ADF4156

Cycle Slips

Cycle slips occur in integer-N/fractional-N synthesizers when

the loop bandwidth is narrow compared to the PFD frequency.

The phase error at the PFD inputs accumulates too fast for the

PLL to correct, and the charge pump temporarily pumps in the

wrong direction. This slows down the lock time dramatically.

The ADF4156 contains a cycle slip reduction circuit to extend

the linear range of the PFD allowing faster lock times without

loop filter changes.

When the ADF4156 detects that a cycle slip is about to occur, it

turns on an extra charge pump current cell. This outputs a

constant current to the loop filter, or removes a constant current

from the loop filter (depending on whether the VCO tuning

voltage needs to increase or decrease to acquire the new

frequency). The effect is that the linear range of the PFD is

increased. Stability is maintained because the current is

constant and is not a pulsed current.

If the phase error increases again to a point where another cycle

slip is likely, the ADF4156 turns on another charge pump cell.

This continues until the ADF4156 detects that the VCO

frequency has gone past the desired frequency. It then begins to

turn off the extra charge pump cells one by one until they have

all been turned off and the frequency is settled.

Up to seven extra charge pump cells can be turned on. In most

applications, it is enough to eliminate cycle slips altogether,

giving much faster lock times.

Setting Bit DB28 in the MOD/R register (R2) to 1 enables cycle

slip reduction. Note that a 45% to 55% duty cycle is needed on

the signal at the PFD in order for CSR to operate correctly.

SPUR MECHANISMS

This section describes the three different spur mechanisms that

arise with a fractional-N synthesizer and how to minimize them

in the ADF4156.

Fractional Spurs

The fractional interpolator in the ADF4156 is a third order Σ-Δ

modulator (SDM) with a modulus (MOD) that is programmable

to any integer value from 2 to 4095. In low spur mode (dither

enabled) the minimum allowable value of MOD is 50. The SDM

is clocked at the PFD reference rate (fPFD) that allows PLL output

frequencies to be synthesized at a channel step resolution of

fPFD/MOD.

In low noise mode (dither off), the quantization noise from the

Σ-Δ modulator appears as fractional spurs. The interval between

spurs is fPFD/L, where L is the repeat length of the code sequence

in the digital Σ-Δ modulator. For the third-order modulator

used in the ADF4156, the repeat length depends on the value of

MOD, as listed in Table 7.

Table 7. Fractional Spurs with Dither Off

Condition (Dither Off)

Repeat

Length

If MOD is divisible by 2, but not 3 2 × MOD

If MOD is divisible by 3, but not 2 3 × MOD

If MOD is divisible by 6

6 × MOD

Otherwise

MOD

Spur Interval

Channel step/2

Channel step/3

Channel step/6

Channel step

In low spur mode (dither enabled), the repeat length is

extended to 221 cycles, regardless of the value of MOD, which

makes the quantization error spectrum look like broadband

noise. This can degrade the in-band phase noise at the PLL

output by as much as 10 dB. Therefore, for lowest noise, dither

off is a better choice, particularly when the final loop BW is low

enough to attenuate even the lowest frequency fractional spur.

Integer Boundary Spurs

Another mechanism for fractional spur creation are interactions

between the RF VCO frequency and the reference frequency.

When these frequencies are not integer related (which is the

whole point of a fractional-N synthesizer) spur sidebands

appear on the VCO output spectrum at an offset frequency that

corresponds to the beat note or difference frequency between

an integer multiple of the reference and the VCO frequency.

These spurs are attenuated by the loop filter and are more

noticeable on channels close to integer multiples of the

reference where the difference frequency can be inside the loop

bandwidth, hence the name integer boundary spurs.

Reference Spurs

Reference spurs are generally not a problem in fractional-N

synthesizers as the reference offset is far outside the loop

bandwidth. However, any reference feed-through mechanism

that bypasses the loop can cause a problem. One such

mechanism is feed through of low levels of on-chip reference

switching noise out through the RFIN pin back to the VCO,

resulting in reference spur levels as high as –90 dBc. Care

should be taken in the PCB layout to ensure that the VCO is

well separated from the input reference to avoid a possible feed

through path on the board.

SPUR CONSISTENCY AND FRACTIONAL SPUR

OPTIMIZATION

With dither off, the fractional spur pattern due to the

quantization noise of the SDM also depends on the particular

PHASE word with which the modulator is seeded. Setting the

SD reset bit to zero (DB14 in Register 3) ensures that the SDM

is seeded with the PHASE word on every write to Register 0.

The PHASE word can be varied to optimize the fractional and

subfractional spur levels on any particular frequency. Thus, a

look-up table of PHASE values corresponding to each frequency

can be constructed for use when programming the ADF4156.

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