11

S and Noise Parameter Measurements

The position of the reference planes used for the mea-

surement of both S and Noise Parameter measurements is

shown in Figure 23. The reference plane can be described

as being at the center of both the gate and drain pads.

S and noise parameters are measured with a 50 ohm

microstrip test fixture made with a 0.010” thickness

aluminum substrate. Both source pads are connected

directly to ground via a 0.010” thickness metal rib which

provides a very low inductance path to ground for both

source pads. The inductance associated with the addition

of printed circuit board plated through holes and source

bypass capacitors must be added to the computer circuit

simulation to properly model the effect of grounding the

source leads in a typical amplifier design.

a matching network that will present

minimal associated circuit losses. The noise figure of the

completed amplifier is equal to the noise figure of the

device plus the losses of the matching network preceding

the device. The noise figure of the device is equal to Fmin

only when the device is presented with

coefficient of the matching network is other than

the noise figure of the device will be greater than Fmin

based on the following equation.

|

Zo (|1 +

Where Rn/Zo is the normalized noise resistance,

optimum reflection coefficient required to produce Fmin

and

actually presented to the device.

The losses of the matching networks are non-zero and

they will also add to the noise figure of the device creating

a higher amplifier noise figure. The losses of the matching

networks are related to the Q of the components and asso-

ciated printed circuit board loss.

higher frequencies and increases as frequency is lowered.

Larger gate width devices will typically have a lower

as compared to narrower gate width devices. Typically for

FETs, the higher

higher than 50

isrequiredforthedevicetoproduceFmin.

At VHF frequencies and even lower L Band frequencies,

the required impedance can be in the vicinity of several

thousand ohms. Matching to such a high impedance

requires very hi-Q components in order to minimize circuit

losses. As an example at 900 MHz, when air wound coils

(Q>100)are used for matching networks, the loss can still

be up to 0.25 dB which will add directly to the noise figure

of the device. Using multilayer molded inductors with Qs

in the 30 to 50 range results in additional loss over the air

wound coil. Losses as high as 0.5 dB or greater add to the

typical 0.15 dB Fmin of the device creating an amplifier

noise figure of nearly 0.65 dB.

Figure 23. Position of the Reference Planes.

Gate

Pin 2

Source

Pin 3

Drain

Pin 4

Source

Pin 1

Reference

Plane

Microstrip

Transmission Lines

Px

Noise Parameter Applications Information

The Fmin values are based on a set of 16 noise figure mea-

surements made at 16 different impedances using an ATN

NP5 test system. From these measurements, a true Fmin

is calculated. Fmin represents the true minimum noise

figure of the device when the device is presented with an

impedance matching network that transforms the source

impedance, typically 50

, to an impedance represented

by the reflection coefficient