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Cobham Tactical Communications and Surveillance
1916 Palomar Oaks Way, Suite 100, Carlsbad, CA 92008
Tel: 760-496-0055
FAX: 760-496-0057
GMS Inc. doing business as Cobham Tactical Communications and Surveillance
www.cobham.com/tcs
Technical Operations Manual
The most important thing we build is trust.
VEPA-2W Linear Power Amplifier
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REVISION HISTORY
Version
Date Author Comments
X1
10/01/11
Nathan Moore / Marcio Koptcke Initial release.
X2 02/15/12 Nathan Moore Release to include C2 Band
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TABLE OF CONTENTS
1. CAUTIONS AND WARNINGS ...................................................................................................................................................... 5
1.1. ELECTROMAGNETIC RADIATIONS ................................................................................................................................................. 5
1.2. SUPPLY VOLTAGE AND CURRENT .................................................................................................................................................... 5
1.3. PA ENABLE (Fast Switching) ................................................................................................................................................................. 5
1.4. RF LOAD TERMINATION (50 Ohm)ΩΩ ....................................................................................................................................... 5
1.5. HEATSINK .......................................................................................................................................................................................................... 6
1.6. RF OVERDRIVE ............................................................................................................................................................................................... 6
1.7. OVER TEMPERATURE AUTOMATIC SHUTDOWN.................................................................................................................. 6
1.8. POWER MEASUREMENT .......................................................................................................................................................................... 6
2. ABSOLUTE MAX RATINGS ............................................................................................................................................................ 7
3. SPECIFICATIONS ................................................................................................................................................................................ 7
4. POWER AMPLIFIER FEATURES ................................................................................................................................................. 9
4.1. MODE OF OPERATIONS ........................................................................................................................................................................... 9
4.1.1. HIGH LINEARITY MODE (OR HIGH MER MODE) .......................................................................................................... 9
4.1.2. HIGH EFFICIENCY MODE (OR LOW CURRENT MODE) ............................................................................................ 9
4.2. PA ENABLE (Fast Switching Capable) ............................................................................................................................................. 9
4.3. VSWR, RF OVERDRIVE AND OVER-TEMERATURE PROTECTION CIRCUITS ...................................................... 9
5. OPERATIONAL INSTRUCTIONS ............................................................................................................................................ 10
5.1. EXTERNAL CONNECTIONS ................................................................................................................................................................. 10
5.2. CONNECTING & DISCONNECTING THE POWER AMPLIFIER .................................................................................... 11
5.3. APPLICATION EXAMPLES ................................................................................................................................................................... 11
5.3.1. BASIC CONFIGURATION ........................................................................................................................................................... 11
5.3.2. VEPA-2W / VMT COMBINATION .......................................................................................................................................... 13
5.4. USING A TRANSMITTER / PA COMBINATION .................................................................................................................... 13
5.4.1. The old FM approach .................................................................................................................................................................... 13
5.4.2. The NEW Digital Modulation approach ........................................................................................................................... 14
6. TYPICAL POWER AMPLIFIER PERFORMANCES ......................................................................................................... 16
6.1. GAIN @ 2W COFDM Output .............................................................................................................................................................. 16
6.2. Current @ 2W COFDM Output, 12 Vdc ....................................................................................................................................... 18
6.3. MER @ 2W COFDM Output ................................................................................................................................................................. 19
6.4. Small Signal S-Parameter (High Efficiency Mode) .............................................................................................................. 20
LIST OF TABLES
Table 1 – ABSOLUTE MAX RATINGS ............................................................................................................................................. 7
Table 2 – General Specifications: ..................................................................................................................................................... 7
Table 3 – Frequency Specific Specifications: .......................................................................................................................... 8
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LIST OF FIGURES
Figure 1 – VEPA-2W PA External Connections .................................................................................................................... 10
Figure 2 – PA on small Heat-sink with Test Cable and Isolator (Isolator Optional) ................................ 12
Figure 3 – VEPA-2W PA with VMT ................................................................................................................................................ 13
Figure 4 – LD Band Gain ....................................................................................................................................................................... 16
Figure 5 – LD Band Gain Change over Temp ......................................................................................................................... 16
Figure 6 – L2 Band Gain ........................................................................................................................................................................ 16
Figure 7 – L2 Band Gain Change over Temp .......................................................................................................................... 16
Figure 8 – W7 Band Gain ..................................................................................................................................................................... 16
Figure 9 – W7 Band Gain Change over Temp ....................................................................................................................... 16
Figure 10 – S2 Band Gain .................................................................................................................................................................... 16
Figure 11 – S2 Band Gain Change over Temp ...................................................................................................................... 16
Figure 12 – SK Band Gain .................................................................................................................................................................... 16
Figure 13 – SK Band Gain Change over Temp ...................................................................................................................... 16
Figure 14 – C2 Band Gain .................................................................................................................................................................... 17
Figure 15 – C2 Band Gain Change over Temp ...................................................................................................................... 17
Figure 16 – LD Band Current ............................................................................................................................................................. 18
Figure 17 – L2 Band Current ............................................................................................................................................................. 18
Figure 18 – W7 Band Current ........................................................................................................................................................... 18
Figure 19 – S2 Band Current ............................................................................................................................................................. 18
Figure 20 – SK Band Current ............................................................................................................................................................. 18
Figure 21 – C2 Band Current ............................................................................................................................................................. 18
Figure 23 – L2 Band MER ..................................................................................................................................................................... 19
Figure 24 – W7 Band MER .................................................................................................................................................................. 19
Figure 25 – S2 Band MER .................................................................................................................................................................... 19
Figure 26 – SK Band MER .................................................................................................................................................................... 19
Figure 29 – L2 Band S-Parameters ................................................................................................................................................ 20
Figure 30 – W7 Band S-Parameters .............................................................................................................................................. 20
Figure 31 – S2 Band S-Parameters ................................................................................................................................................ 20
Figure 32 – SK Band S-Parameters ............................................................................................................................................... 20
Figure 33 – C2 Band S-Parameters ............................................................................................................................................... 20
APPENDICES
APPENDIX 1 - ACRONYMS ................................................................................................................................................................. 21
REFERENCES
( 1 ) MIL-STD-810 Department of Defense Test Method Standard for Environmental Engineering
Considerations and Laboratory Tests
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1. CAUTIONS AND WARNINGS
1.1. ELECTROMAGNETIC RADIATIONS
There is no consensus in the scientific community about the potential harmful effect of
electromagnetic radiations. This power amplifier will radiate an average 2 W power with peak powers
14 dB higher (when operating with a COFDM –DVBT transmitter). It is recommended that the operator
minimizes exposure to electromagnetic radiations by:
A) Avoid direct proximity to the antenna when the transmitter is turned ON.
B) Use a low leakage attenuator when the transmitter is operated for testing on a bench.
1.2. SUPPLY VOLTAGE AND CURRENT
The amplifier is designed for a wide input voltage (9-32 Vdc). Make sure that the power supply has a
voltage within this range and is capable of providing the current needed with some margin. At 12 Vdc
the amplifier will draw less than 1.6 A in High Linearity mode when the input power is properly set to
deliver 2 W. The efficiency of the amplifier remains about constant as the voltage changes. Therefore
the current draw at 24 Vdc will be about half and the current draw at 9 Vdc will be about 30% more.
Make sure your power supply is capable of delivering the current needed. Current limiting should be
avoided as it may induce oscillatory voltage fluctuations that might damage the amplifier.
The Power Amplifier is reversed polarity protected.
1.3. PA ENABLE (Fast Switching)
The VEPA-2W incorporates a fast switching PA Enable (PIN-5) function. This PIN is LVTTL compatible
(3-5Vdc) and must be tied logic high for the VEPA-2W to operate. The +5VDC output (PIN-4) can be
tied to the PA_ENABLE (PIN-5) if the user does not need to access the PA Enable functionality. The
voltage on this pin should not exceed 5.5VDC.
1.4. RF LOAD TERMINATION (50 Ohm)ΩΩ
Always terminate properly the RF output port into a well matched antenna. The amplifier has an
internal protection circuit which will cause an internal input switch to open and disconnect the RF input
when there is a RL lower than 5-6 dB (VSWR protection is not available in C2 band VEPA). The switch
will remain in an open latched condition until the poor load condition is corrected and the DC input
power is cycled OFF and ON (the external LED will monitor the protection circuit activation, will remain
green during normal operation and turn red when the PA has entered protection mode, also FAULT 1
(PIN2) will toggle logic high). The protection circuit may not be 100% effective in protecting the
amplifier for infinite mismatch at full Pout at all angles of reflection.
A well matched antenna should always be used. The reflected power will decrease the link range and
will be dissipated internally in the PA, thus increasing the size of the heat-sink needed.
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On the bench make sure to use a 20 dB attenuator rated for 2W (or more).
1.5. HEATSINK
Use an adequate heat-sink to keep the base plate temperature of the PA below 70
O
C (149 F).
For operation in extreme conditions or under direct sun exposure larger heat-sink and/or forced cooling
will be required. An over temperature detector sticker is applied externally and internally to the PA.
The sticker has five white dots which turn black when the temperature reaches 60, 65, 71, 77, 82
O
C.
(140, 149, 160, 171, 180 F). A black 71
O
C. (160 F) dot will void the warranty.
1.6. RF OVERDRIVE
The PA is designed to operate at up to 2W with COFDM signals having PAR ratios up to 14 dB. To
achieve the best efficiency the PA operates at the highest power that will allow the desired linearity
without damaging the devices. Therefore it is important not to overdrive the PA or damage might
occur.
This PA will survive and continue to operate with a momentary 2-3 dB overdrive. Higher overdrives will
trigger the protection circuit opening an internal RF switch which disconnects the RF input from the
rest of the circuit. The switch will remain in an open latched condition until the overdrive input is
removed and the DC input power is cycled OFF and ON (the external LED will monitor the protection
circuit activation, will remain green during normal operation and turn red when the PA has entered
protection mode, also FAULT 1 (PIN2) will toggle logic high). This circuit will provide a certain level of
protection from accidental overdrives. High and sudden overdrives will damage the PA and therefore all
care should be taken to avoid overdrive conditions. The operator should carefully set the input power
before connecting to the PA. During set up it is recommended that the output power be monitored
with a power meter.
1.7. OVER TEMPERATURE AUTOMATIC SHUTDOWN
The VEPA-2W is designed and specified to operate up to a maximum of 70°C baseplate temperature.
Operating the PA at higher temperatures than specified can damage the PA or cause the MTTF to drop
significantly. The VEPA-2W incorporates an over-temperature automatic shutdown feature. When the
baseplate temperature reaches ~75°C the PA will toggle to standby mode (~110mA current draw at
12Vdc) until the baseplate temperature drops below 70°C at which point the PA will resume normal
operation (the external LED will monitor the protection circuit activation, will remain green during
normal operation and turn red when the PA has entered protection mode, also FAULT 2 (PIN1) will
toggle logic high). No power cycle is required, although the operator should take steps to better heat-
sink the VEPA-2W if the protection state is reached during normal operation.
1.8. POWER MEASUREMENT
Using a reliable power meter is recommended for initial power set up. Choose a power head that can
handle the power to be measured. In most cases you will need to connect a suitable attenuator
between the PA RF output port and the Power meter. Using in-line power meter is not recommended
especially with antennas for initial set up because an antenna will create a standing wave. The power
meter readings will vary depending on the position of the power meter in the line and therefore it is
possible to have readings lower or higher than the actual power delivered by the PA.
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2. ABSOLUTE MAX RATINGS
Table 1 – ABSOLUTE MAX RATINGS
MAX MOMENTARY RF INPUT COFDM 6 dBm
MAX SUPPLY VOLTAGE 35 Vdc
MAX CONTINUOS COFDM PWR 2 W
MAX CONTINUOUS FM PWR 3 W
MAX PA_ENABLE VOLTAGE 6.5 Vdc
CAUTION: 2-3 dBm is the typical input PWR for operation at 2 W. The PA will survive a momentary 3 dB
overdrive. Larger overdrives will trigger the protection circuit (see section 4.2)
3. SPECIFICATIONS
Table 2 – General Specifications:
Parameter Specification
DC input voltage (Vdc) 9-32
Mechanical Dimension
(without heat-sink)
2.1 "L x 3.0 "W x 0.6 "H
5.3 cm x 7.6 cm x1.52 cm
Weight lbs 0.300
grams 136
Base-plate Temperature -20 °C to + 70 °C
Operating Humidity 95% Non-condensing
Control Connector (Header)
9pin 1.25mm JST
SM09B-GHS-TB
FAULT 2 PIN 1
FAULT 1 PIN 2
MODE SELECT PIN 3
+5 VDC (OUTPUT) PIN 4
PA ENABLE (3-5Vdc) PIN 5
Vcc (9-32Vdc) PIN 6
Vcc (9-32Vdc) PIN 7
GND PIN 8
GND PIN 9
RF Connectors SMA female
NOTES:
To select High Efficiency MODE (LOW CURRENT) leave Pin 3 open.
To select High Linearity MODE (HIGH MER) connect Pins 3 and 4. Or connect 3-5 Vdc to Pin3.
PA ENABLE is LVTTL compatible.
Pins 8 and 9 are internally connected to chassis.
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Table 3 – Frequency Specific Specifications:
Parameter Specification
Frequency (GHz) 1.4 - 1.6 1.7 - 1.85 1.7 - 2.4 1.99 - 2.5 2.2 - 2.4 2.2 - 2.7 4.4 – 5.0
Frequency Band LD L2 W7 S2 S1 SK C2
HIGH EFFICIENCY
MODE
1
:
MIN AVERAGE MAX MIN AVERAGE MAX MIN AVERAGE MAX MIN AVERAGE MAX MIN AVERAGE MAX MIN AVERAGE MAX MIN AVERAGE MAX
Liner Gain (dB)
29.0 30.3 31.7 29.8 30.6 31.1 28.5 29.7 30.8 29.0 30.2 30.9 29.8 30.5 30.9 29.2 30.5 31.6 29.3 30.7 32.8
Flatness (dB)
+/- 1.3 +/- 1.5
+/- 0.7 +/- 0.8
+/- 1.2
+/-
1.4
+/- 1.0 +/- 1.1
+/- 0.5 +/-0.6 +/- 1.2 +/- 1.3 +/- 1.3 +/- 1.6
Current @12 Vdc on
50 Load (A)
1.06 1.15
0.96 1.04
1.1 1.28
1.04 1.17
1.0 1.08 0.97 1.01 1.07 1.11
MER (dB)
2
21.7 28.0 25.3 26.9 19.3 21.9 21.0 23.0 22.2 23.4 19.7 22.8 22.6 25.1
EVM (%)
3
4.0 8.2
4.5 5.4
8.0 10.8 7.1 8.9 6.8 7.8 7.2 10.4 5.5 7.4
ACPR (-dB)
22.5 29.0
26.2 27.8
20.1 22.7 21.8 23.9 23.0 24.3 20.5 23.6 27 29.5
HIGH LINEARITY MODE
1
: MIN AVERAGE MAX MIN AVERAGE MAX MIN AVERAGE MAX MIN AVERAGE MAX MIN AVERAGE MAX MIN AVERAGE MAX MIN AVERAGE MAX
Liner Gain (dB)
28.7 30.1 31.8 29.4 30.6 31.6 28.7 30.2 31.5 29.7 31.0 31.8 30.8 31.3 31.8 29.2 31 32.5 30.8 32.3 34.3
Flatness (dB)
+/- 1.2 +/- 1.4
+/- 0.6 +/- 0.7
+/- 1.1
+/-
1.2
+/- 0.8 +/- 0.9
+/- 0.5
+/-
0.6
+/- 1.0 +/- 1.1 +/- 1.4 +/- 1.7
Current @12 Vdc on
50Ω Load (A)
1.33 1.44
1.18 1.28
1.32 1.52
1.25 1.4
1.2 1.3 1.15 1.22 1.32 1.39
MER (dB)
2
27.3 29.9 29.1 30.5 25.2 26.2 26.0 27.2 26.5 27.2 24.6 26.7 26.3 28.7
EVM (%)
3
3.2 4.3 3.0 3.5 4.9 5.5 4.4 5.0 4.4 4.7 4.6 5.9 3.7 4.8
ACPR (-dB)
28.5 31.0 30.0 32.1 26.2 27.3 27.1 28.5 27.3 28.5 25.5 27.8 30.5 33
Input RL (dB)
-25 -20 -20 -14 -14.7 -9.6 -12.3 -9.6 -12.3 -9.6 -16 -10.5 -13 -12
Output RL (dB)
-17 -12 -17 -12 -17.3 -9.6 -15.5 -10 -15.5 -10 -15.8 -10 -19 -15
Harmonic (dBm /MHz)
-29 -25 -35 -29 -37 -24 -40 -31 -45 -37 -40 -32 -34 -23
Gain Change (dB) (Over
Temperature -20 to 70 °C
+/- 0.25 +/- 0.5
+/- 0.25 +/- 0.5
+/- 0.25
+/-
0.5
+/- 0.25 +/- 0.5
+/- 0.25
+/-
0.5
+/- 0.25 +/- 0.5 +/- 1.25 +/- 1.5
Standby Current @ 12 Vdc
(A) (PA_ENABLE = 0V)
0.11 0.12 0.11 0.12 0.11 0.12 0.11 0.12 0.11 0.12 0.11 0.12 0.08 0.12
Switching Speed T
ON
(ns)
90 100 90 100 90 100 90 100 90 100 90 100 150 160
Switching Speed T
OFF
(ns)
40 50 40 50 40 50 40 50 40 50 40 50 100 110
RF Isolation (PA_ENABLE
= 0Vdc) (dB)
65 70 65 70 65 70 65 70 65 70 65 70 65 70
Notes:
1.
Measurements performed at 2 W output power (33 dBm), PA_ENABLE = 3.0Vdc
2.
MER measurements were performed using a Rhode & Schwartz EFA Test RCVR mod 2067 in Stationary mode. DVB-T test signal generated with an Agilent MXG Vector Signal Generator.
3.
EVM specification valid for QPSK modulation only.
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4. POWER AMPLIFIER FEATURES
4.1. MODE OF OPERATIONS
The VEPA-2 W amplifier was designed to give the best possible combination of Linearity and Efficiency.
There are applications where High Linearity is required (i.e. 64 QAM) and applications where lower
linearity is acceptable and High Efficiency is more important (e.g. 16 QAM and QPSK). The VEPA-2W
amplifier allows the operator to select the preferred mode of operation by external cable (cable can be
provided with jumpers installed according to the desired operation mode) or by installing jumpers in
the connector.
4.1.1. HIGH LINEARITY MODE (OR HIGH MER MODE)
To select this mode jumper pin 3 to pin 4. This will result in higher linearity with approx. 200 mA
increase in current (at 12 Vdc).
4.1.2. HIGH EFFICIENCY MODE (OR LOW CURRENT MODE)
To select this mode simply leave pin 3 of the connector open. This will result in a current about
200mA lower than the high linearity mode and about 2-3 dB decrease in MER when operating at
2W full power. There is almost no MER penalty when operating at 1W. Therefore for operation at
1W or lower the best MODE of operation is the HIGH EFFICIECNCY MODE
4.2. PA ENABLE (Fast Switching Capable)
The VEPA-2 W amplifier was designed have a LVTTL compatible enable pin (PIN 5). The VEPA-2W was
designed for fast switching applications with an ON time <100ns and an OFF time of <50ns (For C2
band: ON time <150ns and an OFF time of <100ns). To achieve this fast switching functionality the PA
will draw a small bias current when in standby (PA_ENABLE = 0V or LVTTL logic low), less than 120mA.
The VEPA-2W has >65dB isolation when in standby mode.
4.3. VSWR, RF OVERDRIVE AND OVER-TEMERATURE PROTECTION CIRCUITS
The VEPA-2W amplifier implements a circuit which senses VSWR and overdrive. When the RL of the
load is lower than 7-8 dB and/or when the input overdrive exceeds 3 dB the protection circuit triggers a
RF switch placed in series with the RF input line (VSWR protection is not available in C2 band VEPA).
The switch remains open until the faulty condition is removed and the DC input power is cycled OFF
and ON. In Normal operation the green LED (see fig. 1) is ON. When the RF switch opens the LED color
turns red, as well as the FAULT 1 (PIN2) toggling logic high.
CAUTION: The protection circuit increases the chances for the PA to survive excessive VSWR and/or
overdrive is NOT 100% protection in all cases. An output mismatch that occurs at the output port
because the antenna match is degraded (poor antenna, antenna degraded by rain, antenna placed in
proximity of metal objects etc.) will likely trigger the protection circuit and protect the amplifier from
damage. But a sudden open or short at full power could damage the amplifier. Similarly the protection
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circuit will be effective in protecting from limited overdrives. Sudden large overdrives will damage the
amplifier.
The VEPA-2W also incorporates an OVER TEMPERATURE PROTECTION circuit. This circuitry operates
slightly differently than the VSWR and OVERDRIVE protection in that it does not need a power cycle to
correct. The OVER TEMPERATURE shutdown will allow normal operation when the VEPA-2W baseplate
temperature has dropped by 5°C from the trip point of 75°C. In Normal operation the green LED (see
fig. 1) is ON. When the RF switch opens the LED color turns red, as well as the FAULT 2 (PIN2) toggling
logic high. If this fault is being toggled during normal operation of the amplifier, the operator needs to
take steps to improve the heat-sinking of their system.
5. OPERATIONAL INSTRUCTIONS
Read this section carefully and the section in Section 1 (CAUTIONS AND WARNINGS)
before operating the Power Amplifier
.
5.1. EXTERNAL CONNECTIONS
Figure 1 – VEPA-2W PA External Connections
9 pin 1.25mm JST I/O
SM09B-GHS-TB
RF OUT
Antenna Port
MODE
RF INPUT
GREEN LED
DC PWR indicator
and protection circuit
status
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5.2. CONNECTING & DISCONNECTING THE POWER AMPLIFIER
When Connecting the Power Amplifier, follow the following steps for best results and to avoid damaging
the amplifier.
1) Apply the load to the amplifier (make sure a good load is ALWAYS present when working with any
power amplifier).
For operational setup of the amplifier, use an acceptable antenna with a good VSWR (< 1.5:1
recommended). Reflected power will decrease the efficiency of the link and will also cause extra heat
to be dissipated internally to the PA. For bench tests use a 20 dB attenuator rated at least at 2 W
connected directly at the RF Out port.
2) Measure the signal level from your transmitter or signal generator before connecting it to the input
port. EXCESSIVE INPUT POWER WILL DAMAGE THE PA. The input level should be conservatively set
lower and then raised while monitoring the output power and/or current.
When using a COFDM transmitter such as COBHAM/GMS VMT, SDMT, M2T or M2TE YOU MUST
RECALIBRATE THEM TO REDUCE THE OUTPUT POWER. It is necessary to attenuate the power
internally to the transmitter rather than placing an attenuator at the transmitter output because the
linearity of the transmitter will be better. Consult the transmitter’s manual to reduce the output power.
If needed add external attenuators to the transmitter or signal generator until the desired power is
obtained. Start with an input power set at -5 dBm.
3) Set your supply at the desired operation voltage within the 9-32 Vdc range. Make sure the power
supply is capable of supplying the current required. The VEPA-2W will draw ~110mA when in standby
mode (LVTTL logic low). The VEPA-2W PA draws about 1.5 A at 12 Vdc when in 2W High Linearity
mode, with the PA_ENABLE logic high (Current will vary with frequency band). At lower voltages the
current will be higher. A 2A power supply will suffice for all voltages from 9-32 Vdc.
4) Connect the power supply to the PA using adequate AWG wire, connect the PA_ENABLE pin to 3.0Vdc
(LVTTL compatible, should not exceed 5.5V). PA_ENABLE voltage should not be applied before the
main supply voltage. Keep in mind that the voltage drop in the wire will reduce the voltage at the PA
supply terminal and therefore will cause an increase in current. The current consumption indicated in
the specification sheet is valid when the voltage is measured at the PA input terminals.
5) Connect a power meter at the RF port (after the attenuator). If a power meter is not available use the
current meter to get an (approximate) indication of power.
6) Set the mode of operation desired. If high efficiency mode is desired leave the connector pin 3 open.
If High linearity mode is desired jumper pin 4 to pin 3.
7) Connect the RF input to the Power Amplifier. The power should be less than desired. If the power
exceeds 2W an error was made in the setup of input power: DISCONNECT THE RF INPUT
IMMEDIATELY and recheck the power at the output of the transmitter.
5.3. APPLICATION EXAMPLES
5.3.1. BASIC CONFIGURATION
Fig. 2 shows the VEPA-2W PA mounted on a small heat-sink which is adequate for laboratory
testing and operation in mild environment. With this heat-sink the temperature will raise about
25
O
C (45 F) when the heat-sink is kept with the fins down on a test bench.
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Figure 2 – PA on small Heat-sink with Test Cable and Isolator (Isolator Optional)
This heat-sink is typically shipped to customer for evaluation purposes. Fig. 2 shows also a test
cable which includes a switch to select the mode of operation (High Linearity or High Efficiency).
The picture also shows an external isolator, the isolator is an optional addition, and is generally not
needed. The C2 band VEPA-2W incorporates an internal isolator.
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5.3.2. VEPA-2W / VMT COMBINATION
Fig. 3 shows the VEPA-2 W PA in combination with a VMT (Very Miniature Transmitter). This
combination offers a full 2W transmitting capability in a very compact package.
Figure 3 – VEPA-2W PA with VMT
CAUTION: Changes to the VMT attenuator settings should be made only by well-trained
operators. Before making changes to the VMT attenuator settings disconnect the RF input to the
PA and measure the output power of the VMT. In normal operation the VMT RF output power
should be about 3 dBm or less.
For a detailed explanation on the use of the VMT transmitter refer to the applicable manuals.
5.4. USING A TRANSMITTER / PA COMBINATION
5.4.1. The old FM approach
The methodology of interfacing a digital transmitter with a linear PA is somewhat different that
the methodology used for constant envelope type modulation such as FM.
In the days of FM modulation the last stage of a transmitter was driven into compression to obtain
the best efficiency. The nonlinearity caused by this approach did not affect the quality of the link
because the information was contained in the frequency and deviation of the carrier and not in its
amplitude and phase. The only undesirable effect was an increase in harmonics emission, problem
which was solved by using a low pass filter.
VEPA-2W PA
VMT
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In the FM world it made sense to mate a transmitter operating at its RF rated output power to an
external Power Amplifier in order to extend its range of operation. There was no need to modify
the Power output of the transmitter if the PA had the correct gain. For example GMS sold a 200
mW FM transmitter and an additional 10 dB gain amplifier to boost the Power to 2 W. The
operator had only to connect the TX output to the PA RF input. THIS APPROACH IS NO LONGER
VALID FOR DIGITAL MODULATION.
5.4.2. The NEW Digital Modulation approach
Cobham/GMS transmitters are designed for operation with COFDM–DVBT type signal. In this type
of modulation the information is contained in the phase and amplitude of multiple carriers which
are transmitted simultaneously. These carriers, being spaced at regular frequency interval,
combine in amplitude thus creating very large amplitude peaks. The result is that the Peak to
Average Amplitude Ratio (PAR) is very large (typically limited to 14 dB by the DAC in the
modulator). Unlike the FM world neither the amplitude nor the phase can be distorted or the
quality of the information will be degraded.
In order to avoid this distortion, in the digital modulation world the Power Amplifier operates at a
certain level of back OFF meaning that the average power transmitted is several dB lower than the
P1dB of the various stages of the amplifier. But this has a price in term of power consumption and
therefore transmitters are always designed with the least amount of back off which will give the
required linearity.
For the sake of example a 200 mW (23 dBm) amplifier deigned to perform with a 25 dB MER will
use as final stage a device which has a P1 dB around 29 dBm. If we had chosen to use a device
with greater P1 dB the MER would have been greater at the expense of extra current.
It would be convenient if we could couple this transmitter with a 10 dB gain amplifier and obtain a
2W transmitting system, but this is NOT the approach that provides the best results. Let’s say that
out amplifier has an MER of 28 dB. If we connect a 25 dB MER signal into it, the 3
rd
order product
and harmonics of the transmitter will further re-grow in the PA. The resulting MER will be lower
than 25 dB and therefore the system will no longer be capable of transmitting higher order
constellations. In order to obtain a resulting 25 dB MER (same MER as the transmitter by itself) we
would be forced to overdesign the transmitter by considerably degrading its efficiency.
The proper approach to solve this problem and obtain the best linearity (MER) and efficiency in all
cases is to:
1) Make a transmitter which has 25 dB MER when operating at 200 mW
2) Use a PA with higher gain and an MER a little higher than 25 dB
3) When the transmitter is used in combination with the PA, the transmitter Power is “Backed OFF”
(i.e. an internal attenuator is set higher).
By doing so the Transmitter is operating at a larger Back OFF from the P1 dB and the linearity
increases. (typically around 31 dB MER). With a 31 dB MER input the PA will see little third order
product at its input and the total MER combination will be higher than 25 dB as required for best
system performance.
Cobham/GMS transmitters are factory calibrated with the proper amount of Back OFF when
shipped in combination with the PA.
100-M0167X2 15 of 21
GMS Inc. doing business as Cobham Tactical Communications and Surveillance
www.cobham.com/tcs
CAUTION: WHEN COMBINING A TRANSMITTER WITH A PA FOR THE FIRST TIME THE
CUSTOMER SHOULD ALWAYS MAKE SURE THAT THE POWER OUTPUT OF THE TRANMITTER IS
COMPATIBLE WITH THE MAX ABSOLUTE RATING OF THE POWER AMPLIFIER.
100-M0167X2 16 of 21
GMS Inc. doing business as Cobham Tactical Communications and Surveillance
www.cobham.com/tcs
6. TYPICAL POWER AMPLIFIER PERFORMANCES
This chapter gives typical power amplifier performances for the VEPA-2W frequency bands. The PA has repeatable performances from unit to unit.
6.1. GAIN @ 2W COFDM Output
Figure 4 – LD Band Gain
Figure 5 – LD Band Gain Change over
Temp
Figure 6 – L2 Band Gain
Figure 7 – L2 Band Gain Change over
Temp
Figure 8 – W7 Band Gain
Figure 9 – W7 Band Gain Change over
Temp
Figure 10 – S2 Band Gain
Figure 11 – S2 Band Gain Change over
Temp
Figure 12 – SK Band Gain
Figure 13 – SK Band Gain Change over
Temp
100-M0167X2 17 of 21
GMS Inc. doing business as Cobham Tactical Communications and Surveillance
www.cobham.com/tcs
Figure 14 – C2 Band Gain
Figure 15 – C2 Band Gain Change over
Temp
100-M0167X2 18 of 21
GMS Inc. doing business as Cobham Tactical Communications and Surveillance
www.cobham.com/tcs
6.2. Current @ 2W COFDM Output, 12 Vdc
Figure 16 – LD Band Current
Figure 17 – L2 Band Current
Figure 18 – W7 Band Current
Figure 19 – S2 Band Current
Figure 20 – SK Band Current
Figure 21 – C2 Band Current
100-M0167X2 19 of 21
GMS Inc. doing business as Cobham Tactical Communications and Surveillance
www.cobham.com/tcs
6.3. MER @ 2W COFDM Output
Figure 22 – LD Band MER
Figure 23 – L2 Band MER
Figure 24 – W7 Band MER
Figure 25 – S2 Band MER
Figure 26 – SK Band MER
Figure 27 – C2 Band MER
100-M0167X2 20 of 21
GMS Inc. doing business as Cobham Tactical Communications and Surveillance
www.cobham.com/tcs
6.4. Small Signal S-Parameter (High Efficiency Mode)
Figure 28 – LD Band S-Parameters
Figure 29 – L2 Band S-Parameters
Figure 30 – W7 Band S-Parameters
Figure 31 – S2 Band S-Parameters
Figure 32 – SK Band S-Parameters
Figure 33 – C2 Band S-Parameters
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