CD-700, VCXO Based PLL
Loop Filter
A PLL is a feedback system which forces the output frequency to lock in both phase and frequency to the
input frequency. While there will be some phase error, theory states there is no frequency error. The loop
filter design will dictate many key parameters such as jitter reduction, stability, lock range and acquisition
time. Be advised that many textbook equations describing loop dynamics, such as capture range are
based on ideal systems. Such equations may not be accurate for real systems due to nonlinearities, DC
offsets, noise and do not take into account the limited VCXO bandwidth. This section deals with some
real world design examples. Also, there is loop filter software on the Vectron web site, plus a full staff of
experienced applications engineers who are eager to assist in this process. Common CD-700 PLL
applications are shown in Figures 8, 9 (frequency translation), Figure 10 (clock recovery) and
Figure 11 (clock smoothing).
Of primary concern to the designer is selecting a loop filter that insures lock-in, stability and provides
adequate filtering of the input signal. For low input frequencies, a good starting point for the loop filter
bandwidth is 10 Hz (typical). An example would be translating an 8 kHz signal to 44.736 MHz.
Figures 8 and 9 show 8kHz to 44.736 MHz and 8kHz to 19.440 MHz frequency translation designs.
For high input frequencies, a good starting point for the loop filter bandwidth is 100 ppm times the input
frequency.
It’s fairly easy to set a low loop bandwidth for large frequency translations such as 8kHz to 44.736MHz,
but becomes more difficult for clock smoothing applications such as 19.440 MHz input and 19.440MHz
output. In this example, 100ppm * 19.440MHz is approximately 2kHz and this loop filter bandwidth may
be too high to adequately reject jitter. A good way to resolve this is to lower the DATAIN frequency such
as dividing the input frequency down. The loop filter bandwidth becomes lower since 100ppm * DATAIN
is lowered. Figure 11 shows an example for clock smoothing on a relatively high input frequency signal
and maintaining a wide lock range.
There is no known accurate formula for calculating acquisition time and so the best way to provide
realisitc figures is to measure the lock time for a CD-700. By measuring the control voltage settling time,
acquisiton time was measured in the range of 3-5 seconds for applications such as 8kHz to 34.368 MHz
frequency translation which is similar to the application in Figures 8 and 9, to sub 10 milliseconds for NRZ
data patterns such as Figure 10. It may be tempting to reduce the damping factor to 0.7 or 1.0 in order
to improve acquisition time; but, it degrades stability and will not signifigantly improve acquisition time. A
damping factor of 4 is fairly conservative and allows for excellent stability.
Some general quidelines for selecting the loop filter elements include: Values should be less than
1Megohm and at least 10kohm between the PHO and OPN, the capacitor should be low leakage and a
polarized capacitor is acceptable, the R/C’s should be located physically close to the CD-700 .The loop
filter software available on the web site was written for 5 volt operation. A simple way to calculate values
for 3.3 volt operation is to multiply the data density by 0.66 (3.3V / 5V).
SPICE models are another design aid. In most cases a new PLL CD-700 design is calculated by using
the software and verified with SPICE models. The simple active
Π
model is shown in Figure 7.
Loop filter values can be modified to suit the system requirements and application. There are many
excellent references on designing PLL’s, such as “Phase-Locked Loops, Theory, Design and
Applications”, by Roland E Best (McGraw-Hill).
Vectron International, 267 Lowell Rd, Hudson NH 03051-4916
Page 7 of 14
Tel: 1-88-VECTRON-1
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Web:
www.vectron.com
Rev : 06Apr08
VECTRON [ Vectron International, Inc ]
