ADRF6601

Rev. 0 | Page 15 of 24

THEORY OF OPERATION

The ADRF6601 integrates a high performance downconverting

mixer with a state-of-the-art fractional-N PLL. The PLL also inte-

grates a low noise VCO. The SPI port allows the user to control the

fractional-N PLL functions and the mixer optimization functions,

as well as allowing for an externally applied LO or VCO.

The mixer core within the ADRF6601 is the next generation of

an industry leading family of mixers from Analog Devices, Inc.

The RF input is converted to a current and then mixed down to

IF using high performance NPN transistors. The mixer output

currents are transformed to a differential output voltage.

The high performance active mixer core results in an excep-

tional IIP3 and IP1dB, with a very low output noise floor for

excellent dynamic range. Over the specified frequency range,

the ADRF6601 typically provides an IF input P1dB of 14.3 dBm

and an IIP3 of 31 dBm.

Improved performance at specific frequencies can be achieved

with the use of the internal capacitor DAC (CDAC), which is

programmable via the SPI port, and through the use of a resistor

to a 5 V supply from the IP3SET pin (Pin 29). Adjustment of

the capacitor DAC allows increments in phase shift at internal

nodes in the ADRF6601, thus allowing cancellation of third-

order distortion with no change in supply current. Connecting

a resistor to a 5 V supply from the IP3SET pin increases the

internal mixer core current, thereby improving overall IIP2 and

IIP3, as well as IP1dB. Using the IP3SET pin for this purpose

increases the overall supply current.

The fractional divide function of the PLL allows the frequency

multiplication value from REF_IN to LO output to be a frac-

tional value rather than be restricted to an integer value as

in traditional PLLs. In operation, this multiplication value is

INT

+ (FRAC/MOD)

where:

INT

is the integer value.

FRAC

is the fractional value.

MOD

is the modulus value.

The INT, FRAC, and MOD values are all programmable via

the SPI port.

In other fractional-N PLL designs, fractional multiplication

is achieved by periodically changing the fractional value in a

deterministic way. The disadvantage of this approach is often

spurious components close to the fundamental signal. In the

ADRF6601, a Σ-Δ modulator is used to distribute the fractional

value randomly, thus significantly reducing the spurious content

due to the fractional function.

PROGRAMMING THE ADRF6601

The ADRF6601 is programmed via a 3-pin SPI port. The timing

requirements for the SPI port are shown in Figure 2. Eight pro-

grammable registers, each with 24 bits, control the operation of

the device. The register functions are listed in Table 8.

Table 8. ADRF6601 Register Functions

Register

Function

Register 0

Integer divide control for the PLL

Register 1

Modulus divide control for the PLL

Register 2

Fractional divide control for the PLL

Register 3

Σ-Δ modulator dither control

Register 4

PLL charge pump, PFD, reference path control

Register 5

PLL enable and LO path control

Register 6

VCO control and VCO enable

Register 7

Mixer bias enable and external VCO enable

Note that internal calibration for the PLL must be run when the

ADRF6601 is initialized at a given frequency. This calibration is

run automatically whenever Register 0, Register 1, or Register 2 is

programmed. Because the other registers affect PLL performance,

Register 0, Register 1, and Register 2 should always be programmed

in the order specified in the Initialization Sequence section.

To program the frequency of the ADRF6601, the user typically

programs only Register 0, Register 1, and Register 2. However,

if registers other than these are programmed first, a short delay

should be inserted before programming Register 0. This delay

ensures that the VCO band calibration has sufficient time to com-

plete before the final band calibration for Register 0 is initiated.

Software is available on the ADRF6601 product page of the

Analog Devices website (www.analog.com) that allows easy

programming from a PC running Windows XP or Vista.

INITIALIZATION SEQUENCE

To ensure proper power-up of the ADRF6601, it is important to

reset the PLL circuitry after the VCC supply rail settles to 5 V ±

0.25 V. Resetting the PLL ensures that the internal bias cells are

properly configured, even under poor supply start-up conditions.

To ensure that the PLL is reset after power-up, follow these steps:

1.

Disable the PLL by setting the PLEN bit to 0 (Register 5,

Bit DB6).

2.

After a delay of >100 ms, set the PLEN bit to 1.

After this procedure is followed, the other registers should be

programmed in this order: Register 7, Register 6, Register 4,

Register 3, Register 2, Register 1. Then, after a delay of >100 ms,

Register 0 should be programmed.