RF/IF Amplifiers with OIP3 of 47dBm/50dBm at 240MHz Ease Implementation, Guarantee Performance

October 2013
I N
T H I S
I S S U E
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converts a positive input
to a negative output with a
single inductor 20
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Volume 23 Number 3
RF/IF Amplifiers with OIP3 of
47dBm/50dBm at 240MHz
Ease Implementation,
Guarantee Performance
Greg Fung
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Our communication infrastructure’s limited bandwidth is nearly
filled to capacity by our increasing thirst for data transmitted
via smartphone, TV, GPS and Wi-Fi. To quench this thirst,
communications architects define systems that pack increasingly
more data into limited bandwidth,
but data rate improvements
come at a price: the need for
increasingly higher fidelity transmit
and receive signal chains.
When it comes to amplifiers, low noise and high
linearity are required to faithfully reproduce a signal
without degrading the original signal. At low signal powers, undesired noise must be low enough to
allow the intended signal to rise above the noise floor.
At high signal levels, a linear amplifier must prevent
unwanted harmonics and intermodulation products
from masking the intended signal. The LTC®6431-15
and LTC6430-15 achieve both of these goals.
The LTC6431-15 and LTC6430-15 are two fixed gain
amplifiers that feature very high OIP3 (linearity) with
very low associated noise. The LTC6431-15 is a singleended radio frequency (RF)/intermediate frequency
The LT®3795 LED driver reduces peak EMI without incurring LED flicker. See page 12.
Caption
w w w. li n ea r.com
(continued on page 4)
The LTC6431-15 boasts a typical OIP3 of 47dBm at 240MHz—
essentially hammering the intermodulation products (IM3) into the
noise floor so they don’t interfere with the intended signals.
(LTC6430/1-15 continued from page 1)
FUNDAMENTAL
SOURCE
x2
y = a1x + a2
x3
+ a3
SOURCE
LOAD
y = a1x + a2x2 + a3x3
FREQUENCY
0
–10
DESIRED TONE
DESIRED SIGNAL
–20
0
–40
IM3 PRODUCT
–30
–40
–50
–60
–70
–80
–120
–140
10 20 30
–30 –20 –10 0
INPUT POWER (dBm)
2ND ORDER
10
OIP3 IN dBm
–20
–80
UNDESIRABLE
IM3 PRODUCTS
For instance, if a single tone is injected
into a nonlinear amplifier, the result is the
desired tone plus its harmonics. Normally,
these harmonics can be filtered out, as
they are far enough in frequency from the
desired tone. If two tones are injected into
40
–60
3RD ORDER
of an issue, but an amplifier’s linearity becomes increasingly important.
Linearity limits the ability to isolate the
desired signal from unwanted signals
in the frequency domain. At high input
signal levels, the desired signal rises far
above the noise floor, so noise is less
20
AMPLITUDE
NONLINEAR
AMP
MULTIPLE
TONES
IMPRESSIVE OIP3 HAMMERS
DOWN IM PRODUCTS
80
2ND ORDER
OUTPUT
FREQUENCY
60
FUNDAMENTAL
FREQUENCY
INPUT
Figure 2. Two
tones at the input
of a nonlinear
device create
intermodulation
product at the output.
–100
–90
40
50
Figure 3. Output 3rd order intercept point (OIP3)
4 | October 2013 : LT Journal of Analog Innovation
LOAD
AMPLITUDE
AMPLITUDE
NONLINEAR
AMP
AMPLITUDE (dB)
Noise limits communication system sensitivity at low input signal levels. Noise
in a communication system is characterized by the noise figure (NF), which is the
signal-to-noise power ratio at the output
divided by the signal-to-noise power
ratio at the input expressed in decibels.
There is always noise at the input of an
amplifier and it is gained up along with
desired signal. The NF is an indicator of
how much unwanted noise the amplifier itself adds to the signal. Ideally, the
amplifier would have a NF of 0dB, but
any real amplifier adds noise, so the goal
is to minimize noise impairment. Typical
IF amplifiers have noise figures of 3dB to
12d B. The LTC6431-15 and LTC6430-15
both exhibit a 3.3dB NF at 240MHz.
OUTPUT
FREQUENCY
AMPLITUDE
LOW NF FOR LOW INPUT SIGNALS
INPUT
Figure 1. A single
tone at the input of
a nonlinear device
creates harmonics
at the output.
OUTPUT POWER (dBm)
(IF) gain block that can directly drive
a 50W load, whereas the LTC6430-15
is a differential RF/IF gain block with
higher power and an even wider linear
bandwidth. These gain blocks combine state-of-the-art performance with
ease of use—eliminating implementation difficulties by internally handling
of biasing, impedance matching, temperature compensation and stability.
–100
200 210 220 230 240 250 260 270 280
FREQUENCY (MHz)
Figure 4. The LTC6431-15 boasts an OIP3 of 47dBm
at 240MHz—essentially hammering the IM3 products
of a 2-tone signal into the noise floor so that they
don’t interfere with the intended signals.
design features
The single-ended LTC6431-15 excels as an IF amplifier to overcome filter
losses, or as an ADC driver when used with a balun transformer. With
its wide bandwidth, the LTC6431-15 can cover the entire CATV band.
Z1
Figure 5. Adding matching networks
to the input and output
Z2
Z1
INPUT
MATCH
Z2
OUTPUT
MATCH
INPUT
Z = 50Ω
OUTPUT
Z = 50Ω
TRADITIONAL
RF AMPLIFIER
f = 240MHz
a nonlinear amplifier, the result is a far
more complicated mix of the two desired
tones and a multitude of unwanted tones,
including harmonics of the two tones, the
sum and difference of the two input tones,
and other intermodulation products.
Figure 6. Single-ended IF amplifier
Intermodulation (IM3) products
(2f1 – f2 and 2f2 – f1) are a subset of these
unwanted tones and they are particularly
onerous. IM3 products can fall very close
to the intended signal’s frequency, making them nearly impossible to filter out.
5V
VCC = 5V
1000pF
Noise (characterized by NF) limits an
amplifier’s sensitivity at low input signal
amplitudes, while linearity (characterized
by OIP3) limits sensitivity at high input
amplitudes. Taken together, these two
metrics, NF and OIP3, define the amplifier’s useful dynamic range for a signal.
RF
CHOKE,
560nH
1000pF
LTC6431-15
RSOURCE
50Ω
Amplifier linearity is most often characterized by the 3rd order output
intercept point (OIP3)—the hypothetical point where the power of the IM3
products intersects the fundamental
power (Figure 3). The LTC6431-15 exhibits very small IM3 products and thus its
OIP3 is very good. Minimizing the IM3
product is especially important when a
blocker (interferer) or an adjacent channel is nearby. Figure 3 shows that IM3
products grow three times faster than the
desired tones. This limits the acceptable
output power, and therefore the input
power, that the amplifier can handle
without distorting the desired signal.
RLOAD
50Ω
20
54
15
50
10
MAGNITUDE (dB)
OIP3 (dBm)
46
42
38
34
0
–5
–10
–15
–20
30
26
S PARAMETER
S11
S21
S12
S22
5
–25
0
200
400
600
FREQUENCY (MHz)
800
LTC6431-15 OIP3 vs frequency
1000
–30
0
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
FREQUENCY (GHz)
2
LTC6431-15 S parameter vs frequency
Figure 7. LTC6431-15 100MHz–1700MHz single-ended evaluation circuit and performance
October 2013 : LT Journal of Analog Innovation | 5
The LTC6430-15 excels as an ADC driver for high speed, high resolution
ADCs. The challenge in these applications is to drive the unbuffered ADC
inputs to their required input voltage levels while preserving the signal-tonoise ratio (SNR) and spurious free dynamic range (SFDR) of the ADC.
Figure 8. Simplified schematic of
wideband differential 14-bit ADC driver
560nH
0402AF
60pF
GUANELLA
BALUN
1:1
1nF
VCC = 5V
150Ω
HIGH LINEARITY SOLVES THE
TOUGHEST COMMUNICATION
PROBLEMS
VCM
5V
49.9Ω
350Ω
•
•
The LTC6431-15 boasts a typical OIP3 of
47dBm at 240MHz —essentially hammering
the IM3 products into the noise floor so
that they don’t interfere with the intended
signals (Figure 4). Not to be outdone, the
LTC6430-15 features an OIP3 of 50dBm
at 240MHz. Both amplifiers offer a very
wide dynamic range when combined with
their 3.3dB NFs —addressing the high data
rate challenge by maintaining high fidelity at both high and low signal levels.
1nF
100nH
0402CS
LTC6430-15
LTC2158
200ps
EASY TO INSERT
Implementing an RF/IF gain stage has
not always been easy. Traditionally, the
designer must first consider circuit biasing. The LTC6431-15 has an internal bias
circuit that requires only 90m A from a
single 5V supply, while the LTC6430-15
draws 160m A from a single 5V supply.
Figure 9. LTC6430-15 driver
and LTC2158-14, dual
14-bit ADC combination
evaluation circuit
1000pF
60pF
0.1µF
1000pF
DNC
GND
DNC
T_DIODE
LTC6430-15
DNC
120nH
0402CS
–OUT
1000pF
560nH
348Ω
BALUN = MaCom 1:1 TRANSFORMER MABA-007159
6 | October 2013 : LT Journal of Analog Innovation
1000pF
AIN+
LTC2158-14
AIN–
DNC
DNC
–IN
GND
DNC
60pF
1000pF
1000pF
VCM
DNC
DNC
DNC
–IN
0.1µF
49.9Ω
+OUT
VCC
100Ω
DIFFERENTIAL
560nH
DNC
DNC
VCC
DNC
DNC
GND
+IN
1:1
BALUN
+IN
348Ω
GND
49.9Ω
The internal bias circuit optimizes the
device operating point for maximum linearity. A temperature compensation circuit
maintains performance over environmental changes and prevents current runaway at high temperature. These devices
also include an internal voltage regulator
to minimize performance changes due
to imperfections in the power supply.
0.1µF
VCC = 5V
GND
An RF/IF amplifier must also be impedance
matched at the input and the output to
maximize power transfer and minimize
reflections. This is traditionally a timeconsuming iterative task. Typically the
designer must add input and output networks to match the amplifier impedance
design features
Figure 10. 500MHz single
tone SFDR and SNR of
LTC6430-15 and LTC2158
driver/ADC combo board
(SNR = 61.5dB, SFDR =
75.7dB)
to the system impedance, normally
50Ω (Figure 5). These matching networks
in turn alter the amplifier’s NF and OIP3—
often compromising the NF and OIP3 to
achieve a reasonable impedance match.
The LTC6431-15 and LTC6430-15 amplifiers
internally match their input and output
impedance over the 20MHz –1700MHz
band, simplifying design while preserving their NF and OIP3. The single ended
LTC6431-15 is internally input and output
matched to 50W, whereas the LTC6430-15
is internally matched to 100W differential impedance at the input and the
output. This is allows the devices to be
easily inserted into various applications
without additional matching elements.
Figure 11. 500MHz 2-tone
measurement of IM3
products of LTC6430-15
and LTC2158 driver/ADC
combo board (IM3 low
= –101dBfs, IM3 high =
–102dBfs)
GUARANTEED STABILITY AND
PERFORMANCE
The LTC6431-15 and LTC6430-15 are
unconditionally stable when implemented with our applications circuits.
A-grade versions of the LTC6431-15
are individually characterized for OIP3
at 240MHz, guaranteeing a minimum
OIP3 of 44dBm. Similarly, A-grade versions of the LTC6430-15 are individually
characterized for OIP3 at 240MHz, guaranteeing a minimum OIP3 of 47dBm.
Table 1. Summary of results over frequency for ADC driver evaluation circuit
LTC6430/LT2158 COMBO CIRCUIT
LT2158 ADC ALONE
FREQ.(MHz)
1M
SFDR
SNR
1M
SFDR
SNR
250
–87
73.8
63.1
–95
78
66.5
300
–86
77.5
62.8
–94
78
65.5
A NEW BREED OF RF AMPLIFIER
400
–87
75.0
62.3
–92
78
64.5
Linear Technology has a long history of
producing superior op amp style amplifiers that handle low frequency signals
with minimal noise and distortion. While
the LTC6431-15 and LTC6430-15 are not
capable of amplifying DC signals like an
op amp, they are capable of amplifying
500
–101
75.7
61.5
–84
70
63.0
600
–88
72.0
60.7
–88
62.5
62.5
700
–92
67.5
60.0
–86
62.0
61.0
800
–94
84.0
59.5
–85
61.5
60.0
900
–82
73.0
58.6
–80
61.0
59.0
1000
–85
61.4
58.1
–83
60.5
58.0
October 2013 : LT Journal of Analog Innovation | 7
Using an appropriate pair of 2:1 balun transformers, the LTC6430-15 provides wideband
amplification with low noise and low distortion. In this balanced configuration, the amplifier is
matched to 50Ω at the input and output. The balanced configuration also has the advantage
of suppressing 2nd order distortion which is critical in multi-octave wideband applications.
BALUN_A = ADT2-1T FOR 50MHz TO 300MHz
BALUN_A = ADT2-1P FOR 300MHz TO 400MHz
BALUN_A = ADTL2-18 FOR 400MHz TO 1300MHz
ALL ARE MINI-CIRCUITS CD542 FOOTPRINT
signals up to 2GHz. Op amps typically
struggle to operate above 200MHz.
With an op amp, feedback typically needs
to be added to set the gain. Increasing the
gain of a voltage feedback op amp further
decreases its operational bandwidth. On
the other hand, our RF style amplifiers
offer a fixed power gain of 15dB. The
RF solution lacks the versatility of gain
adjustment, but the usable bandwidth far
exceeds that attainable from an op amp.
Op amps are designed to drive high
impedance loads, while the LTC6430/31
amplifiers can drive a 50Ω load and deliver
real power over a wide frequency range
(20MHz –1700MHz). Unlike an op amp,
this RF-focused design does not require
termination resistors at the input nor
at the output, as impedance matching
is done internally. Termination resistors at the input add noise and termination resistors at the output attenuate the
power delivered to the load. Therefore,
DNC
DNC
–OUT
R2
350Ω
C5
1000pF
OPTIONAL STABILITY
NETWORK
the RF amplifier solution results in better
overall noise and linearity. The LTC6430-15
and LTC6431-15 amplifiers offer a superior
solution for AC signal applications that
do not require DC-coupled performance.
LTC6431-15 SINGLE-ENDED 50Ω
AMPLIFIER
The single-ended LTC6431-15 is an ideal
solution for a number of applications.
It excels as an IF amplifier to overcome
filter losses, or as an ADC driver when
used with a balun transformer. With
its wide bandwidth, the LTC6431-15
can cover the entire CATV band.
Figure 6 shows a single-ended
IF amplifier, while Figure 7 shows an
LTC6431-15 100MHz –1700MHz evaluation board and performance.
DNC
GND
DNC
–IN
C2
1000pF
• •
100Ω
DIFFERENTIAL
C4
1000pF
BALUN_A
DNC
DNC
C3
1000pF
T2
2:1
T_DIODE
LTC6430-15
DNC
C8
60pF
VCC
GND
DNC
BALUN_A
8 | October 2013 : LT Journal of Analog Innovation
+OUT
DNC
DNC
100Ω
DIFFERENTIAL
RFIN
50Ω, SMA
L1
560nH
DNC
DNC
T1
1:2
VCC
PORT
INPUT
+IN
R1
350Ω
DNC
C7
60pF
GND
C1
1000pF
GND
Figure 12. 50Ω input/output balanced amplifier
PORT
OUTPUT
RFOUT
50Ω, SMA
L2
560nH
C6
0.1µF
VCC = 5V
LTC6430-15 DIFFERENTIAL
APPLICATIONS
The differentially configured inputs and
outputs of the LTC6430-15 lend themselves to a variety of system applications. In the following examples, the
LTC6430-15 linearity, low noise and
wideband performance are put to the test.
In the first example, its differential
outputs mate well to the differential
inputs of an ADC. The LTC6430-15 is
internally input/output matched to
100W differential impedance. 100W is a
convenient impedance for driving high
speed ADCs. Next, using 2:1 balun transformers in a balanced configuration, the
LTC6430-15 delivers wideband amplification with low distortion into 50W.
Finally, using 1.33:1 balun transformers, the LTC6430-15 can be matched to a
75W system to deliver wideband amplification across the entire CATV band.
design features
A single balun cannot cover the entire LTC6430-15 band of operation. Linear
offers several evaluation circuits that cover the amplifier’s intended bandwidth.
Conveniently transformed to 50Ω at the input and output(s) for ease of bench
characterization, these evaluation circuits also demonstrate the performance of the
LTC6430-15 when used in a purely differential application without the baluns.
Figure 13. Evaluation circuit of balanced amplifier shown in Figure 12: 50MHz–300MHz (ADT2-1T baluns)
54
15
10
50
MAGNITUDE (dB)
OIP3 (dBm)
46
42
38
34
0
–5
–10
–15
–20
30
26
S PARAMETER
S11
S21
S12
S22
5
–25
0
100
200
300
FREQUENCY (MHz)
400
–30
500
0
100
200 300 400 500
FREQUENCY (MHz)
600
700
Figure 14. Evaluation circuit of balanced amplifier shown in Figure 12: 300MHz–1100MHz (ADTL2 baluns)
20
50
15
46
10
MAGNITUDE (dB)
OIP3 (dBm)
42
38
34
0
–5
–10
–15
–20
30
26
S PARAMETER
S11
S21
S12
S22
5
–25
0
200
400
600
FREQUENCY (MHz)
800
–30
1000
0
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
FREQUENCY (GHz)
2
Figure 15. Evaluation circuit of balanced amplifier shown in Figure 12: 200MHz–1500MHz (TCM2-43X baluns)
50
15
10
46
5
MAGNITUDE (dB)
OIP3 (dBm)
42
38
34
–5
–10
–15
–20
30
26
S PARAMETER
S11
S21
S12
S22
0
–25
0
0.25
1
0.5
0.75
FREQUENCY (GHz)
1.25
1.5
–30
0
100
200 300 400 500
FREQUENCY (MHz)
600
700
October 2013 : LT Journal of Analog Innovation | 9
Cable TV offers unique challenges for an amplifier. A high channel count requires
excellent 3rd order linearity and due to the multiple octave environment, 2nd order
products must be suppressed as well. The LTC6430-15 meets these challenges using
a pair of 1.33:1 baluns to transform its inherent 100Ω differential impedance to 75Ω.
ADC Driver
Table 1 displays minimal degradation
of SNR and SFDR for this high speed,
high resolution ADC. The LTC6430-15’s
high linearity (Figures 10 and 11) and
low noise allow the designer to drive
the ADC with minimal filtering at the
ADC input. All measurements are taken
from a single application circuit without adjusting the matching networks.
This highlights the LTC6430-15 wide
bandwidth and linearity performance.
Balanced Amplifier Drives 50Ω Loads
Using an appropriate pair of 2:1 balun
transformers, the LTC6430-15 provides
wideband amplification with low noise
and low distortion (Figure 12). In this
balanced configuration, the amplifier is
matched to 50Ω at the input and output. The balanced configuration also
has the advantage of suppressing 2nd
order distortion which is critical in
multi-octave wideband applications.
10 | October 2013 : LT Journal of Analog Innovation
DNC
DNC
VCC
GND
DNC
•
100Ω
DIFFERENTIAL
C4
0.047µF
DNC
–OUT
DNC
GND
DNC
MINI-CIRCUITS 1:1.33
T2
1.33:1
DNC
DNC
BALUN_A = TC1.33-282+
FOR 50MHz TO 1000MHz
C3
0.047µF
T_DIODE
LTC6430-15
DNC
C2
0.047µF
DNC
+OUT
DNC
DNC
BALUN_A
GND
+IN
100Ω
DIFFERENTIAL
L1
560nH
DNC
VCC
RFIN
75Ω,
CONNECTOR
T1
1:1.33
GND
PORT
INPUT
–IN
The LTC6430-15 excels as an ADC driver for
high speed, high resolution ADCs (Figure 8).
The challenge in these applications is to
drive the unbuffered ADC inputs to their
required input voltage levels while preserving the signal-to-noise ratio (SNR) and
spurious free dynamic range (SFDR) of the
ADC. As shown by the performance results
for the evaluation circuit in Figure 9, the
LTC6430-15 is able to drive the LTC2158
(dual 14-bit, 310Msps ADC) over its full
input bandwidth with very little degradation in SFDR and SNR (Figure 10).
C1
0.047µF
C5
1000pF
•
BALUN_A
PORT
OUTPUT
RFOUT
75Ω,
CONNECTOR
L2
560nH
C6
0.1µF
VCC = 5V
Figure 16. 50MHz to 1000MHz CATV push-pull amplifier with 75Ω input and 75Ω output
Unfortunately, a single balun cannot cover
the entire LTC6430-15 band of operation.
Linear Technology offers a number of
evaluation circuits that cover the amplifier’s intended bandwidth (Figures 13–15).
Conveniently transformed to 50Ω at the
input and output(s) for ease of bench
characterization, these evaluation circuits
also demonstrate the performance of the
LTC6430-15 when used in a purely differential application without the baluns.
The results reveal the importance of
selecting the correct balun transformer
for the frequency of interest. Due to their
limited bandwidth, the balun transformers limit the LTC6430-15 performance.
Together, these three balanced circuits
demonstrate the linearity and wide bandwidth attainable with the LTC6430-15.
CATV Application
A CATV application circuit is the final
example of the LTC6430-15’s versatility (Figure 16). Cable TV offers unique
challenges for an amplifier. Often the
required frequency band covers more
than four octaves and the amplifier must
have flat gain and impedance matching
to a 75Ω environment. A high channel
count requires excellent 3rd order linearity
and due to the multiple octave environment, 2nd order products must be suppressed as well. The LTC6430-15 meets
these challenges using a pair of 1.33:1
baluns to transform its inherent 100Ω differential impedance to 75Ω (Figure 17).
Given its low noise, low 2nd and 3rd
order distortion, and flat gain, this circuit
can handle CATV demands while consuming only 800mW from a 5V supply.
design features
The LTC6431-15 and LTC6430-15 are manufactured using a high performance
SiGe BiCMOS process, compared to other RF gain blocks manufactured using
GaAs transistors. Using a silicon-based process yields better reproducibility over
comparable GaAs processes. A BiCMOS process also allows Linear to integrate
distortion cancellation, bias control and voltage regulator functions into the devices.
SILICON-BASED PROCESS FOR
BETTER REPRODUCIBILITY
The LTC6431-15 and LTC6430-15 are
manufactured using a high performance
SiGe BiCMOS process, compared to other
RF gain blocks manufactured using Ga As
transistors. Using a silicon-based process
yields better reproducibility over comparable Ga As processes. A BiCMOS process
also allows Linear to integrate distortion
cancellation, bias control and voltage
regulator functions into the devices.
Figure 17. LTC6430-15
50MHz–1000MHz CATV
evaluation circuit and
performance results
CONCLUSION
50
15
10
46
MAGNITUDE (dB)
OIP3 (dBm)
38
34
–10
–15
–25
0
0
200
400
600
FREQUENCY (MHz)
800
–30
1000
–20
NOISE FIGURE (dB)
–40
–50
–60
–70
–80
–90
HD2 AVG
HD3 AVG
0
200
400
600
FREQUENCY (MHz)
0.25
1
0.5
0.75
FREQUENCY (GHz)
1.25
1.5
5
–30
–100
0
6
VCC = 5V
T = 25°C
POUT = 8dBm/TONE
–10
HD2 & HD3 (dBc)
0
–5
–20
30
–110
S PARAMETER
S11
S21
S12
S22
5
42
26
To meet the demands of modern communications standards, and simplify
RF/IF designs, the LTC6431-15 and the
LTC6430-15 achieve best-in-class noise
and linearity at the lowest DC power
dissipation. They are easy to use, versatile, and guarantee performance
over a wide range of conditions. n
800
1000
4
3
2
VCC = 5V
T = 25°C
INCLUDES BALUN LOSS
1
0
0
200
400
600
FREQUENCY (MHz)
800
1000
October 2013 : LT Journal of Analog Innovation | 11