MLX71122_16 Datasheet, PDF (26/59 Pages) Melexis Microelectronic Systems – 300 to 930MHz FSK/FM/ASK Receiver

MLX71122
300 to 930MHz FSK/FM/ASK Receiver
Datasheet
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 f_FB will catch up to the reference signal f_R (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.
External
VCC
Loop Filter
TNK1
LF TNK2 VCCVCO
5k
5k
Charge Pump
VCOCUR
VEE
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, I_CP
-
VCO gain, K_VCO
-
desired phase margin of the open PLL loop transfer function, Φ_M
-
desired closed PLL loop bandwidth, f_C
-
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.
REVISION 014 – AUGUST, 2016
390 10 71122 01
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