adf7012 Analog Devices, Inc., adf7012 Datasheet - Page 13

• Manufacturer Part Number: adf7012
• Description: Multichannel Ism Band Fsk/gfsk/ook/gook/ask Transmitter
• Manufacturer: Analog Devices, Inc.

The output buffer to CLK
the function register high. On power-up, this bit is set high.
The output buffer can drive up to a 20 pF load with a 10% rise
time at 4.8 MHz. Faster edges can result in some spurious
feedthrough to the output. A small series resistor (50 Ω) can be
used to slow the clock edges to reduce these spurs at F
LOOP FILTER
The loop filter integrates the current pulses from the charge
pump to form a voltage that tunes the output of the VCO to the
desired frequency. It also attenuates spurious levels generated by
the PLL. A typical loop filter design is shown in Figure 29.
In FSK, the loop should be designed so that the loop bandwidth
(LBW) is a minimum of two to three times the data rate.
Widening the LBW excessively reduces the time spent jumping
between frequencies, but results in reduced spurious
attenuation. See the section Tips on Designing the Loop Filter.
For OOK/ASK systems, a wider loop bandwidth than for FSK
systems is desirable. The sudden large transition between two
power levels results in VCO pulling (VCO temporarily goes to
incorrect frequency) and can cause a wider output spectrum.
By widening the loop bandwidth a minimum of 10 × data rate,
VCO pulling is minimized because the loop settles quickly back
to the correct frequency. The free design tool ADIsimPLL™ can
be used to design loop filters for the ADI family of transmitters.
VOLTAGE-CONTROLLED OSCILLATOR (VCO)
The ADF7012 features an on-chip VCO with an external tank
inductor, which is used to set the frequency range. The center
frequency of oscillation is governed by the internal varactor
capacitance and that of the external inductor combined with the
bond-wire inductance. An approximation for this is given in the
Equation 4. For a more accurate selection of the inductor, see
the section Choosing the External Inductor Value.
The varactor capacitance can be adjusted in software to increase
the effective VCO range by writing to bits VA1 and VA2 in the
R register. Under typical conditions, setting VA1 and VA2 high
increases the center frequency by reducing the varactor
capacitance by approximately 1.3 pF.
F
VCO
=
PUMP OUT
π 2
CHARGE
(
L
INT
+
L
EXT
OUT
Figure 29.
)
1
is enabled by setting Bit DB4 in
×
(
C
VAR
+
C
FIXED
VCO
)
CLK
.
(4)
Rev. 0 | Page 13 of 28
Figure 32 shows the VCO gain over temperature and frequency.
VCO gain is important in determining the loop filter design—
predictable changes in VCO gain resulting in a change in the
loop filter bandwidth can be offset by changing the charge-
pump current in software.
VCO Bias Current
VCO bias current may be adjusted using bits VB1 to VB4 in the
function register. Additional bias current will reduce spurious
levels, but increase overall current consumption in the part. A
bias value of 0x5 should ensure oscillation at most frequencies
and supplies. Settings 0x0, 0xE ,and 0xF are not recommended.
Setting 0x3 and Setting 0x4 are recommended under most
conditions. Improved phase noise can be achieved for lower
bias currents.
VOLTAGE REGULATORS
There are two band gap voltage regulators on the ADF7012
providing a stable 2.25 V internal supply: a 2.2 µF capacitor
(X5R, NP0) to ground at C
should be used to ensure stability. The internal reference
ensures consistent performance over all supplies and reduces
the current consumption of each of the blocks.
The combination of regulators, band gap reference, and biasing
typically consume 1.045 mA at 3.0 V and can be powered down
by bringing the CE line low. The serial interface is supplied by
Regulator 1, so powering down the CE line causes the contents
of the registers to be lost. The CE line must be high and the
regulators must be fully powered on to write to the serial
interface. Regulator power-on time is typically 100 µs and
should be taken into account when writing to the ADF7012
after power-up. Alternatively, regulator status may be monitored
at the MUXOUT pin once CE has been asserted, because
MUXOUT defaults to the regulator ready signal. Once
Regulator_ready is high, the regulator is powered up and the
serial interface is active.
FSK MODULATION
FSK modulation is performed internally in the PLL loop by
switching the value of the N register based on the status of the
TxDATA line. The TxDATA line is sampled at each cycle of the
PFD block (every 1/F
to-high transition, an N value representing the deviation
frequency is added to the N value representing the center
frequency. Immediately the loop begins to lock to the new
frequency of F
makes a high-to-low transition, the N value representing the
deviation is subtracted from the PLL N value representing the
center frequency and the loop transitions to F
CENTER
+ F
PFD
DEVIATION
seconds). When TxDATA makes a low-
REG1
and a 470 nF capacitor at C
. Conversely, when TxDATA
CENTER
ADF7012
− F
DEVIATION
REG2
.