HMC832LP6GETR datasheet(27/49 Pages) AD

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HMC832

Data Sheet

translates to a 1.2 Hz error. The exact frequency mode of the

HMC832 addresses this issue and can eliminate quantization

error by programming the channel step size to fPD/10 in

feature can be used whenever the desired frequency, fVCO, can be

exactly represented on a step plan where there are an integer

number of steps (<214) across integer-N boundaries.

Mathematically, this situation is satisfied if













≥

and

)

(

where

)

mod(

GCD

VCO

GCD

VCOk

GCD

(16)

where:

GCD means greatest common divisor.

fPD = frequency of the phase detector.

fVCOk is the channel step frequency where 0 < k < 224−1, as shown

in Figure 46.

Some fractional PLLs are able to achieve these exact frequencies

by adjusting (shortening) the length of the phase accumulator

(the denominator or the modulus of the Δ-Σ modulator) so that

the Δ-Σ modulator phase accumulator repeats at an exact

period related to the interval frequency (fVCOk − fVCO(k−1)) in

Figure 46. Consequently, the shortened accumulator results in

more frequent repeating patterns and as a result often leads to

spurious emissions at multiples of the repeating pattern period,

or at harmonic frequencies of fVCOk − fVCO(k−1). For example, in

some applications, these intervals might represent the spacing

between radio channels, with the spurious occurring at

multiples of the channel spacing.

In comparison, the Analog Devices method is able to generate

exact frequencies between adjacent integer-N boundaries while

still using the full 24-bit phase accumulator modulus, thus

achieving exact frequency steps with a high phase detector

comparison rate, which allows Analog Devices PLLs to maintain

excellent phase noise and spurious performance in the exact

frequency mode.

Figure 46. Exact Frequency Tuning

INTEGER

BOUNDARY

INTEGER

BOUNDARY

fN + 1 – fN = fPD

fVCO1

fVCO2

fVCO3

fVCO = fVCO2

fVCO4

fN + 1

fVCO14

fVCO14 – 1

fVCO14 – 2

Rev. A | Page 26 of 48

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