Batch53MHzBuffered'

This mode measures the magnitude of each of the 53 MHz batches
integrated over an interval corresponding to about 1.6 microseconds (84
bunches), the full extent of the single batch beam structure.

It is used to measure the "average" position of each of the six batches
during a single pass. Data s collected is triggered by the BSYNC \$74
event. This event occurs 200 microseconds before the kicker fires. The
1250 points of buffered data is searched starting from the newest data
toward the earliest data in the buffer. MI BPM data is searched for the
six batches corresponding to the last turn and all batch positions
previous to the last turn are disregarded. Signals measured in the beam
line will be processed in the same manner, the only difference is the
found batches are not followed by earlier beam since it is a single pass
system even though 537 microseconds worth of data is collected.

# 6 Batch 53 MHz -- '6Batch53MHzTimed'

This mode measures the magnitude of each of the 53MHz batches integrated
over 1.6 microseconds (84 bunches). The only difference with the
6Batch53MHzBuffered mode is the acquisition specification for this
measurement includes an eighteen-turn delay before the data is
collected. This is done to make a precise time-referenced measurement
with respect to the BSYNC event, fixing the time between the BSYNC and
each measured batches. This mode collects only 32 points or 13.7
microseconds worth of data.

# Signal Processing Hardware and Signal Processing Characteristics

The NuMI BPM system is based on a digital down-converter system (Echotek
814 board). The board consists of 8 separate channels, each of which has
an 80 MHz 14 bit digitizer, 80 MHz digital down-converter chip, and a
128 Kword FIFO. The digital down-converter is the heart of the system
and it has a numerical mixer and three filters, two of which are FIR
filters with 21 and 64 taps respectively and a five tap Cascaded
Integrated Comb filter (CIC).

The digitized batch signal v(t)

$$v(t) = \begin{matrix}
\frac{\text{nq}}{\sqrt{2\pi}\sigma_{t}} & e^{- \frac{1}{2}\frac{t^{2}}{\sigma^{2_{t}}}} & \frac{l}{2c} & R
\end{matrix}$$

gets down converted by the numerical mixer, then filtered and decimated
in the CIC filter. Then the CFIR and PFIR perform more filtering and
decimation before the data is stored in the FIFO as in-phase (I) and
quadrature (Q) pair.

This pair is transferred through the VME backplane from the DDC-814
board to the front-end processor where square root of the sum of the
squares

$A = \sqrt{\begin{matrix}
(I)^{2} & + & (Q)^{2}
\end{matrix}}$ ; $B = \sqrt{\begin{matrix}
(I)^{2} & + & (Q)^{2}
\end{matrix}}$

is performed on the signal from each of the BPM plates then the peak
search algorithm seeks for each of the batches. Once found the raw data
is stored in local memory.

The batch position is computed for each BPM from the I and Q data pair
for plate A and plate B only when the user requests the batch position.

$$

Position is $P = \frac{\begin{matrix}
A & - & B
\end{matrix}}{\begin{matrix}
A & + & B
\end{matrix}}\begin{matrix}
 & \text{ScaleFactor}
\end{matrix}$, The batch intensity $I = A + B$

The scale factor for MI for horizontal BPM = 36.55 mm/V, vertical BPM =
41.77 mm/V. The scale factor for an 8 GeV BPM = 75 mm/V

# DDC Signal Processing Parameter Setup

The greychip parameters are read from a file and loaded through the
front-end processor.

The files contain the necessary parameters to setup the down convert
frequency, the filter coefficients and gains. The table below lists two
different files used in NuMI BPM system with some of the most important
settings.

  ----------------------- ----------------------- -----------------------
  PARAMETERS              6Batch53MhzEnsemble     6BatchNarrowBand

  Burst                   128                     512

  Dataskip                5                       3

  CIC decimation          7007                    7007

  CIC Scale               7B                      7B

  CFIR                    11                      01

  PFIR                    03                      01

  Fine Gain               0500                    0400

  Clock                   74.3447320              74.3447320

  Fdownconv               21.2414820              21.2414820

  CFIR taps               11 tap Gaussian filter  2 tap Boxcar filter

  CFilter Sum             55968                   64000

  PFIR taps               14 tap boxcar           15 tap boxcar

  Pfilter Sum             56000                   60000

  \# Channels used        4                       2

  CIC decimation          4                       4

  CFIR decimation         2                       2

  PFIR decimation         1                       2

  Final Shift             17                      16

  Sec/tap                 107.6 nanosec.          107.6 nanosec.

  Sec/point               107.6 nanosec.          215.2 nanosec.

  PFIR length             1.5064 microsec         1.614 microsec.
  ----------------------- ----------------------- -----------------------

# DDC Clock Synchronization

The Echotek digital down converter board requires a clock signal for the
A/D's and DDC's. A PMC clock generator card (ECSG-1R3ADC) mounted on the
front-end processor (MVME 3401) generates any frequency from 10 MHz up
to 100 MHz and the generated frequency is available on the card's front
panel.

The clock signal can be an asynchronous signal or it can be a locked
signal referenced to an external signal through a phase lock loop (PLL).
The main injector clock (53.10435 MHz) is used as an external reference
signal to the PLL, reducing or eliminating the arbitrary phase
difference between the clock phase and sampled signal phase when locking
the frequencies of these two signals. The PLL is programmed to produce a
clock signal that is 7/5 of the reference signal (74.34609MHz) and is
used as the sampling clock by the DDC board.

The PLL setup parameters in the startup script specify the reference
clock frequency and the PLL counter values. Ecsg (53103380, 743, 8, 60,
92, 20, 3, 3).