AN178 Philips, AN178 Datasheet - Page 14

• Manufacturer Part Number: AN178
• Description: Modeling the PLL
• Manufacturer: Philips

DataSheet4U.com
www.DataSheet4U.com
DataSheet
Philips Semiconductors
FREE-RUNNING FREQUENCY
Free-running frequency measurements are easily made by
connecting a frequency counter or oscilloscope to the VCO output of
the loop. The loop should be connected in its final configuration with
the chosen values of input, bypass, and low-pass filter capacitors.
No input signal should be present. As the free-running frequency is
read out, it can be adjusted to the desired value by the adjustment
means selected for the particular loop. It is important not to make
the frequency measurement directly at the timing capacitor, unless
the capacity added by the measurement probe is much less than the
timing capacitor value, since the probe capacity will then cause a
frequency error.
When the frequency measurement is to be converted to a DC
voltage for production readout or automated testing, a calibrated
phase-locked loop can be used as a frequency meter.
CAPTURE AND LOCK RANGES
Figure 14a shows a typical measurement setup for capture and lock
range measurements. The signal input from a variable frequency
oscillator is swept linearly through the frequency range of interest
and the loop FM output is displayed on a scope or (at low
frequencies) X-Y recorder. The sweep voltage is applied to the X
axis.
Figure 14b shows the type of trace which results. The lock range is
given by the outer lines on the trace, which are formed as the
incoming frequency sweeps away from the center frequency. The
inner trace, formed as the frequency sweeps toward the center
frequency, designates the capture range. Linearity of the VCO is
revealed by the straightness of the trace portion within the lock
range. The slope ( f/ V) is the conversion gain K
the particular free-running frequency.
1988 Dec
4
U
Modeling the PLL
.com
Figure 14. Capture and Lock Ranges
b. Oscilloscope Display
a. Measurement Setup
o
for the VCO at
SL01024
DataSheet4U.com
14
By using the sweep technique, the effect on free-running frequency,
capture range, and lock range of the input amplitude, supply voltage,
low-pass filter and temperature can be examined.
Because of the lock-up time duration and variation, the sweep
frequency must be much lower than the free-running frequency,
especially when the capture range is below 10% of the free-running
frequency. Otherwise, the apparent capture and lock range will be
functions of sweep frequency. It is best to start sweeping as slowly
as possible and, if desired, increase the rate until the capture range
begins to show an apparent reduction — indicating that the sweep is
too fast. Typical sweep frequencies are in the range of 1/1000 to
1/100,000 of the free-running frequency. In the case of the 567, the
quadrature detector output may be similarly displayed on the Y axis,
as shown in Figure 15, showing the output level versus frequency
for one value of input amplitude.
Capture and lock range measurements may also be made by
sweeping the generator manually through the band of interest.
Sweeping must be done very slowly as the edges of the capture
range are approached (sweeping toward center frequency) or the
lock-up transient delay will cause an error in reading the band edge.
Frequency should be read from the generator rather than the loop
VCO because the VCO frequency gyrates wildly around the center
frequency just before and after lock. Lock and unlock can be readily
detected by simultaneously monitoring the input and VCO signals,
the DC voltage at the low-pass filter, or the AC beat frequency
components at the low-pass filter. The latter are greatly reduced
during lock as opposed to frequencies just outside of lock.
FM AND AM DEMODULATION DISTORTION
These measurements are quite straight-forward. The loop is simply
set up for FM detection and the test signal is applied to the input. A
spectrum analyzer or distortion analyzer (HP333A) can be used to
measure distortion at the FM output.
For FM demodulation, the input signal amplitude must be large
enough so that lock is not lost at the frequency extremes. The data
sheets give the lock (or tracking) range as a function of input signal
and the optional range control adjustments. Due to the inherent
linearity of the VCOs, it makes little difference whether the FM
carrier is at the free-running frequency or offset slightly as long as
the tracking range limits are not exceeded.
The faster the FM modulation in relation to the center frequency, the
lower the value of the capacitor in the low pass filter must be for
Figure 15. Quadrature-Phase Detector and Phase Comparator
DataSheet4U.com
Outputs of the NE567 PLL
Application note
AN178
SL01025