100-M0167X2
9 of 21
GMS Inc. doing business as Cobham Tactical Communications and Surveillance
www.cobham.com/tcs
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