AD6190

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PRODUCT DESCRIPTION

The AD6190 is a complete RF/IF transceiver for operation in

the 902 MHz–928 MHz Industrial, Scientific and Medical

(“ISM”) frequency band. Together with a suitable spread-

spectrum controller, the AD6190 can be used to design

a spread-spectrum system compliant with FCC “Part 15”

(47CFR15.247) regulations. The AD6190 is a fully compatible

companion chip to the Zilog Z87L00 “ZPhone” frequency-

hopping spread-spectrum controller.

The AD6190 includes a receive path of LNA, image-reject

mixer, IF amplifier and limiter amplifier with RSSI. The trans-

mit path accepts a 10.7 MHz IF input signal, and uses image-

reject upconversion to the 902 MHz–928 MHz band. Frequency

control is achieved using an on-chip VCO and dual-modulus

prescaler connected to an inexpensive low frequency PLL for

channel selection and frequency hopping.

Additionally, an on-chip voltage regulator stabilizes the VCO to

prevent LO pushing due to power supply variations.

APPLYING THE AD6190

Receive Signal Path

The AD6190 Low Noise Amplifier (LNA) and image-reject

mixer together provide downconverter with a total gain of 24 dB

and a typical Noise Figure (NF) of 4.2 dB.

The LNA input port exhibits an impedance of 320-j61 at

915 MHz. In order to provide an optimum match to a 50

Ω

source, the network shown in Figure 8 should be used.

RF IN

82pF

2.2pF

15nH

50

AD6190

Figure 8. LNA Input Matching Circuit

The frequency plan of the AD6190 provides the lowest possible

RF implementation cost. A single conversion design is used with

a 10.7 MHz IF to take advantage of the very low cost filters

available. However, since the 902 MHz–928 MHz band is wider

than twice the IF, it is possible that undesired in-band signals

will be mixed down to the IF. These images could cause inter-

ference to the desired signal. It is thus necessary to provide

tunable filtering before the receive mixer, or some other ap-

proach to eliminate interference from image signals.

In the AD6190, a technique known as “image-reject” (or SSB)

mixing is used. This technique suppresses image interference by

using a pair of mixers with quadrature local oscillators. See

Figure 9.

90

LO

90

IF

OUT

RF

IN

AD6190

Figure 9. Image-Reject Mixer

applied to two mixers in parallel. These mixers are driven by

local oscillator signals in quadrature. The mixer outputs at the

two mixer IF ports contain both the desired signal and the

image signal. However, the outputs of the two mixers are in

quadrature (shifted 90 degrees relative to each other). The

outputs of the two mixers are then shifted another 90 degrees

relative to each other in a phase-shift network. The two mixer

outputs thus contain the desired signal and the image signal

exactly 180 degrees out of phase. By adding (or subtracting) the

two signals, the undesired image signals cancel, the desired

signal components add, and image-rejection occurs. Local oscil-

lator leakage is suppressed by the use of doubly-balanced mixers.

The quality of the image rejection is a function of the phase and

amplitude matching of the quadrature branches of the LO and

IF phase-shift networks. In the AD6190, image-rejection is

typically 33 dB.

The mixer output that drives the input side of the first

10.7 MHz filter should also be connected through a parallel

RLC network of 6.8 pF, 1 k

Ω, and 7 pF to the power supply to

match the 330

Ω filter impedance.

The 10.7 MHz IF signal is then filtered and amplified by a

24 dB fixed gain. The output of this stage is further filtered, and

applied to a 6-stage limiting amplifier. The limiter output signal

is typically 450 mV p-p into a 30 k

Ω, 30 pF load, with a dc

offset level of approximately 1.76 V dc.

All 10.7 MHz IF filters are assumed to be standard 330

Ω imped-

ance ceramic types. The AD6190 RX IF signal chain and TX IF

input includes internal matching resistors for this impedance.

When used with the Zilog Z87L00 Spread-Spectrum Controller

IC, the 10.7 MHz IF signal contains the received data encoded in

FSK modulation with approximately a

±33 kHz deviation. The

Z87L00 performs the FSK demodulation in the digital domain.

The RSSI (Received Signal Strength Indicator) signal represents

the strength of the received signal, linear in dB, and scales with

supply voltage. With a 3.3 V supply (through a 10

Ω resistor on

the VCCIF pin), an RF signal level of –100 dBm at the LNA

input will produce an RSSI voltage of approximately 900 mV.

The RSSI voltage will increase with increasing RF input level,

at approximately 22 mV/dB to approximately 2.4 V at

–30 dBm input. The RSSI output voltage remains above 2.4 V

for input levels up to +15 dBm.