Summer Undergraduate Laboratory Internship
(SULI)
Richard Houanche
SLAC
Joe Frisch
8/30/12
1
Abstract
Phase Lock Loop circuit board is made to amplify and match a 476 MHz frequency input
from Stanford Synchrotron Radiation Lightsource (SSRL). The SSRL sends 10ps X-Ray pulses at
a spin valve sample, so that pictures can be made of the precession of spin waves within it. The
PLL board directs the spin waves to oscillate at the correct frequency and phase needed to aim
them in front of the X-rays. Knowledge of how spin waves behave at the nanoscale is a
fundamental one for understanding how to build efficient spintronic devices.
Introduction
To learn about spin waves an electrical circuit that can keep synchronization to a signal
between five to ten GHz needs to be engineered. Spin waves are a collective excitation of atoms
that are believed to propagate in a wave function. Magnetic spin waves in thin film materials will
give us the potential to make advancements in flash memory. Phase Lock Loop (PLL) is
electronic system that has a modifiable frequency oscillator, which can independently stay
synchronized to a signal.
The PLL is constructed of many components, amplifiers, mixers, frequency multipliers,
frequency synthesizer, and the main component is a PID (proportional–integral–derivative)
Controller. Our PLL will be able to take the frequency from Stanford Synchrotron Radiation
Lightsource (SSRL), which is 476 MHz and multiply by an integer number n (varying from 12 to
20), than shifting it by 476 MHz / m, where m is any integer from 2 to 17. The PID is the
locking system that attaches the frequency synthesizer to the wave and permits the tracking of
2
the moving wave. All of the other parts in the system permit the ability for the PLL to have the
correct parameters set for in which we want the frequency, timing, and power to be passing
throughout the system.
Spin waves that we are looking at are within the ferromagnetic material in spin valve
technology. The PLL will direct the spin waves to be oscillating at the correct frequency and
phase needed. As we excite the spin waves we can track the amplitude in spatial and time
domain. And then using pictures taken by the X-rays of the SSRL at 10ps picoseconds time
resolution, we can make a video of how these spin waves move because we control the period in which
they move. Knowledge of how spin waves move throughout the thin film layers of the spin
valve is the first step to eventually being able to control the spin waves, efficiently.
Materials
Spectrum Analyzer
Amplifiers
Signal Generator/ Frequency synthesizer
Multipliers
Power Supply
Divider
Power meter
Filters
Digital Phosphor Oscilloscope
Splitters
PID (proportional–integral–derivative)
controller
Mixers
Attenuators
Comb Generator
3
Background
Stanford Synchrotron Radiation Lightsource (SSRL) is a machine that gives off X-rays
that are produced by electrons circulating in a storage ring at close to the speed of light. These
extremely bright X-rays can be used to investigate materials at the nanoscale. With the SSRL and
other technology we will contribute to the enhancement of a thin film structure, called spin
valve. Spin valves interpret minimal adjustments in applied magnetic field to a great change in
resistance, which is very useful in storing memory with hard drives today. The spin valves are
made up of two ferromagnetic layers, with a non magnetic layer in between. When magnetic
fields are applied and in the ferromagnetic layers there is a precession of electrons as they
interact with each other and that propagation is called spin waves. By taking soft X-rays done at
the SSRL of spin valves we will be able to see the spin waves precession within a ferromagnetic
material, such as iron.
To enhance spin valve technology there has to be a better understanding of how spin
waves behave. In the thin film structure, we are analyzing a 5 nm thick layer of nickel-iron, 250
nm of silicon nitride, and on a centrally etched potassium hydroxide substrate. At beam line 131, we use a STXM (Scanning Transmission X-ray Microscopy) with the SSRL this allows us to
take images of the spin waves at about 70 picoseconds time resolution, or even down to 10 ps
with a special operation mode of SSRL. Our resolution timing is quick but unfortunately the
separation between pulses take about 2.1 nanoseconds. Since the process of taking pictures is
relatively slow we need to know the precession these spin waves in phase. From using the
frequency given by the synchrotron’s x-rays we are able to build a machine that will apply the
magnetic field needed to have the sample in the precise place. And with the correct magnetic
4
field being put onto the spin waves the goal of being able to create a video of their travelling
precession is possible.
The spin waves are mainly excited by the magnetic field, which allows us to direct the
motion in a way that will let us see the precession. As we excite the spin waves we can track the
amplitude in spatial and time domain. Knowing how spin waves move through this system is the
first step to eventually control the spin waves and work on the spin valve technology. For
example, spintronic devices exploit both the spin and charge of its electrons, which allows them
to be smaller and more energy efficient than conventional electronic circuits.
Methods
Being able to examine the way spin valves rotate will not be possible without the PLL
circuit. The PLL will have an output frequency ranging from about 5700-9600 GHz. It is
necessary for frequency levels to be very high because that is the range in which we have
predicted the spin waves oscillate at due to many factors, mainly the magnetic field. To be able
to increase the frequency in the system we need a series of frequency multipliers and amplifiers.
The frequency multipliers increase the signals by factor of 2, 3, or 5 in our circuit and are
arranged in a specific manner. In between those multipliers are amplifiers that can handle the
correct frequencies along with bringing the power up to the correct power level that the next
frequency multiplier can handle. Throughout this multiplier chain we also added attenuators an
electronic device that reduces the amplitude of an electronic signal, when needed. We also will
needed to have the multiplier chains set up with labels should anyone need to change the configurations, it
will be fairly simple to adjust.
5
Table 1. Multiplier chain loses and gains in power
Config1
2x
attenuator
2x
attenuator
3x
last
17.2
14.2
17.2
14.2
13.7
17.8
Power before connecting to the Multipliers in (dBM)
Config 2
Config 3
Config 4
5x
17 2x
17.2 2x
3x
9.4 attenuator
14.2 attenuator
last
14.9 2x
17.2 3x
attenuator
14.2 attenuator
2x
13.6 3x
2x
11.9 last
last
17.2
Config 5
5x
2x
2x
last
17.2
14.2
17.2
14.2
10.5
16
17
9.2
9.6
15.7
(last) is the output power of the last amplifier in the chain, the amplifiers are assumed
Diagram 1. Multiplier Chain
Diagram to the left represents the multiplier chain.
The all of the amplifiers are in blue squares and are
labeled with an “A”. The multipliers are in the
circles and are labeled 1-8. The adjustable cables
are the arrows, if the arrows come from the same
amplifier or splitter than they are the same cable.
Table 2. Multiplier Chain Speed Sheet
Cables
A
B
Cables stay connected to the Amps
C
D
Synthesizer Freq
5791.333
7222.333
7695.333
8647.333
9599.333
Multipliers
1 to
E
Result Freq (MHz)
5712 A
7143 A
7616 A
8568 A
9520 A
2
1
2
2
1
B
C
B
B
C
4 C
D
D
5 E
4 D
3 C
D
7 E
6
D
6
8
8 E
5 E
8 E
The table above is the directions needed to change to different configurations of the system.
6
The prospected power that is going to be given from the synchrotron to our system is
about zero dBm. That initial power is amplified to about 21 which will go into a splitter, one
signal will go to our multiplier chain and, the other will go into our divider. The divider will
divide the signal by six and the multiplier chain will increase it to a signal of five to ten GHz and
then combine with a mixer after. In doing this process we will then have a high multiple of 476
MHz only of set by on sixth 476 MHz, this is a very important part of the process because it
allows for the phase to be slightly offset with the spin waves. If signals of the spin waves and
the synthesizer were precisely in phase than the experiment would have the same picture being
made constantly, instead different pictures being made at different points in the phase.
After the frequency is set to our intended values then it enters the loop of the PID
controller and the frequency synthesizer. The frequency initially hits the PID controller which
takes that signal and locks it to the signal of the synthesizer which we have set to be the
configuration’s predicted output. When everything is set up correctly the PID controller
proportional and integral need only be adjusted and the phase lock loop should be operating.
The phase lock loop signal will then go to the RF output which will be driving the microwave
field.
Results
An accurate measurement of jitter is necessary for ensuring the reliability of the PLL
system. Jitter is the variation of a signal with respect to its ideal position in time. It causes an
increase in system noise, uncertainty in the actual phase of the sample, and inter-symbol
interference. The lower the jitter the more accurate PLL circuit works.
Equations use to calculate Jitter
7
𝑉(𝑡) = 𝐴 sin(ωt) or
𝑑𝑉
𝑑𝑡
𝑉(𝑡) = 𝐴 sin(𝜙)
= 𝐴𝜔 cos(𝜔𝑡)
(1)
(2)
𝜔 = 2𝜋f
(3)
𝜙 = 2𝜋𝑓𝑡
(4)
𝑡=
𝜙
(5)
2𝜋𝑓
sin⁡(𝜔1 𝑡) 𝑠𝑖𝑛(𝑤2 𝑡 + 𝜙) = sin 𝜔(𝑡 ± Δ𝑡)
𝑉(𝑡) = 𝐴𝑠𝑖𝑛(𝜔𝑡 + 𝜙)
Δ𝑡 =
(6)
(7)
Δ𝑉
(8)
𝜋𝑓𝐴
Table 3. List of the PID parameters needed for locking the PLL to the synchrotron
Configuration
Freq (Hz)
Gain
P(gain)
I=1/T
Voltage (V)
Amplitude (A)
Jitter (s)
PID controller parameters
223
53
2222
1
2
3
5.79E+09 7.22E+09 7.70E+09
5
5
5
0.1
0.1
0.1
20
20
50
3.30E-03 4.30E-03 1.50E-03
0.245
0.7
0.36
7.40E-13 2.71E-13 1.72E-13
233
4
8.65E+09
5
0.1
5
3.03E-03
0.17
6.56E-13
522
5
9.60E+09
5
0.1
20
1.30E-03
0.18
2.39E-13
Figure 1. Representation of Jitter, Ideal and Exaggerated
Jitter is the timing variation from start to finish. If this was a before and after the distance in
between the red dashed lines in the out of phase image would represent the jitter.
8
Discussion
740 fs is the biggest amount jitter in the five configurations; this is remarkable because
we were expecting it to be in the picoseconds range. The smallest amount of jitter is 172 fs
which came from the third configuration; this may be due to the fact that this multiplier chain
had only X2 multipliers. As shown by Table 3 the configurations that started with a X5
multiplier also had considerably low jitter time frames which may be caused two of the X3
multipliers may not be used at their appropriate power levels.
A useful attachment that was not integrated into the circuit board was the comb
generator. A comb generator is a signal generator that produces multiple harmonics of its input
signal, ranging from times 2-25. The entire multiplier chain could be replaced with that single
part, with some draw backs. The multiplier chain is less accurate but more convenient pertaining
to design and switching frequencies. The main reason that it was not put into the circuit was
waiting to receive it in the mail took long than building the entire circuit board.
The difference in drift between comb generator and the multiplier chain which is about 4
ps over a 14 hour measurement. This is a great because it verifies that the PLL circuit board will
not have a drift that effects our measurements. The measurements have a can only be taken at the
speed of the pulses given by the synchrotron, which is about 10 ps. This proves that using the
comb generator or the multiplier chain our PLL will control the lock so that it stays within a
domain where the phase is continuous trough out the 10 hour long experiments.
9
Summary
The Phase Lock Loop circuit board works better than expected. It was used for the spin
valve experiment once during the program, and was working fine but unfortunately there was
something else wrong with the experiment. The samples that were being used in the experiment
were all bad, which did not allow for any pictures to be taken of the spin waves. Knowledge of
how spin waves behave at the nano-scale is a fundamental one for understanding how magnetism
works at the nano-scale, and hence it allows for the possibility of building efficient spintronic
devices.
Future work- To create a control loop feedback mechanism so that the scientist running the
experiment would not have to borrow the labs PID controller. It would also enhance my
understanding of control theory, and how it is works. Also it would be great to continue with the
spin valve experiment, interpreting the pictures of the spin waves.
10
http://people.ccmr.cornell.edu/~buhrman/theses/FAlbert.pdf
THE FABRICATION AND MEASUREMENT OF CURRENT PERPENDICULAR TO THE PLANE
MAGNETIC NANOSTRUCTURES FOR THE STUDY OF THE SPIN TRANSFER EFFECT
A Dissertation Presented to the Faculty of the Graduate School
of Cornell University in Partial Fulfillment of the Requirements for the Degree of
Doctor of Philosophy
by
Frank Joseph Albert
January 2003
http://marcuslab.harvard.edu/theses/DOUWESthesis.pdf
This thesis was carried out at the Information Storage Technology Group, MESA Research
Institute, University of Twente, P.O. Box 217, 7500AE Enschede
Title: The Spin-valve Transistor
Author: Douwe Johannes Monsma
11