MICRF405 datasheet(22/46 Pages) MICREL

22 / 46 page

Micrel

MICRF405

April 2006

M9999-041906

(408) 955-1690

VCO

Adr

Data

A6..A0

0001011

LowBatt_en=1

Freq_Band1=0

Freq_Band0=1

VCO_freq2=0

VCO_freq1=1

VCO_freq0=1

Modulation1=1

Modulation0=0

0010110

INT_LF_TEST=0

VCO_IB2=0

VCO_IB1=0

VCO_IB0=0

VCO_by=0

OUTS2=0

OUTS1=0

OUTS0=0

0011000

VCO_freq_O2

VCO_freq_O1

VCO_freq_O0

VC_HI

VC_LO

LOW_BATT

The VCO has no external components. It oscillates

at 1.8 GHz and is divided by 2, 4 and 6 in the 900

MHz, 450 MHz or 315MHz band respectively. This

divide ratio is controlled by the Freq_Band[1:0] bits,

as shown in Table 12.

FreqBand1 FreqBand0

Comments

RF frequency 290-325 MHz

RF frequency 430-490 MHz

RF frequency 860-980 MHz

Table 12. Frequency Band.

The VCO_IB setting is automatically set when

VCO_IB[2:0]=0. If VCO_IB[2:0] are programmed <>

0, it will overrule the automatic setting. Default and

recommended

for

automatic

settings,

VCO_IB[2:0]=0 for all frequencies.

The bias bits will optimize the phase noise, and the

frequency bits will control a capacitor bank in the

VCO. The tuning range, the RF frequency versus

varactor voltage, is dependent upon the VCO

frequency setting, and is shown in Figure 10. When

the tuning voltage is in the range from 1.2 to 1.6V,

then the VCO gain is at its maximum, approximately

60-80 MHz/V. It is recommended that the varactor

voltage is kept within this range. Table 15 shows the

recommended settings of VCO_freq and FreqBand

for various frequencies.

To ensure correct settings over variations, a circuit

monitoring the varactor voltage on start up is added.

When the PLL has locked, or after a timeout has

occurred if it doesn’t lock, this circuit will control the

varactor voltage. This will be performed if either the

VCO_Fr_Chk

VCO_Fr_Auto

bit

set.

VCO_Fr_Chk set will set the interrupts VC_HI, in

case of a too high VCO_freq setting creating a too

high varactor voltage, and VC_LO, in case of a too

low VCO_freq setting creating a too low varactor

voltage. If the VCO_Fr_Auto bit is set, then the

transmitter will, if the varactor voltage is out of range,

change the programmed VCO_freq setting until the

voltage is within the range. A new setting will remain

active as long as power is on, VCO_Fr_Auto is set

and the programmed VCO_freq[2:0] bits are not

altered. The tuned VCO_freq setting of the

automatic tune circuit can be read out in the interrupt

and VCO_Fr_Auto are set, each step is done by the

automatic tuning circuit that will be flagged with a

VC_LO or VC_HI interrupt. The limits of varactor

voltage used by this control circuit are between

350mV and 350mV below AVDD. The check is

performed

when

entering

mode.

PA_LDc_en=1, the control will also be executed for

each programming creating an internal load pulse

(load_en=1) while staying in TX mode. This means

that when changing frequency, the MICRF405 will

check that the VCO_Freq[2:0] settings are correct

for the new frequency. Refer to Table 13 and Table

14 for further details.

The MICRF405 must be programmed with the

recommended settings for FreqBand and VCOfreq,

as given in Table 15, even if the VCO_Fr_Chk and

VCO_Fr_Auto are enabled. This is needed to give

the VCO the correct starting values.

The VCO can be bypassed by applying a differential

local oscillator (LO) signal to the device on pin

CPOUT and VARIN. A resistor of 18kΩ to ground

and a series capacitor of 47pF are needed on both

pins for proper biasing. The bit VCO_by must be set

to 1.

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