Circuit Note

CN-0518

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Rev. 0 | 2 of 8

CIRCUIT DESCRIPTION

RF LOW NOISE AMPLIFIER

The CN0518 uses cascaded HMC376 low noise amplifiers for its

RF signal path, achieving an overall gain of approximately 25

dB. The HMC376 is a gallium arsenide (GaAs), pseudomorphic

high electron mobility transistor (pHEMT), high MMIC, low noise

amplifier that operates from 700 MHz to 1000 MHz.

Figure 2 shows the typical S-Parameter performance, with a typical

gain of 15 dB and return losses of more than 10 dB throughout

the entire frequency range. The HMC376 has a typical noise figure

of 0.7 dB, a 1 dB compression point (P1dB) of 21.5 dBm, and a

third-order intercept point (OIP3) of 36 dBm.

Figure 2. HMC376 Typical S-Parameters

Impedance Matching Network

The HMC376 RF input and output ports are internally matched for

a single-ended 50 Ω impedance. This internally matched topology

allows the HMC376 to be readily ported between any printed

circuit board material regardless of its dielectric constant, thickness,

or composition. The RFOUT also has an integrated DC-blocking

capacitor that eliminates the need for an external capacitor. The

only requirement needed is an inductor in parallel with the RF input

with respect to the ground.

Figure 3 shows the basic configuration and implementation of two

cascaded HMC376 amplifiers.

Figure 3. Basic Connections for Cascading HMC376 Amplifiers

RF SAW FILTER

The first stage amplifier output of the CN0518 is filtered by a

surface wave acoustic (SAW) filter, which eliminates unwanted

out-of-band amplification. When selecting a filter, a balance must

be struck between the band flatness and the out-of-band rejection.

SAW filters are also a source of insertion loss, which reduces the

overall gain of the signal chain and needs to be considered during

selection.

The filter used has a pass band of 902 MHz to 928 MHz, with a

return loss of 21 dB and an insertion loss of 1.9 dB at 915 MHz

center frequency.

RF DIRECTIONAL COUPLER

The CN0518 includes a multilayer RF directional coupler. This

device has an operating frequency of 698 MHz to 2.62 GHz with a

coupling factor of 25.5 at 915 MHz. It has an input and output port

impedance of 50 Ω and a typical insertion loss of 0.2 dB.

RF SWITCH

The ADG901 is a wideband RF switch that uses a CMOS process

to provide high isolation and low insertion loss. It is an absorptive

(matched) switch with 50 Ω terminated shunt legs. The switch

enables the user to pass DC signals up to 0.5 V without the use of

a DC-blocking capacitor.

The ADG901 operates at DC up to 4.5 GHz at -3 dB frequency. At

the 915 MHz center frequency, this device has a typical insertion

loss of approximately 1.8 dB at "ON state" as shown in Figure 4,

and a typical isolation loss of approximately 30 dB at "OFF state" as

shown in Figure 5.

Figure 4. ADG901 Insertion Loss Performance at ON State