MRFIC1502 View Datasheet(PDF) - Motorola => Freescale

Part Name

Description

Manufacturer

MRFIC1502

Integrated GPS Downconverter

Motorola => Freescale 

circuit, the VCO is centered at 1527.7 MHz, and the gain of

the VCO is approximately 20 MHz/Volt.

The above performance is heavily dependent on the

capacitive structure that is used as the emitter bypass on pin

6. The total capacitance should be approximately 1.0 pF; that

can be achieved using either a discrete element or a

microstrip open circuited stub. The evaluation circuit shown

uses a 0.4 pF capacitor.

Phase–locked Loop Design

The VCO signal at 1527.68 MHz is divided by 40 to get the

second LO frequency of 38.19 MHz. In addition to providing

the LO to the second mixer, the 38 MHz signal is output

through a translator and is used as the sampling clock for the

digital correlator and decimator circuits. There is an addition-

al divide by two so the signal used by the phase detector is at

19.096 MHz. The reference input to the phase detector (pin 18)

has an input sensitivity of 400 mVpp minimum and 2.5 Vpp

maximum.

The loop filter design is the standard op–amp loop filter,

resulting in a type 2 second order loop. The layout of the

discrete components around the loop filter and VCO is very

critical to the performance of the phase–locked loop. Care

should be taken in routing the VCO control voltage line from

the output of the loop filter to the varactor diode.

The output of the divide by 40 is buffered by a clock

translator that converts the low level sine wave into a TTL

level square wave. The loading on the buffer is high so the

peak currents can reach as high as 50 mA with the maximum

load of 1.0 kΩ in parallel with 40 pF on the output. Therefore,

the translator has a dedicated VCC supply, pin 28, which

requires external bypassing and isolation. The recom-

mended bypassing uses two capacitors in parallel, a 47 pF

and a 0.01 µF capacitor.

Conclusion

The MRFIC1502 offers a highly integrated downconverter

solution for GPS receivers. For more detailed applications

information on GPS system design refer to application note

AN1610, “Using Motorola’s MRFIC1502 in Global Positioning

System Receivers.”

MOTOROLA RF DEVICE DATA

MRFIC1502

5

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