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| 型號(hào)： | MC145181 |
| 廠商： | Motorola, Inc. |
| 英文描述： | Dual 550/60MHz PLL Frequency Synthesizers with DACs and Voltage Multipliers(帶DACs和電壓乘法器的雙550/60MHzPLL頻率合成器) |
| 中文描述： | 雙550/60MHz鎖相環(huán)頻率合成器的DAC和電壓倍增器（帶數(shù)模轉(zhuǎn)換器和電壓乘法器的雙550/60MHzPLL頻率合成器） |
| 文件頁(yè)數(shù)： | 34/71頁(yè) |
| 文件大小： | 907K |
| 代理商： | MC145181 |
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MC145181
34
MOTOROLA RF/IF DEVICE DATA
7B. MAIN LOOP FILTER DESIGN — CONVENTIONAL
The current output of the charge pump allows the loop
filter to be realized without the need of any active
components. The preferred topology for the filter is illustrated
in Figure 29.
The Ro/Co components realize the primary loop filter. Ca
is added to the loop filter to provide for reference sideband
suppression. If additional suppression is needed, the Rx/Cx
realizes an additional filter. In most applications, this will not
be necessary. If all components are used, this results in a
fourth order PLL, which makes analysis difficult. To simplify
this, the loop design will be treated as a second order loop
(Ro/Co), and additional guidelines are provided to minimize
the influence of the other components. If more rigorous
analysis is needed, mathematical/system simulation tools
should be used.
Component
Guideline
Ca
Rx
Cx
<0.1 x Co
>10 x Ro
<0.1 x Co
The focus of the design effort is to determine what the
loop’s natural frequency,
ω
o, should be. This is determined by
Ro, Co, Kp, Kv, and Nt. Because Kp, Kv, and Nt are given, it is
only necessary to calculate values for Ro and Co. There are
three considerations in selecting the loop bandwidth:
1. Maximum loop bandwidth for minimum tuning speed.
2. Optimum loop bandwidth for best phase noise
performance.
3. Minimum loop bandwidth for greatest reference
sideband suppression.
Usually a compromise is struck between these three
cases, however, for a fixed frequency application, minimizing
the tuning speed is not a critical parameter.
To specify the loop bandwidth for optimal phase noise
performance, an understanding of the sources of phase
noise in the system and the effect of the loop filter on them is
required. There are three major sources of phase noise in the
phase–locked loop — the crystal reference, the VCO, and
the loop contribution. The loop filter acts as a low–pass filter
to the crystal reference and the loop contribution. The loop
filter acts as a high–pass filter to the VCO with an in–band
gain equal to unity. The loop contribution includes the PLL IC,
as well as noise in the system; supply noise, switching noise,
etc. For this example, a loop contribution of 15 dB has been
selected.
The crystal reference and the VCO are characterized as
high–order 1/f noise sources. Graphical analysis is used to
determine the optimum loop bandwidth. It is necessary to
have noise plots from the manufacturers of both devices.
This method provides a straightforward approximation
suitable for quickly estimating the optimal bandwidth. The
loop contribution is characterized as white–noise or
low–order 1/f noise, given in the form of a noise factor which
combines all the noise effects into a single value. The phase
noise of the crystal reference is increased by the noise factor
of the PLL IC and related circuitry. It is further increased by
the total divide–by–N ratio of the loop. This is illustrated in
Figure 30. The point at which the VCO phase noise crosses
the amplified phase noise of the crystal reference is the point
of the optimum loop bandwidth. In the example of Figure 30,
the optimum bandwidth is approximately 15 kHz.
To simplify analysis further, a damping factor of 1 will be
selected. The normalized closed loop response is illustrated
in Figure 31 where the loop bandwidth is 2.5 times the loop
natural frequency (the loop natural frequency is the
frequency at which the loop would oscillate if it were
unstable). Therefore, the optimum loop bandwidth is
15 kHz/2.5 or 6.0 kHz (37.7 krads) with a damping
coefficient,
ζ
1. T(s) is the transfer function of the loop
filter.
where
Nt = Total PLL Divide Ratio — 8 x N
where (N = 25 ... 40),
Kv = VCO Gain – 2
π
Hz/V,
Kp = Phase Detector/Charge Pump Gain – A
= ( |IOH | + |IOL | ) / 4
π
.
Technically, Kv and Kp should be expressed in radian units
[Kv (rad/V), Kp (A/rad)]. Since the component design
equation contains the Kv x Kp term, the 2
π
cancels and the
value can be expressed as AHz/V (amp hertz per volt).
Figure 29. Loop Filter
Charge
Pump
Ph/Frq
Det
N Counter
Xtl
Osc
PLL
VCO
Co
Ca
Ro
Cx
Rx
R Counter
0
0
0
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