New Beam Position Monitor System Design for the APS Injector\*

R. Lill, O. Singh, N. Arnold

Advanced Photon Source, Argonne National Laboratory

9700 South Cass Avenue, Argonne, Illinois 60439 USA

**Abstract.** Demands on the APS injector have evolved over the last few
years to the point that an upgrade to the existing beam position monitor
(BPM) electronics is required. The injector is presently being used as a
source for both the low-energy undulator test line (LEUTL) project and
the top-up mode of operation. These new requirements and the fact that
many new rf receiver components are available at reasonable cost make
this upgrade very desirable at this time. The receiver topology selected
is a logarithmic processor, which is designed around the Analog Devices
AD8313 log amplifier demodulation chip. This receiver will become the
universal replacement for all injector applications measuring positions
signals from 352 to 2856 MHz with minimum changes in hardware and
without the use of a downconverter. The receiver design features
integrated front-end gain and built-in self test. The data acquisition
being considered at this time is a 100-MHz, 12-bit transient recorder
digitizer. The latest experimental and commissioning data and results
will be presented.

# Introduction

The Advanced Photon Source injector has taken on a more demanding role
over the last few years. The new demands involve operating in top-up
mode 1. In this mode of operation the injector refills the storage ring
every two minutes in order to maintain a current of 102 1.0 mA. In order
to inject efficiently into the targeted storage ring bucket, the
injector beam position becomes critical. The new beam position monitor
(BPM) will provide improved dynamic range, resolution, and reduction of
other systematic errors to ensure reliable injector operation.

The low-energy undulator test line (LEUTL) project 2 has also created a
new set of demands on the injector. The Linac and several transport
lines are shared by both the injection process and LEUTL, requiring new
and improved diagnostic capability. These new requirements and the fact
that many new rf receiver components are available at reasonable cost
make this upgrade very desirable at this time.

\*Work supported by U.S. Department of Energy, Office of Basic Energy
Sciences under Contract No. W-31-109-ENG-38.

# System Design

The system design philosophy was to simplify the topology so the same
basic design can be used as the universal replacement for all injector
position monitoring applications. The other general design constraint
was to make it as reliable and maintainable as possible. The design
choice was a log-ratio system with the subtraction function being done
in software. The basic expression for the log-ratio beam position 3,4
is:

$\begin{array}{r}
\text{Np} = \lbrack(\text{log}(A) - \text{log}(B)\rbrack = \text{log}(\frac{A}{B}) = \text{tanh}^{- 1}\left\lbrack \frac{(A - B)}{(A + B)} \right\rbrack \\
2\frac{A - B}{A + B},
\end{array}$

where Np is the normalized position and A and B is the induced voltages
on the stripline pick-up. This technique adds some flexibility to the
system design. The algorithm to calculate the beam position can be
changed or adjusted in the future as new techniques develop.

The BPM system was partitioned into subsystems as shown in Figure 1. The
stripline detector, bandpass filters, and front-end board make up the
major differences between applications. This partitioning allows the use
of most of the components for operation at either 352 MHz or 2856 MHz.
The stripline sensitivity and system calibration factor can be
calculated by the following equations 3,4:

$S \approx \frac{\text{80}}{\text{ln}(\text{10})} \times \frac{1}{b},$

where S = stripline sensitivity (dB/mm) and b = half aperture (mm), and

$X(\text{mm}) \approx \frac{1}{S \times G_{\text{system}}} \times V_{\text{out}},$

where X(mm) = normalized position (mm), $G_{\text{system}}$ = system
gain (which is set to 36mV/dB), and $V_{\text{out}}$ = log amplifier
module output (mV).

Table 1 describes the location and number of BPMs that will be upgraded.
These applications are transport lines between the linac and the storage
ring. This table also describes the type of detector, sensitivity, and
system calibration factor.

![](media/image1.wmf)

**Figure 1.** System block diagram.

**Table 1.** **System Applications.**

+-----------+---------+----------+----------+----------+------------+
| **L       | *       | **Fre    | **Half   | **S      | **         |
| ocation** | *Number | quency** | Ap       | tripline | Normalized |
|           | of      |          | erture** | Sen      | Position   |
|           | BPMs**  | *        |          | sitivity | (m/mV)**   |
|           |         | *(MHz)** | **(mm)** | (        |            |
|           |         |          |          | dB/mm)** |            |
+-----------+---------+----------+----------+----------+------------+
| Linac     | 15      | 2856     | 17       | 2.0      | 13.6$V_{\t |
|           |         |          |          |          | ext{out}}$ |
+-----------+---------+----------+----------+----------+------------+
| LEUTL     | 20      | 2856     | 17 and   | 2.0      | 13.6$V_{\t |
|           |         |          | button   |          | ext{out}}$ |
|           |         |          | type     |          |            |
+-----------+---------+----------+----------+----------+------------+
| Linac to  | 4       | 2856     | 17       | 2.0      | 13.6$V_{\t |
| PAR       |         |          |          |          | ext{out}}$ |
+-----------+---------+----------+----------+----------+------------+
| PAR to    | 5       | 352      | 25       | 1.4      | 20.0$V_{\t |
| Booster   |         |          |          |          | ext{out}}$ |
+-----------+---------+----------+----------+----------+------------+
| Booster   | 8       | 352      | 25       | 1.4      | 20.0$V_{\t |
| to        |         |          |          |          | ext{out}}$ |
| Storage   |         |          |          |          |            |
| Ring      |         |          |          |          |            |
+-----------+---------+----------+----------+----------+------------+

## 

## RECEIVER DESIGN

The receiver is partitioned as shown in Figure 1. There is a front-end
board, four log amplifier boards, and a control and regulation board for
each receiver. The boards are housed in an EMI-shielded aluminum case.
The receivers are installed in a 19-inch-wide, 4-U height card crate
where up to eight receivers can be installed.

### Front-End and Self Test Board

The rf front-end board provides the gain and self-test capabilities for
the system. The four signals from the input bandpass filters are sampled
via 15-dB directional couplers, which are printed on the circuit board.
This provides the ability to trouble-shoot the system without
disconnecting any cables. The directional couplers also serve as feeds
for the self-test oscillators. In the self-test mode the coupler is
switched from a 50- termination to a voltage-controlled oscillator. The
oscillator drives a two-way equal power divider that is also printed on
the circuit board and provides an equal input to one plane of the BPM.
This same layout is duplicated for each plane. The board employs a
selectable gain stage to shift the operating range by 20 dB. Presently
we are using a low-noise Stanford Microdevices SGA-3586 cascadeable gain
block with a noise figure of 2.5 dB. There are other amplifiers in this
series with different gain and noise parameters that could be
implemented in the future. This amplification will shift the input
operating range while maintaining the same system gain
($G_{\text{system}}$). This feature will be used in some applications
and has the effect of extending the dynamic range.

The front-end and self-test rf board was designed using microstrip
transmission lines and components laid out on Rogers RO3006 microwave
board materials. The impedance was calculated using Eq. (4) and was
optimized in the lab 5. The dielectric constant of the ceramic PTFE
composite material is 6.15 and the loss tangent is 0.0025 @ 10 GHz. One
of the design goals was keeping the rf board construction process as
simple as possible by avoiding bonding substrates. This equates to a
two-layer board with a thickness of 0.025 inch to insure the trace width
of 0.036 inch for 50- lines. The boards are designed to be drop-in
replacements for 352- or 2856-MHz applications.

$Z \approx \frac{\text{377}}{2\pi\left( \frac{\left( \varepsilon_{r} + 1 \right)}{2} \right)^{\frac{1}{2}}} \times \left\lbrack \text{ln}\left( \frac{8h}{W} \right) + \frac{1}{8}\left( \frac{W}{2h} \right)^{2} - \frac{1}{2}\frac{\varepsilon_{r} - 1}{\varepsilon_{r} + 1}\left( \text{ln}\frac{\pi}{2} + \frac{1}{\varepsilon_{r}}\text{ln}\frac{4}{\pi} \right) \right\rbrack$,

where Z=microstrip impedance, $\varepsilon_{r}$=substrate dielectric
constant, h=substrate thickness, and W=microstrip width.

### Logarithmic Amplifier and Video Driver Board

The block diagram in Figure 2 shows one of the four logarithmic
amplifier video driver boards. The input broadband matching network
enables us to use the same hardware for both the 352- and 2856-MHz
applications. The AD8313 is one of the main motivations for upgrading
the existing 2856-MHz BPMs that require downconverters. The AD8313 uses
a cascade of eight amplifier/limiter cells, each having a gain of 8 dB
typically and a --3 dB bandwidth of 3.5 GHz, for a total gain of 64 dB.

The ADC video driver is the AD8002 dual current feedback amplifier that
provides offset and gain adjustment to the log amplifier. In order to
evaluate an off-the-self ADC board, the output was designed to drive a
0-2 volt range with a 1-M load impedance. The board is a two-layer
microstrip design with a thickness of 0.025 inch to insure the trace
width of 0.036 inch for 50- lines.

The log linearity plot shown in Figure 3 illustrates that each gain
range has a dynamic rage of about 60 dB with an overlap of 40 dB. The
combination of both ranges enables operation over an 80-dB dynamic range
with a maximum of 0.6 dB errors in log linearity for 352-MHz
applications. The S-band application is expected to be similar with some
roll-off at the band edges. The overall pulse rise time for 10 to 90% of
the input is 45 ns without the lowpass filter implemented for the
overall system.

![](media/image2.wmf)

**Figure 2.** One channel log amplifier block diagram.

### Control and Regulator Board

The control and regulator board supplies and distributes regulated power
to the five boards in the receiver. It also provides the common point
for the control interface. The board employs a hybrid coaxial connector
that allows a blind mate connection to the main frame.

![](media/image3.wmf)

**Figure 3.** 352-MHz receiver logarithmic error.

## Striplines and Bandpass Filters

The 2856-MHz BPMs employ shorted S-band /4 stripline pickup electrodes,
which were recently installed 6, and have a 34-mm-diameter vacuum
chamber aperture. The bandpass filters being considered for this
application are three-pole /4 combline filters. The 3-dB bandwidth is 3
MHz with an in-band insertion loss of 6.5 dB.

The 352-MHz BPM applications also utilize a shorted /4 stripline
detector with a vacuum chamber aperture size of 50 mm. They differ from
the S-band design not only in blade length but also in width (1.4 rads).
The system input bandwidth is limited to a 3-dB bandwidth of 3.0 MHz by
Surface Acoustic Wave (SAW) bandpass filters. The SAW filters have
matching networks on the input and output with an in-band insertion loss
of 7.5 dB. The filter sets are matched to 0.2 dB across the 3-MHz band
centered at 350 MHz. The SAW filter is a very economical (\$7/filter)
solution since it is a standard RFM product.

## DATA ACQUISITION

We are presently evaluating several options for data acquisition at this
time. One option is a 12-bit, 100-MHz transient recorder. The VME module
contains eight channels, each having 256K of memory. Most of the data
quoted in this paper was recorded with the 12-bit, 100-MHz digitizer.
The other option is a 14-bit, 50-MHz digitizer. The general approach for
using the transient digitizer is to reduce the error or standard
deviation by $\frac{1}{\sqrt{N}}$, where N is the number of samples
collected during the video pulse. In the 352-MHz application we can take
as many as 30 samples or 300 ns from the start of the event.

## PERFORMANCE

We have built and are currently testing prototypes for a 352-MHz
receiver and a 2856-MHz receiver at this time. The plot shown in Figure
4 is the output of the 352-MHz system operating with the 12-bit, 100-MHz
digitizer. The input to the BPM system is a simulated beam pulse with
the data acquisition set to acquire 30 samples from a single pulse. The
top trace is an overlay of all four-blade signals as a function of input
power. The second trace shows the position intensity dependence with
error bars representing the noise. The third trace shows the
peak-to-peak noise of each of the four blades as a function of input
power.

The 352-MHz system was also tested in the lab with a 50-mm stripline
chamber installed in a wire test set-up. The wire test accuracy was 19 m
rms over a 5mm range and 8 m rms over a 1 mm range. The lab testing
indicates that the overall electronic system drift stability is \<15 m
over a 48-hour period.

The 352-MHz system testing is currently moving from the lab to the
accelerator for further evaluation. The single-shot resolution for this
system installed in booster-to- storage ring (BTS) transport line is 15
m over a 30-dB dynamic range. Further testing will continue after the
May shut down.

![](media/image4.png){width="5.795833333333333in"
height="3.9319444444444445in"}

**Figure 4.** 352-MHz performance.

**Table 2.** **Design Specifications.**

+-----------------------+-----------------------+-----------------------+
| ####                  |                       |                       |
+-----------------------+-----------------------+-----------------------+
| **Parameter**         | **PC Gun**            | **RF Gun**            |
+-----------------------+-----------------------+-----------------------+
| Dynamic Range         | 0.1-2 nC (26 dB)      | 0.1-10 nC (40 dB)     |
+-----------------------+-----------------------+-----------------------+
| Single Shot           | 15 m rms              | 100 m rms             |
| Resolution            |                       |                       |
+-----------------------+-----------------------+-----------------------+
| Drift                 | 15 m rms              | 100 m rms             |
+-----------------------+-----------------------+-----------------------+
| Accuracy              | 100 m 5mm             | 100 m 5mm             |
+-----------------------+-----------------------+-----------------------+

## DISCUSSION

The BPM system was designed to meet the parameters given in Table 2. The
352-MHz system satisfies the given requirements; however some
enhancements warrant investigation. We will explore reducing the
single-shot noise by implementing a differentially driven log amplifier
output. This will reduce the common mode broadband noise.

The S-band receiver will be more challenging. However, if the
signal-to-noise ratio remains relatively constant, we should see about a
32% improvement in the resolution due to the fact that the chamber
aperture is smaller.

The plan is to complete testing on the two prototypes by mid-summer,
followed by ramping-up production of the first-article units by late
summer.

# Acknowledgments

The author would like to acknowledge Glenn Decker for many helpful
discussions, Tom Buffington for the mechanical design work, and Robert
Keane and Chuck Gold for providing valuable electrical measurement and
assembly expertise.

# References

1\. Emery, L., "Recent Operational Data on Continuous Top-Up Operation
at the Advanced Photon Source," PAC 2001 Conference Proceedings,
Chicago, pp. 2599-2601 (2001).

2\. Milton, S., "Measurements of Exponential Gain and Saturation of SASE
at the APS LEUTL," PAC 2001 Conference Proceedings, Chicago, pp. 236-240
(2001).

3\. Shafer, R., "Beam Position Monitoring," AIP Conference Proceedings
No. 212, Upton, NY. pp. 26-58 (1989).

4\. Aiello, G. and Mills M., "Log-Ratio Technique For Beam Position
Monitor Systems," AIP Conference Proceedings No. 281, Berkeley, CA, pp.
301-310 (1992).

5\. Gupta, K. C. et. al., *Microstrip Lines and Slotlines*, Artech House
Publishers, Boston, London 1996.

6\. Gorski, A. and Lill, R., "Construction and Measurement Techniques
for the APS LEUTL Project RF Beam Position Monitors," Proceedings of the
1999 Particle Accelerator Conference, New York, pp. 1411-1413 (1999).