MLX71122_16 datasheet(26/59 Pages) MELEXIS

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MLX71122

300 to 930MHz FSK/FM/ASK Receiver

Datasheet

REVISION 014 – AUGUST, 2016

Page 26 of 59

390 10 71122 01

3.2.8. Voltage Controlled Oscillator (VCO)

The receiver includes an LC-based voltage controlled oscillator with an external inductor connected

between pins TNK1 and TNK2. Two internal varactor diodes in series combination are forming the tuneable

part of the oscillator tank. The oscillation frequency is adjusted by the DC voltage at pin LF. The tuning

sensitivity of the VCO is approximately 83MHz/V for 433MHz operation and 105MHz/V at 868MHz,

respectively. Since the cathodes of the varactors is tied to VCC, a higher voltage at pin LF or an Up-signal of

the PFD forces the capacitance to decrease and the VCO frequency to increase. With positive phase

detector polarity (PFDPOL = HIGH) the edges of the signal fFB will catch up to the reference signal fR (see Fig.

4).

The VCO current VCOCUR can be adjusted via the SPI in order to ensure stable oscillations over the whole

frequency range. Also the bias current of the output buffer can be increased with VCOBUF to enhance its

driving capability at the high frequency bands above 800MHz (see section 4.1.2). If the supply voltage is

lower than 5V it is possible to adjust the tuning range of the VCO with VCORANGE

(see 4.1.2). The minimum supply voltage is 3V.

Fig.10: VCO schematic

3.2.9. Loop Filter Calculation

The values of the loop filter elements depend on several parameters which are the:

peak charge pump current, ICP

VCO gain, KVCO

desired phase margin of the open PLL loop transfer function, ΦM

desired closed PLL loop bandwidth, fC

the feedback divider ratio, N

The peak charge pump current of the MLX71122 can be set to 100μA or 400μA (see 4.1.2).

The VCO gain depends on the external tank inductor L0 and the VCORANGE setting (see 4.1.2). The

following table gives typical VCO gain values of the evaluation boards together with the frequency band.

VEE

Charge Pump

External

Loop Filter

VCCVCO

TNK1

TNK2

VCC

VCOCUR

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