HMC703LP4E Datasheet, PDF (33/58 Pages) Hittite Microwave Corporation – 8 GHZ FRACTIONAL SYNTHESIZER

v02.0813
HMC703LP4E
8 GHz fractional synthesizer
With this simplification the total integrated VCO phase noise, Ф²_v, in rads² is given by
Ф²_v= Ф²(f₀) Вπ
where
Ф² is the single sideband phase noise in rads²/Hz inside the loop bandwidth, and
В is the 3 dB corner frequency of the closed loop PLL
The integrated phase noise at the phase detector, Ф²_pd, is just scaled by N² ie. Ф²_pd = Ф²_v/N²
(EQ 10)
The rms phase jitter of the VCO (Ф_v) in rads, is just the square root of the phase noise integral.
Since the simple integral of (EQ 10) is just a product of constants, we can easily do the integral in the log domain. For
example if the phase noise inside the loop is -110 dBc/Hz at 10 kHz offset and the loop band­width is 100 kHz, and the
division ratio is 100, then the integrated phase noise at the phase detector, in dB, is given by;
Ф²_pd = 10log(Ф²(f₀) Вπ/N²) =
-110 + 5
+ 50 - 40 =
-95 dBrads, or
equivalently
�
�95
= 10 ²⁰
= 18 µrads = 1 milli-degrees
rms.
While the phase noise reduces by a factor of 20logN after division to the reference, due to the increased period of the
PD reference signal, the jitter is constant.
The rms jitter from the phase noise is then given by T_jpn=T_pd Ф²_pd/2π
In this example if the PD reference was 50 MHz, T_pd = 20 nsec, and hence T_jpn = 56 femto-sec.
Charge Pump and Phase Detector
The Phase Detector or PD has two inputs, one from the reference path divider and one from the RF path divider. When
in lock these two inputs are at the same average frequency and are fixed at a constant aver­age phase offset with
respect to each other. We refer to the frequency of operation of the PD as f_pd. Most formula related to step size, delta-
sigma modulation, timers etc., are functions of the operating frequency of the PD, f_pd is sometimes referred to as the
comparison frequency of the PD.
The PD compares the phase of the RF path signal with that of the reference path signal and controls the charge pump
output current as a linear function of the phase difference between the two signals. The out­put current varies in a linear
fashion over nearly ±2π radians (±360) of input phase difference.
Charge Pump and Phase Detector Functions
Phase detector register Reg 0Bh allows manual access to control special phase detector features.
Reg 0Bh[2:0] allows fine tuning of the PD reset path delay. This adjustment can be used to improve perfor­mance at very
high PD rates. Most often this register is set to the recommended value only.
Reg 0Bh[5] and [6] enables the PD UP and DN outputs respectively. Disabling prevents the charge pump from pumping
up or down respectively and effectively tri-states the charge pump while leaving all other functions operating internally.
CP Force UP Reg 0Bh[7] and CP Force DN Reg 0Bh[8] allows the charge pump to be forced up or down respectively.
This will force the VCO to the ends of the tuning range which can be useful for testing of the VCO.
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