ADF5355 datasheet(18/38 Pages) AD

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ADF5355

Data Sheet

PROGRAM MODES

Table 5 and Figure 39 through Figure 53 show how the program

modes must be set up in the ADF5355.

The following settings in the ADF5355 are double buffered: main

fractional value (FRAC1), auxiliary modulus value (MOD2),

auxiliary fractional value (FRAC2), reference doubler, reference

divide by 2 (RDIV2), R counter value, and charge pump current

setting. Two events must occur before the ADF5355 uses a new

value for any of the double buffered settings. First, the new value

must latch into the device by writing to the appropriate register,

and second, a new write to Register 0 must be performed.

For example, to ensure that the modulus value loads correctly,

every time that the modulus value updates, Register 0 must be

written to. The RF divider select in Register 6 is also double

buffered, but only if DB14 of Register 4 is high.

VCO

The VCO core in the ADF5355 consists of four separate VCOs,

each of which uses 256 overlapping bands, which allows the

device to cover a wide frequency range without large VCO

sensitivity (KV) and without resultant poor phase noise and

spurious performance.

The correct VCO and band are chosen automatically by the

VCO and band select logic whenever Register 0 is updated and

automatic calibration is enabled. The VCO VTUNE is disconnected

from the output of the loop filter and is connected to an internal

reference voltage.

The R counter output is used as the clock for the band select

logic. After band selection, normal PLL action resumes. The

nominal value of KV is 15 MHz/V when the N divider is driven

from the VCO output, or the KV value is divided by D. D is

the output divider value if the N divider is driven from the

RF output divider (chosen by programming Bits[D23:D21] in

The VCO shows variation of KV as the tuning voltage, VTUNE,

varies within the band and from band to band. For wideband

applications covering a wide frequency range (and changing

output dividers), a value of 15 MHz/V provides the most accurate

KV, because this value is closest to the average value. Figure 36

shows how KV varies with fundamental VCO frequency along with

an average value for the frequency band. Users may prefer this

figure when using narrow-band designs.

Figure 36. VCO Sensitivity, KV vs. Frequency

OUTPUT STAGE

The RFOUTA+ and RFOUTA− pins of the ADF5355 connect to

the collectors of an NPN differential pair driven by buffered

outputs of the VCO, as shown in Figure 37. In this scheme,

the ADF5355 contains internal 50 Ω resistors connected to

the VRF pin. To optimize the power dissipation vs. the output

power requirements, the tail current of the differential pair is

programmable using Bits[D2:D1] in Register 6. Four current

levels can be set. These levels give approximate output power

levels of −4 dBm, −1 dBm, +2 dBm, and +5 dBm, respectively.

Levels of −4 dBm, −1 dBm, +2 dBm can be achieved using a

50 Ω resistor to VRF and ac coupling into a 50 Ω load. A +5 dBm

level requires an external shunt inductor to VRF. Note that an

inductor has a narrower operating frequency than a 50 Ω

resistor. For accurate power levels, refer to the Typical

Performance Characteristics section. Add an external shunt

inductor to provide higher power levels; however, this is less

wideband than the internal bias only. Terminate the unused

complementary output with a circuit similar to the used output.

Figure 37. Output Stage

The doubled VCO output (6.8 GHz to 13.6 GHz) is available on

the RFOUTB pin, which can be ac-coupled to the next circuit.

Figure 38. Output Stage

FREQUENCY (GHz)

3.3

3.8

4.3

4.8

5.3

5.8

6.3

6.8

AVERAGE

VCO SENSITIVITY

LINEAR

TREND LINE

VCO

RFOUTA+

RFOUTA–

VRF

50Ω

BUFFER/

DIVIDE BY

1/2/4/8/

16/32/64

2× VCO

MUX

RFOUTB

Rev. A | Page 18 of 38

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