ADF4350BCPZ datasheet(13/34 Pages) AD

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Data Sheet

ADF4350

Rev. B | Page 13 of 34

The VCO shows variation of KV as the VTUNE varies within the

band and from band-to-band. It has been shown for wideband

applications covering a wide frequency range (and changing

output dividers) that a value of 33 MHz/V provides the most

accurate KV as this is closest to an average value. Figure 21

shows how KV varies with fundamental VCO frequency along

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

this figure when using narrowband designs.

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6

FREQUENCY (GHz)

Figure 21. KV vs. Frequency

In fixed frequency applications, the ADF4350 VTUNE may

vary with ambient temperature switching from hot to cold.

In extreme cases, the drift causes VTUNE to drop to a very low

level (<0.25 V) and can cause loss of lock. This becomes an

issue only at fundamental VCO frequencies less than 2.95 GHz

and at ambient temperatures below 0°C.

In cases such as these, if the ambient temperature decreases

below 0°C, the frequency needs to be reprogrammed (R0 updated)

to avoid VTUNE dropping to a level close to 0 V. Reprogramming

the device chooses a more suitable VCO band, and thus avoids

the low VTUNE issue. Any further temperature drops of more

than 20°C (below 0°C) also require further reprogramming.

Any increases in the ambient temperature do not require repro-

gramming.

OUTPUT STAGE

The RFOUTA+ and RFOUTA− pins of the ADF4350 are connected

to the collectors of an NPN differential pair driven by buffered

outputs of the VCO, as shown in Figure 22. To allow the user to

optimize the power dissipation vs. the output power requirements,

the tail current of the differential pair is programmable by

Bits [D2:D1] in Register 4 (R4). Four current levels may be set.

These levels give output power levels of −4 dBm, −1 dBm, +2

dBm, and +5 dBm, respectively, using a 50 Ω resistor to AVDD

and ac coupling into a 50 Ω load. Alternatively, both outputs

can be combined in a 1 + 1:1 transformer or a 180° microstrip

coupler (see the Output Matching section). If using the outputs

individually, the optimum output stage consists of a shunt

inductor to VVCO. The unused complementary output must

be terminated with a similar circuit to the used output.

An auxiliary output stage exists on Pins RFOUTB+ and RFOUTB−

providing a second set of differential outputs which can drive

another circuit, or which can be powered down if unused. The

auxiliary output must be used in conjunction with the main RF

output. It cannot be used with the main output powered down.

Another feature of the ADF4350 is that the supply current to

the RF output stage can be shut down until the device achieves

lock as measured by the digital lock detect circuitry. This is

enabled by the mute till lock detect (MTLD) bit in Register 4 (R4).

VCO

RFOUTA+

RFOUTA–

BUFFER/

DIVIDE-BY-

1/2/4/8/16

Figure 22. Output Stage

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