ADRF6655ACPZ-R7 datasheet(17/44 Pages) AD

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ADRF6655

Rev. 0 | Page 17 of 44

CIRCUIT DESCRIPTION

The ADRF6655 can be subdivided into a PLL and VCO block

and a mixer block. A detailed circuit description for each block

follows.

PLL AND VCO BLOCK

The PLL and VCO block, shown in Figure 49, is made up of a

reference input block, a phase and frequency detector (PFD), a

charge pump, a VCO, and a divide-by-N modulus block. An

off-chip loop filter completes the loop.

LOOP

FILTER

PRESCALER

ADRF6655 PLL BLOCK DIAGRAM

BAND

SELECT

TO MIXER

BLOCk

VTUNE

PFD

REFIN

FRAC

MOD

INT

PROGRAMMABLE

DIVIDER

THIRD-ORDER

INTERPOLATOR

×2

÷2

÷4

Figure 49. PLL and VCO Block

The VCO is implemented with a single core that consists of 64

overlapping bands, as shown in Figure 50. The correct band is

selected automatically by the VCO band calibration circuit when

VCO band selection takes roughly 4000 PFD cycles. During

calibration, an internal mux is used to disconnect the VCO input

voltage from the VTUNE pin and apply an internal reference

voltage for calibration. When calibration is complete, the VCO

input voltage is reconnected to the VTUNE pin and normal

PLL operation resumes.

1.4

1.6

1.8

2.0

2.2

2.4

0.51.01.52.0

2.5

VTUNE (V)

Figure 50. fVCO/2 vs. Tuning Voltage for All 64 Bands

The VCO operates at twice the LO frequency for improved

isolation. The nominal value of Kv is 75 MHz/V at the VCO

output. As the VCO band is changed from 0 to 63, the size of the

varactor is also changed, thus maintaining a roughly constant

Kv across the entire operating range.

RF MIXER BLOCK

VCC

133

Ω

133

Ω

VCO

ADRF6655 MIXER BLOCK

÷2 OR ÷3

SIF

CDAC

V2I

OUTN

OUTP

RFIN

IP3SET

Figure 51. Mixer Block

The mixer portion of the ADRF6655, shown in Figure 51, consists

of an LO signal chain, an RF voltage-to-current (V-to-I) converter,

and a mixer core. The LO chain receives a signal from either the

internal VCO or an external LO source. This LO signal then passes

through a frequency divider, which can be set to divide-by-2

or divide-by-3, depending on the desired LO frequency. The

differential RF inputs are converted into currents by the V-to-I

converter and fed into the mixer core. A pair of 133 Ω pull-up

resistors are used to present a ~250 Ω source impedance at the

IF output.

LO Signal Chain

The LO chain consists of a mux that selects between the internal

VCO and an external LO source. The LO signal can then be

divided by 2 or divided by 3, providing a wide range of LO

frequencies from 1050 MHz to 2300 MHz. A buffer then drives

this divided down signal to the mixer core. The LO signal can

also be observed via the LO I/O port when the internal VCO

is selected. When the external LO buffer is enabled, the supply

current and die temperature increase, resulting in a slight

degradation of RF performance. In normal operation mode,

the external LO buffer should be disabled to help minimize

power consumption and provide optimal RF performance.

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