ALM-2712
Ultra Low-Noise GPS Amplifier with Pre- and Post-Filter
Application Note 5480
Introduction
The ALM-2712 is a GPS front-end module which consists
of a low noise amplifier with pre- and post-filters. The
module has very low noise figure of approximately 1.3 dB
even with the integration of a pre-filter inside the module.
This is the lowest noise figure among all the GPS modules
in the market with a pre-filter. All three components are
housed in a 12-lead MCOB (multiple-chips-on-board)
package with small dimensions, 3(L) x2.5 (W) x1(H) mm.
The dimensions and the PCB footprint are in ALM-2712
datasheet. Figure 1 shows the simplified internal circuitry.
The low noise amplifier inside the module uses Avago
Technologies’ proprietary GaAs Enhancement mode
pHEMT process to achieve low noise, high gain and high
linearity. The pre- and post-filters are use FBAR (film bulk
acoustic resonator) technology for very low insertion
loss and good out-of-band rejection. Integration of these
three components produces a GPS front end module with
extremely low noise figure and high rejection of out-ofband signals. This helps ensure the receiver’s performance
in concurrent or simultaneous GPS (S-GPS) operation1, 2.
Operating at 2.7 V and 7.5 mA, the measured performance
of the ALM-2712 on the demonstration board shown in
figure 6 was: 1.3 dB Noise Figure, 14 dB gain, +5.3 dBm
Vsd
Vdd
Bias
Control
RFin
RFout
LNA
Module Outline
Figure 1. Simplified ALM-2712 internal circuitry
input IP3, 2.3 dBm input P1dB, 8 dB IRL and greater than
10 dB ORL. By integrating the FBAR filter, the module
achieved excellent out-of-band rejection that was greater
than 90 dBc over 800-900 MHz, greater than 78 dBc over
1620-2100 MHz and greater than 68 dBc over 2300-2700
MHz relative to the 1575 MHz GPS frequency. By using
Avago Technologies’ proprietary GaAs E-pHEMT technology (enhancement-mode pseudomorphic high electron
mobility transistor), the ALM-2712 operates with a supply
voltage down to 1.8 V. ALM-2712 performance at 1.8 V is
shown in Table 2.
Along with the exceptional RF performance, the LNA and
filters integrated solution requires a mimumum number
of external components, which simplifies PCB design and
the manufacturing process. In addition, A CMOS-compatible shutdown pin is included either for turning the LNA
on/off or for bias current adjustment.
Integrated FBAR Filter
A simplified block diagram of a GPS mobile phone receiver
is shown in the Figure 2. During transmit, part of the transmitted signal may leak into the GPS receiver path. Thus, a
good rejection to the transmitting signal band (interferer)
is required at the GPS receiver path to avoid the GPS chipset
from being overloaded by the strong interference power.
The ALM-2712 module includes an integrated input FBAR
filter and an output FBAR filter. FBAR is a breakthrough
resonator technology developed by Avago Technologies.
This technology produces small size filters with excellent
Q. The excellent Q translates into a very steep filter roll-off
and superb out-of-band rejection. With the integrated
filters, the module has exceptional out-of-band rejection
to the interferer signal. This excellent out-of-band rejection
ensures that the receiver’s sensitivity does not degrade in
the presence of strong interferers.
Band Select
GPS Rx
GPS
Chipset
ALM-2712
(Filter + LNA + Filter)
between the Vsd pin and the 2.7 V voltage supply to drop
the voltage and set the bias current at 7.5 mA. The current
drawn by the Vsd pin is in the range of a few hundred microamperes.
Mobile
Satellite
WLAN
Cellular
AWS
PCS
Tx Signal leak to
GPS Rx Path
Tx Chipset
Figure 2. Rx Front end simplified block diagram
Vsd
Gnd
Gnd
(Pin 10) (Pin 11) (Pin 12)
Vdd (Pin 9)
RF in (Pin 1)
RF Out (Pin 8)
Gnd (Pin 2)
Gnd (Pin 7)
Gnd (Pin 3)
Gnd
(Pin 6)
Gnd
(Pin 5)
Gnd
(Pin 4)
TOP VIEW
Figure 3. Pin configuration
The bias current can be reduced by increasing the value
of R2 to save power consumption. Increasing the bias
current by reducing the resistor will improve linearity. Alternatively, the voltage supply to the Vsd pin can be varied
to set the appropriate operating current. The adjustable
current allows a designer to make a tradeoff between
gain, noise figure, and linearity with current consumption. Performance over different bias current levels can
be found in ALM-2712 datasheet along with appropriate
voltage levels and resistor values.
The LNA module also features a shutdown circuitry that is
beneficial for portable devices that have limited battery
life. When the module is in the shutdown mode, the current
consumption is 0.5 μA, and the forward isolation is ~14
dB. The LNA module can be easily turned off by applying 0
V to 0.3 V to Vsd (pin 11), which is CMOS-compatible. The
Vsd pin can be connected to a microcontroller to switch
the LNA module to shutdown mode when it is not needed
to extend the battery life. Figure 4 shows the Gain versus
Vsd plots with different biasing resistor, R2, values.
Gain versus Vsd
Figure 3 shows the pin configuration of the ALM-2712.
The bias circuitry is integrated internally to simplify the
external biasing circuitry of the module. Pin 2 (Ground) is
connect to the center paddle with a thin line to make it
symmetrical to the RF Out pin for good solderability of the
module on PCB.
10
5
Gain, dB
Pin configuration, Biasing, Variable Bias Current and
Shutdown Function
15
0
-5
Vdd=2.7V,
Rbias=6.8kohm
Vdd=1.8V,
Rbias=2.7kohm
-10
Unlike the typical depletion-mode pHEMT LNA, Avago’s
enhancement-mode pHEMT LNA in the ALM-2712
requires only one positive voltage supply to bias the LNA.
In this design, Vdd (pin 9) and Vsd (pin 11) are supplied
with 2.7 V. A 6.8 kΩ resistor ( R2 in Figure 5 and Figure 6) is
2
-15
0
0.5
Figure 4. Gain vs. Vsd.
1
1.5
Vsd, volts
2
2.5
3
Application Circuit for ALM-2712 in GPS Receiver Application
Figure 5 shows the ALM-2712 GPS receiver application
circuit. A DC blocking capacitor is not needed externally
at the device output. The ALM-2712’s input matching and
RF chokes are integrated internally. Thus, only two bypass
capacitors are required on the power supply (Vdd line).
Ultimately, the external components crucial for the actual
application are just two bypass capacitors (C1 and C2)
and one bias resistor (R2). Capacitor C3 is optional as this
component is a supply decoupling capacitor and doesn’t
significantly affect RF performance of the GPS module.
The component placement on the demonstration board
is shown in Figure 6.
The bill of material is shown in Table 1. Performance
measured on the demonstration board is summarized in
Table 2.
Vsd
Vdd
C1
C3
R2/Rbias
C2
Bias
Control
RFin
RFout
LNA
ALM-2712
Module Outline
Figure 5. GPS LNA application circuit schematic
Component Placement, Bill of Material and PCB Layout
Table 1. Bill of Material
Size
Low Noise
Manufacturer & Part Number
Description
Bypass Capacitor
Murata GRM1555C
Bypass Capacitor/Output Matching
C3
0402
6.8 pF
Murata GRM1555C
Bypass Capacitor
R2
0402
2.7 kΩ
Biasing at 5 mA (Vsd=1.8 V)
0402
6.8 kΩ
Biasing at 7.5 mA (Vsd=2.7 V)
Gnd
Murata GRM1555R
47 pF
Vdd
0.1 μF
0402
Vsd
0402
C2
Gnd
C1
zero ohm jumper
RFin
C3
R 2/Rbias
C1
C2
ALM 2712
Figure 6. Demonstration board component placement
3
RFout
Figure 7a and 7b show the layout of each PCB layer and the stacking structure of the demonstration board.
Top view (Top ground metal)
Top view (Inner transmission line/ Inner 2)
Top view (Bottom inner ground metal/Inner 1)
Top view (Bottom ground metal)
Figure 7a. Demonstration board Layout
The demonstration board uses stripline to maximize module performance. The stripline design has the transmission
line/trace sandwiched between two substrate planes that are adjacent to ground planes. As shown in Figure 7a, the
stripline is on the “Inner 2” layer. The stripline is sandwiched between two Rogers 4350 substrates (20 mil and 18 mil).
Each substrate has a ground plane at the side opposite to the stripline – the Inner 1 layer and the bottom layer are
the ground planes. These ground planes shield the transmission line, reducing coupling to adjacent traces, packaging
material and shielding. A comparison between a stripline design and a CPWG (Coplanar Waveguide with Ground) transmission line design that discusses the shielding effect is available in the Avago Technologies Application Note 5414.3
4
Top
0.5 oz copper
Inner 1
Inner 2
Bottom
Total Thickness 62 mils
Stacking Structure
Top
0.5 oz
Metal/ Ground
FR4
~21mils
Support Material, for mechanical strength (adjust to achieve total board thickness of 62 mils)
Inner 1
0.5 oz
Metal/ Ground
Substrate 1
20 mils
Rogers RO4350
Inner 2
0.5 oz
Metal/ Transmission Line
Substrate 2
18 mils
Rogers RO4350
Bottom
0.5 oz
Metal/ Ground
Figure 7b. PCB stacking structure
The ALM-2712 has exceptionally good out-of-band rejection, with attenuation greater than 60 dB. To achieve high attenuation, proper PCB layout is mandatory. One factor that could limit the out-of-band rejection performance is RF
feedthrough between the input port and output port. It is recommended that the PCB layout has plated through
via holes connect to the common ground layer surrounding the input and output ports. This will help minimize the
RF feedthrough and increase the isolation between the input port and output port. Figure 7c shows some “DO’s and
DON’Ts” for the PCB layout.
DON'T
1. Ground surrounding the input port
and output port is separated.
2. No via holes on the ground surrounding
input port and output port.
DO's
1. Ground surrounding the input port
and output port connected completely.
2. Via holes on the ground surrounding
input port and output port.
Figure 7c. PCB layout recommendations.
5
Product Performance
Table 1 summarizes the performance of the ALM-2712. All the parameters are measured on the demonstration board
using the test setup shown in Figure 10.
Table 2. RF Performance for the ALM-2712 at 1.575 GHz.
Supply Voltage, Vdd
Shutdown Voltage, Vsd
Current, Id
Stability Factor, k (30 kHz – 20 GHz)
Gain, S21
Noise Figure, NF
Input Return Loss, IRL
Output Return Loss, ORL
Reverse Isolation
Input 3rd order Intercept Point, IIP3 *
Out of band Input 2nd order Intercept Point, IIP2 **
Input P1dB, IP1dB
Volts
Volts
mA
2.7
2.7
7.5
>1
14
1.3
8
>10
23
+5.3
72
2.3
dB
dB
dB
dB
dB
dBm
dBm
dBm
1.8
1.8
5
>1
12.9
1.4
7.4
>10
21
+3.6
69.9
0
* Test condition: FRF1 = 1.5725 GHz, FRF2 = 1.5775 GHz with input power of -20 dBm per tone measured at the worst case side band.
** Test condition: FRF1= 824.6MHz and input power level of -17 dBm, FRF2 = 2400 MHz and input power level of -40 dBm. IIP2= PinRF2 + [PinRF1 –
(PoutRF2-RF1 – GPS_Gain)]
Figures 8 and 9 show the gain, return loss, reverse isolation, out-of-band rejection and stability of the ALM-2712 on the
demonstration board.
Input Return Loss versus Frequency
Gain versus Frequency
0
16
14
12
-10
Gain, dB
Input Return Loss, dB
-5
-15
10
8
6
4
-20
2
-25
1540
1550
1560
1570 1580
Freq, MHz
1590
1600
0
1540
1610
1550
-20
-5
-30
-10
-15
-20
-25
1540
1590
1600
1610
1600
1610
-40
-50
-60
-70
1550
1560
1570 1580
Freq, MHz
1590
1600
1610
Vdd/Idd=2.7 V/7.5 mA
Figure 8a. Gain, return loss, and reverse isolation vs. frequency.
6
1570 1580
Freq, MHz
Reverse Isolation versus Frequency
0
Reverse Isolation, dB
Output Return Loss, dB
Output Return Loss versus Frequency
1560
-80
1540
1550
1560
Vdd/Idd=1.8 V/5 mA
1570 1580
Freq, MHz
1590
Attenuation Versus Frequency
Attenuation Versus Frequency
0
-10
-20
Attenuation, dB
Attenuation, dB
-74
-75
-76
-77
-78
-79
-80
-81
-82
-83
-84
-30
-40
-50
-60
-70
-80
800
850
900
Freq, MHz
950
-90
1600 1650 1700 1750 1800 1850 1900 1950 2000
Freq, MHz
1000
Attenuation Versus Frequency
0
-10
Attenuation, dB
-20
-30
-40
-50
-60
-70
-80
2000
2100
2200
2300 2400
Freq, MHz
2500
2600
2700
Vdd/Idd=1.8 V/5 mA
Vdd/Idd=2.7 V/7.5 mA
Figure 8b. Out-of-band rejection vs. frequency.
K factor Versus Frequency
30
25
20
K factor
K factor
K factor Versus Frequency
10
9
8
7
6
5
4
3
2
1
0
1540
10
5
1550
1560
1570 1580
Freq, MHz
1590
1600
1610
Vdd/Idd=2.7 V/7.5 mA
Figure 8c. K factor vs. frequency.
7
15
0
0
4000
Vdd/Idd=1.8 V/5 mA
8000
12000
Freq, MHz
16000
20000
0.2
0.18
0.16
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0
-50
Noise Figure Degradation Versus Jamming Signal Strength
850 MHz
890 MHz
915 MHz
1620 MHz
1710 MHz
1885 MHz
1910 MHz
-40
-30
-20
-10
0
Jamming Signal Strength, dBm
10
Noise Figure Degradation, dB
Noise Figure Degradation, dB
Following plots show the degradation of the module’s noise figure at the presence of strong interferer/jamming signal.
The degradation is with reference to the module’s noise figure when there is no interferer/jamming signal presence.
Each plot on the graph represents interferer with different frequency as show by the label on the graph. The plots show
that the degradation is insignificant with interferer strength up to 20 dBm. The test setup for the measurement is illustrated in Figure 10.
20
Figure 9a. Noise Figure degradation versus jamming signal strength at
Vdd/Idd = 2.7 V/7.5 mA.
NF Meter Calibration at
this point
0.2
0.18
0.16
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0
-50
Noise Figure Degradation Versus Jamming Signal Strength
850 MHz
890 MHz
915 MHz
1620 MHz
1710 MHz
1885 MHz
1910 MHz
-40
-30
-20
-10
0
Jamming Signal Strength, dBm
10
20
Figure 9b. Noise Figure degradation versus jamming signal strength at
Vdd/Idd = 1.8 V/5 mA.
Input Power Reference Point
Use power meter to read input
Noise Source
Agilent N4000A
Signal Generator
Agilent MXG N5182A
Jamming Signal
Jammer Frequency
Tunable Bandpass Filter
K&L Microwave
Power
Combiner
Coaxial
Cable
PA
Isolator
Power Amplifier
Mini Circuits
ZHL-42
NF Analyzer
Agilent
N8975A
DUT
GPS
Bandpass Filter
1.575 GHz 1.575 GHz
Tunable Notch Filter
K&L Microwave
• Bandpass Filter will filter the noise generated by the pre-amplifier.
• Notch Filter provides a clean signal at the jammer frequency (The cascaded notch filter gives greater than
100 dB of rejection @1.575 GHz)
Figure 10. Test setup for Noise Figure degradation with presence of jamming signal.
The noise source at the GPS frequency is integrated together with the jamming signal using a power combiner before
the signal is delivered to the device under test. At the jamming signal source, a power amplifier is used to boost the
power level of the jamming signal. An isolator and filters are placed in the jamming signal path to attenuate the noise
(at GPS frequency) generated by the signal generator and power the amplifier from leaking into the noise figure analyzer,
which could cause an inaccurate noise figure measurement.
A GPS bandpass filter is inserted before the noise figure analyzer to prevent the analyzer from being overloaded by the
strong jamming signal.
8
Voltage Supply from RF Output Line
Some applications may require the Vdd voltage supply to come from the RF output line. The schematic in Figure 11
shows such a circuit implemented with an additional inductor at the RF output. In order to minimize the effect on RF
performance, the RF choke should have impedance ten times higher than the RF output trace, or the inductor series
resonant frequency should be close to or higher than the operating frequency.
DC Voltage
Vsd
Vdd
C1
C3
R 2/Rbias
C2
Bias
Control
RFin
RFout
LNA
ALM-2712
Module Outline
Figure 11. Voltage Supply from RF Output Line
Conclusion
This application note demonstrates the performance of the ALM-2712 – a GPS LNA module with very low noise figure,
high linearity and good blocking to the interferer signal, even when operating at low voltage and current. The ALM-2712
needs very few external components and includes a power shutdown circuit and adjustable current capability in a
compact MCOB package.
Note:
1. S. Spiegel et al, “Improving the Isolation of GPS Receivers for Integration with Wireless Communication Systems”, Proc.
IEEE RFIC Symposium, pp563-566, 2003.
2. Yut H. Chow et al, “A 1V, 0.9dB Noise-Figure High Linearity LNA MMIC for Concurrent GPS Handset application”, Proc
Asia Pacific Microwave Conference, 2006
3. Application Note: Application Note 5414 – GPS Low Noise Amplifier Front-end Module with Integrated Pre and Post
Filter and Variable Current/Shutdown Function (http://www.avagotech.com/docs/AV02-2412EN)
For product information and a complete list of distributors, please go to our web site: www.avagotech.com
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Data subject to change. Copyright © 2005-2010 Avago Technologies. All rights reserved.
AV02-2474EN - July 13, 2010