7/30/2019 Frequency Synthesizer Final Report
1/20
Frequency Synthesizer for GSM-900
Group Project for ECE802-607
Instructor : Salem, Fathi M
Group member: Green, ElwoodVelez, Dalimar
Yang, ChaoZiel, Stephen Allen
Design summary.(Directory: /egr/course/personal/ece802/finalproject/PLL)
Sub design Library name Status designer
system design PLL Simulation & schematic Done Chao
PFD&CP&LPF PLL Schematic & Simulation Done
Layout Done
Chao
VCO VCO Schematic & simulation done.
on chip inductor done.
Stephen
Divider Divider Schematic & simulation done.
Layout is partially finished.
Dalimar
Elwood
7/30/2019 Frequency Synthesizer Final Report
2/20
INTRODUCTION
1. Target of the projectThe target of this project is to design a Frequency synthesizer used in a GSM-900
system, including the schematic design as well as layout design. AMI 0.5um CMOS
process is the targeted technology to realize this design.2. Frequency SynthesizerFrequency Synthesizer is used to generate variety of frequency or realize
modulation for many applications. It is built with Phase Locked Loop (PLL). Thus, for
most of the case, PLL and Frequency Synthesizer have the same meaning. Typical PLL
structure is shown below:
PFDCharge
pump
LPF2nd order VCO
Counter
Figure.1 Block diagram of PLL system
A feedback loop forces the output to track the input reference phase so
that the output frequency loop is locked. Based on different configurations in the loop.
The PLL can be categorized into following table.
Table.1 PLL categorization
For many wireless communication applications, Digital PLL are adopted, becauseit can provide high frequency output and easy to be designed.
3. GSM-900MHz system
GSM is the most popular cellphone protocols. GMS-900 is a subset of GSMnetwork. GSM-900 uses 890 - 915 MHz to send information from the and 935 - 960 MHzfor the other direction, providing 124 RF channels spaced at 200 kHz. Duplex spacing of
45 MHz is used.
4. design approach.
7/30/2019 Frequency Synthesizer Final Report
3/20
For simplicity, the integer-N PLL is adopted. other targeted
parameters are list in table-1
Reference
Frequency
200KHz
Loop bandwidth 20KHzVCO gain 40MHz/Volts
Prescalor Two modulus 8/9
Charge PumpCurrent
100uA
order of LPF 2nd
order
Frequency
range
890MHz-915MHz (124
Channel)
7/30/2019 Frequency Synthesizer Final Report
4/20
Design Report
s y s t em s i m u l a t i o n The PLL was modeled and simulated to test its stability and locking time.
Figure.2 shows the behavioral model of the PLL.The parameters in the model is described in the following table
Icp 100uA
C1 14p
C2 211p
R2 155K
K0 40MHz/Volts
Divider radio 4500
The simulation result was shown below:
(a) (b)Figure.2 models of PLL, (a) close loop, (b) open loop
Figure. xxxx (a) freqnency response of open loop structure(b) frequency response of close loop structure
(c) step response of close loop structure.
7/30/2019 Frequency Synthesizer Final Report
5/20
Simulation result shows that
Bandwidth: 20KHz
Looking time:100usPhase Margin: 60
PFDThe PFD was realized in a digital way. additional delay was inserted in the
feedback .The circuit is shown in figure.3
figure.3 PFD schematic
figure.4 simulation result of the pfd.
figure. 5 layout of the pfd.
7/30/2019 Frequency Synthesizer Final Report
6/20
C h a rg e P um p & LP F.
The chcare pump use current mirror to generate sourcing current & sink current of
100uA. For the LPF, the samll capacitor of 15pF was put onchip. The larger one (211pF)was palced offchip. A external pin was claimed for it. Figure.5 shows the schematic of
Charge Pump and LPF.
Figure.5 schematic of charge pump and LPF.
Figure. 6 layout of schematic.
7/30/2019 Frequency Synthesizer Final Report
7/20
Vo l t a g e C on t r o l l e d O s c i l l a t o r
Theory
A typical CMOS voltage controlled oscillator can be seen in figure one below. It consistsof two NMOS and two PMOS transistors hooked up in a negative resistance formation,
which can be modeled as shown in figure two. If both the PMOS and NMOS have thesame gm, then figure three shows the final equivalent circuit, which can be modeled as a
simple RLC tank. All three of these models are using the small signal equivalent model
for the MOSFET analysis.
C1 and C2 really represent MOSFET varactors, where the drain, source (and sometimes,
but not always) the bulk nodes are tied together. In this configuration, the bulk node of
an NMOS would act as the VCTRL pin (to prevent reverse biasing of the internal diodes),and the gate node would act as the output connection. An analysis of this circuit can
show that when the drain and source are tied together, ID is shorted out, and the transistor
is in cutoff. This can be shown visually by using the high frequency model [1].
Figure 1: Typical
VCO Schematic
Figure 2:
Equivalent
circuit of fig. 1
Figure 2: Final
RLC Equivalent
circuit
Figure 4: High Frequency model
for the MOSFET transistor
7/30/2019 Frequency Synthesizer Final Report
8/20
Using this high frequency model, one can derive the expected equivalent capacitance as
follows:
1.) Drain and source tied together; bulk connected to VCONTROL.
++
+= GSGDBSBD
GBTOTAL
CCCCCC 11//
( )( )
+++++
GSGDBSBD
BSBDGSGDGB
CCCC
CCCCC //
EquivalentGB CC //
Therefore, in this configuration it can be seen that CGB is the biggest contributer to theoverall capacitance because CEquivalent comprises of capacitive dividers due to the rest of
the parasitic capacitances of the MOSFET.
2.) Drain, source and bulk all tied together, which are then connected to VCONTROL.
GDGSGB
GDGSGBTOTAL
CCCCCCC
++=
1////
The other capacitances do not show up in this equation because they are all shorted out.
For this project, the first configuration was used, however the above derivations show
that both configurations will work as a variable capacitor.
Resul ts
For this project, we were required to meet the specifications of a GSM receiver. A VCOoperating under these conditions must be able to acquire a tuning frequency from the
range of 890 MHz to 915 MHz. The results of our VCO are shown on the next page.
The extra circuitry to the right of the VCO (N19 and V2) were merely to cause oscillation
in the circuit by offsetting the circuit during initial conditions, thanks to the advice ofProfessor Salem.
Table 1: VCO Transistor Sizes
7/30/2019 Frequency Synthesizer Final Report
9/20
Figure 6: FFT Analysis on VCO Circuit
Figure 5: Final VCO Circuit
7/30/2019 Frequency Synthesizer Final Report
10/20
The inductor was realized using the ASITIC software. A screenshot, along with its
various characteristics, can be seen below.
Figure 7: Plotted results comparing Vctrl to the frequency of the VCO
Figure 8: Layout of on-chip Inductor
Table 2: ASITIC Inductor Parameters
7/30/2019 Frequency Synthesizer Final Report
11/20
F re q u e n c y D iv i d e r
Theory:
The frequency divider divides the output frequency of the VCO until it is
comparable to the reference frequency. The frequency divider closes the feedback path
between the output of the VCO and the phase and frequency detector. The feedback loop
makes possible that the VCO will eventually lock on a multiple of the reference signal.
Figure 1 represents a block diagram of a synthesizer with a frequency divider of ratio .
Using this figure it is determined that the relation of the output and reference frequency
is:
refout fMf *= where M is the division ratio.
) RXW3) ' / 3) 9&2
) UHTXHQF\
' LYLGHU
) UHI
Figure 3: Block diagram of Frequency synthesizer
The frequency divider is implemented using an integer-N architecture as shown in
figure 2. This consists of a dual-modulus prescaler with two parts counter. The two
counters are the program counter and swallow counter. The prescaler divides by
1+N until the swallow counter has counted S cycles. At this point the swallow counter
changes the state of the Modulus control to set the prescaler in divide by mode. After
this change the program counter continues counting until it reaches P cycles. In other
words, the program counter already count S cycles and needs SP cycles at the input,
to reach to P pulses. Expressing the above discussion mathematically, the total number
of VCO cycles for one dual modulus division is:
7/30/2019 Frequency Synthesizer Final Report
12/20
PNSNSPNSM +=++= *)())1(*(
(eq.1)
Frequency toPFD
ProgramCounter
%P
Swallow
Counter
%S
Dual ModulusPrescalar
%N/(N+1)
Frequencyfrom VCO
Modulus
ControlReset
Bit Control
Figure 4: Integer N frequency divider
Design:
The frequency divider will operate at the GSM protocols to send information. The
transmit frequency range for this protocol is between 890 MHz to 915 MHz. We have124 RF channels spaced at 200 kHz.
In the design process we choose the dual modulus prescaler to be 8/9. Then S will
count until the number of channels in this case is 124 cycles. P is calculated using
equation 1 when S is 1 and 125 respectively. The following steps calculate P:
a) refout fMf *= , where M have lowM and highM .
4450200
890==
kHz
MHzMlow 4575
200
915==
kHz
MHzMhigh
b) Use equation 1:
55645758*)125()9*125(then125
55644508*)1()9*1(then1
==+=
==+=
PPS
PPS
7/30/2019 Frequency Synthesizer Final Report
13/20
Table 1 shows a summary of the design specification for the frequency divider.
Prescaler N/(N+1) = 8/9
Program Counter 556
Swallow counter 1 to 125
Table 2: Summary of frequency divider specifications
Dual Modulus Prescaler:
The dual modulus prescaler (DMP) is a simple way of implementing a high
frequency division. In our case N=8 and the circuit was designed to divide by 9 when the
modulus control is low, and by 8 when modulus control is high. The implementation of
the DMP was achieved using four flip-flops with some gate logic. The prescaler consists
of a 2/3 synchronous divider with asynchronous divider circuit. In order to achieve high
speed, the current mode logic (CML) is preferred to static CMOS logic. In this case
differential signals are used at both input and output signals. Figure 3 and Figure 4 are
some of the CML circuits, Dlatch and And/Or gate. The And/Or gate is the same circuit,
but with a different configuration, you can get the 4 operations (AND, NAND, OR,
NOR). For example if you want the operation of OR, the inputs are A and B, and the
output is OUT. This can be shown in Figure 5.
Figure 5: Dlacth schematic
7/30/2019 Frequency Synthesizer Final Report
14/20
Figure 6: And/Or gate schematic
Figure 7: Simulation of the And/Or gate
7/30/2019 Frequency Synthesizer Final Report
15/20
The DMP schematic is shown in Figure 6. From the Figure 7 you can count the cycles of
the clock to verify the function but if you use the markers A and B, take the delta anddivide by the period, you get the number of cycles. For the simulation the circuit is
running with a frequency of 900 MHz or a period of 1.1 ns. In Figure 7a, the cycles are
8.8ns/1.1ns = 8 cycles and in part b 9.9ns/1.1ns = 9 cycles.
Figure 8: DMP 8/9 Schematic
Figure 9: Results of DMP, a) MC = 1, b)MC = 0
7/30/2019 Frequency Synthesizer Final Report
16/20
Program counter:
Program counter is responsible for counting P pulses of output of the prescaler
before outputting a pulse to the phase and frequency detector and resetting itself and the
swallow counter. This program counter will receive frequencies up to 114 MHz. The
program counter will count until 556 so we need 10 bit to implement the circuit. The
circuit uses 10 D Flip-flops and some logic gates to compare the counting with the
desired number, in this case 556. When the circuit reaches 556, it will have a pulse in the
output to the phase and frequency detector, but the same output is the reset for the
swallow counter and the DMP. The schematic circuit is in Figure 8.
Figure 8: Program counter schematic
7/30/2019 Frequency Synthesizer Final Report
17/20
Swallow counter:
The swallow counter, or channel-spacing counter, will count until 125. This input
of 7 bits is controlled from the outside; it can be a microprocessor. The circuit uses 7 D
Flip-flops and some logic gates (XOR, NAND, OR) to compare the counting with the
desired number. When the circuit reaches the desires bits, it will change the state of the
modulus control in order to change the prescaler. After that the swallow counter waits to
be reset from the Program counter. The schematic circuit is in Figure 9 and the resulting
simulations are in Figure 10.
Figure 9: Swallow counter schematic
7/30/2019 Frequency Synthesizer Final Report
18/20
Figure 10: Swallow counter schematic
7/30/2019 Frequency Synthesizer Final Report
19/20
Conclusion and further work
A PLL working at 890-915MHz was developed in this project. Only one external
capacitor was used. All other components are realized on-chip. In the future,
following improvement might be made:
1. Try to integrated the off-chip capacitor on to the chip. (using MOS cap or
capacitor multiplier)
2. Use fractional-N instead of integer-N structure to get better noise performance.
7/30/2019 Frequency Synthesizer Final Report
20/20
Reference
[1] Gray, Hurst, Lewis, and Meyer, Analysis and Design of Analog Integrated
Circuits, 4th
edition, pp. 55.[2] M. Rachedine, D. Kaczman, A. Das, M. Shah, J. Mondal, and C. Shurboff,
Performance Review of Integrated CMOS VCO Circuits for WirelessCommunications,IEEE Radio Frequency Integrated Circuits Symposium, pp.77
80, 2003.[3] P. Andreani and S. Mattisson, On the Use of MOS Varactors in RF VCOs,
IEEE Journal of Solid-State Circuits, pp.905 910, June, 2000.
[4] K. Waheed, K. Desai, F. Salem, A Low Power Frequency Synthesizer with anIntegrated Negative Transconductance LC-Tuned VCO,Proc. IEEE
International Conference on Robotics, Intelligent Systems and Signal Processing,
Changsha, China, October, 2003.[5] B. Razavi, RF Microelectronics, Upper Saddle River: Prentice Hall, Inc., 1998.
[6] A.S. Sedra and K.C. Smith, Microelectronic Circuits, Oxford: Oxford University
Press, 1998.[7] T. Lee, The Design of CMOS Radio-Frequency Integrated Circuits, 2nd edition
Cambridge University Press, 2004.
[8] M. Houlgate, A Frequency Synthesizer for a Radio-Over-Fiber Receiver,
Carleton University, April 2003.[9] R, Desikachari, High-Speed CMOS Dual Modulus Prescalers for Frequency
Synthesis, PhD. Thesis, Oregon State University, 2003