GaAs MMIC Based Control Components with Integral Drivers

Rev. V5

Application Note

M537

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Figure 5. Switching Parameters

Switching Transients

When a device is pulsed, the leading and falling edge

of

the

pulse

contains

many

higher

frequency

components.

The number of components and their

contribution is determined by the sharpness of the

pulse edges.

In addition, the MESFETs are non-

linear devices that generate harmonics of the incident

signals.

The fundamental switching structure filters

out some of these frequency components; however,

some component of these switching signals will be

present in the switch output. These signals can be

specified as a maximum voltage amplitude, or in

terms of spectral content with a maximum power

level.

Power Handling

Any RF signal has an associated RF voltage. This RF

voltage is sinusoidal with a period related to the

frequency. As we have discussed, the MESFET can

be thought of as a voltage variable resistor. As the

RF voltage swing gets high enough, it can counteract

the DC bias level. If this happens, the resistance of

the MESFET will change. If the MESFET is in its low

resistance state, the increase in resistance appears

as increased loss.

Power handling is measured in many ways.

One

convenient benchmark is to specify the power level at

which the device has an additional 1dB of insertion

loss. This quantity, known as the 1dB compression

point, is not a maximum power level. It is a standard

measurement point that provides information about

the range of usable input power levels.

The 1dB compression point is frequency dependent,

reflecting the increasing period of the RF voltage with

decreasing frequency.

At very low frequencies, the

RF voltage is closely approximated as a DC signal

that more efficiently increases the resistance than a

high

frequency,

time-varying

signal.

The

1dB

compression point can be increased by increasing the

negative bias to the MMIC chip from –5 volts to –8

volts. This increases the DC bias level that the RF

voltage swing must counteract, increasing the power

level

required

to

get

the

same

amount

of

compression.

Another measure of power handling is the maximum

power, which is the highest input power that is

guaranteed

without

damage.

Implicit

in

this

specification is that the device is “cold switched”.

Cold switching means the device is switched in the

absence of power. This is an important consideration.

If the device changes state in the presence of power

(“hot switching”), the MESFETs will transition through

a resistance region where a significant portion of the

incident power will be dissipated. This can easily lead

to damage.

Intermodulation Intercept Points

Intermodulation intercept points are measured with

two equal amplitude input signals spaced closely

together in frequency. Since MESFETs are non-linear

devices, they exhibit “mixing” or frequency-generation

characteristics.

In any kind of receiving system,

signals that are generated internal to the device can

degrade

overall

system

performance.

This

measurement is used to determine the intercept

points (typically second and third order) of a device.

The intercept points are calculated values and are

defined as the power at which the intermodulation

product amplitude is equal to the input. This quantity

plays a major role in system dynamic range and

sensitivity performance.

The intercept points are

calculated as follows:

product (dBc)

n = order