Common Problems Of High-Power Measurements; Amplifiers With Agc Loops; On-Wafer Devices (Pulsed Measurements) - Agilent Technologies AN 1287-6 Application Note

Common problems of high-power
measurements
Some amplifiers contain an automatic-gain control
(AGC) loop. AGC loops attempt to keep the output
power of the amplifier constant by adjusting gain
to account for variations at the input of the ampli-
fier. Amplifiers with AGC loops can pose a problem
when measured on some network analyzers, espe-
cially at high power levels.

Amplifiers with AGC loops

Network analyzers sweep across the selected
frequency range while holding the power at the
desired constant power level. At the end of the
sweep, some network analyzers might blank, or
turn off the source, during the time it takes the
analyzer to reset itself and set up for another
sweep — the retrace time. Turning off the source is
a problem for amplifiers containing an AGC loop.
When an amplifier with an AGC loop is measured
and the input signal is turned off at the end of the
sweep, the AGC loop of the amplifier compensates
for the turned-off signal by increasing its gain to
keep the output power level constant. When the
sweep begins again, the network analyzer restores
its signal, and there is power again at the input of
the amplifier, which has ramped-up its gain. The
momentary high output power can cause damage
or destroy the amplifier or the analyzer's receiver
if the AGC loop cannot respond quickly enough.
Using a network analyzer that keeps its power
constant during retrace will reduce the possibility
of destroying the device or damaging the analyzer.
The 8753E keeps power constant except when
switching frequency bands at 300 kHz and 3 GHz.
The 8720E family of network analyzers allows the
user to keep the power constant or to blank during
retrace (the default is to have the power remain
constant). The 8720E family will briefly blank
during band changes at 2.55 GHz. The 8722D blanks
at 20.05 GHz as well. Be aware that this blanking
occurs if the analyzer sweeps across these bands.

On-wafer devices (pulsed measurements)

A problem commonly encountered when meas-
uring high-power on-wafer devices is the heating
up of the DUT. Devices on-wafer tend to heat up
quickly because they lack sufficient heatsinking.
This heating up requires that the temperature
of the DUT be controlled in some way since the
response of a DUT may change as the temperature
of the DUT increases. Two common ways to control
heating up is to pause the network analyzer between
measurements, or pulse the RF and/or DC bias
signals so that a constant DUT temperature is
maintained.
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