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IMPORTANT NOTE FOR USERS OF
ACTIVE ANTENNAS
Radiated emissions measurements are sometimes
made with active antennas. Some of these antennas are not compatible with Sonoma
Instrument amplifiers and are likely to damage them. Please read this document
to familiarize yourself with the problem and prevent damage to your amplifier.
(Download this document in Adobe PDF format)
An active antenna consists of an antenna element and a battery powered built-in amplifier.
Theoretically,
there should be no need for an additional external amplifier, but in practice
many users prefer some additional gain.
Sonoma Instrument amplifiers are low noise,
high gain devices designed to handle very weak signals. The incompatibility with
some active antennas arises from the switching transients produced by the active
antenna when it is powered on or off.
The transient signals from one such antenna are
shown below. Figures 1 and 2 show the antenna output voltage into a high
impedance (1 MOhm) oscilloscope input. The vertical scale is set to 2 V/div.
The
switching transients shown below are by no means the strongest examples available.
Fig. 1 The power-on transient reaches almost 6
Volts.
Fig. 2 The power-off transient adds another 3
Volts to the previous transient, for a total of 9 Volts at the peak.
The following figures show the power-on and
power-off transients when the same active antenna is loaded with a 50 Ohm oscilloscope input.
It is important to note that the amplifier input impedance
approximates 50 Ohm only while the amplifier is operating in its linear range.
As
the amplifier becomes overloaded, its input impedance will change and the
voltage at the amplifier input will be closer to that shown in figures 1 and 2.
In addition, most Sonoma Instrument amplifiers do not present a 50 Ohm input
resistance at d.c.
Fig. 3 Power-on transient, 50
Ohm oscilloscope input.
Fig. 4 Power-off transient,
50 Ohm oscilloscope input.
It is in theory possible to follow a sequence of power-up and
power-down procedures which will protect the amplifier from the
transients. In practice, it is difficult to guarantee that the procedures
will be accurately followed at all times. The preferred method of
protection is by means of the Agilent 11947A Transient
Limiter. It is
specified for operation between 9 kHz and 200 MHz. Shown below are the
power-on transients after they have passed through the limiter. The
power-off transients are negligible. Note the great reduction in peak
amplitude and duration.
Fig. 5 Power-on transient with Agilent 11947A limiter and 1
MOhm oscilloscope input.
Fig. 6 Power-on transient with Agilent 11947A limiter and 50 Oscilloscope input.
Active antennas
are not recommended for use with Sonoma Instrument amplifiers, except
in conjunction with a protective device such as the Agilent 11947A Transient
Limiter. Damage caused by excessive signals at the amplifier input is not
covered under the warranty.
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