Philips PM 6670 - Ratio and Count Measurement Principles; Time Interval Measurement Techniques

15
The number of cycles (N) is:
effective measuring time
N =
period duration
rounded to the nearest higher multiple of 10 for
frequency measurements or higher integer for period
measurements.
When the measuring time has elapsed, the micro
computer calculates the measuring result with a
10-digit resolution. However, the number of digits
displayed, is limited only to the significant digits,
depending on the measuring resolution. This
measuring resolution is defined by the input frequency
and the measuring time.
The number of digits is selected in such a way
that the measuring resolution is equal to 0.2...2 units
of the least-significant digit (LSD), where:
LSD = 2.5 X Frequency
Measuring timex 10^Hz
rounded to the nearest decade.
2.5 X Period
Measuring time x 10'Hz
Ratio Measurements
The counter measures the frequency ratio between
signals connected to input A and input B. The
frequency range is 0...10MHz.
To obtain full input frequency range on PM 6671 ...72,
a ratio measurement can be done in the FREQ A or C
mode. Connect the signal with the highest frequency
to input A (0.1H2...120MHz) and the other signal
to input D set to EXT STD IN. The frequency range
at input D is 50kHz... 10MHz. However, this arrange
ment does not give correct setting of the decimal
point. The display will indicate a frequency. To get
the correct ratio, divide the displayed value by 10' Hz,
e.g. when the display shows 215.513MHz, it
represents a ratio of 21.5513.
A ratio measurement is useful, for instance, when
calibrating oscillator with an awkward frequency. For
example, say that the fequency should be
4.3625872MHz. This is difficult to read on the
display. By connecting such a reference signal to
input B and measuring the ratio instead, the oscillator
is correctly calibrated when the display shows
1.0000000, which is much easier to read.
Count measurements
There are three different count modes:
Manual
The counter totalizes events
at input A, during the time
interval between releasing
and depressing the DISPL
HOLD pushbutton. An event
is defined as a positive-
going slope.
Gated by B
_l 1_
Start and stop by B
The counter totalizes events
at input A, between the
leading and trailing edge of
the input B signal.
The counter totalizes events
at input A, between the start
and stop event at input B.
Time Interval single measurements
In the time interval single mode, the time (i.e. number
of 100ns clock pulses) is measured between a start
event at channel A and a stop event at channel B.
The start and stop triggering can be individually set
with respect to coupling, trigger level, slope and
attenuation (x1 orxlO).
To perform single source measurements, such as
rise-time and pulse width, only channel A has to
be connected. The input B connector is disconnected.
However, channel B is internally connected by
means of the COM via A pushbutton. In this case,
the coupling and attenuation of channel B are dis
connected (identical as for channel A). The trigger
level and slope in channel B can still be set independ
ently of the channel A setting. The resolution of the
measurement is ± 1 clock pulse ( ± 100ns).
— Trigger level A
Channel A.
Trigger level B 4- C honnel B
f
Gale signal
I I
. JUllllUUUl fUUllUULJ''"''''
The measured time interval -
'number of clock pulses totalized
Fig. 3.8. Time interval single mode.
Time interval average measurements
By using the time interval average technique, which
means multiple measurements of a repetitive signal,
the measuring accurancy and resolution are greatly
improved. Compared to single time interval measure
ments, the basic 100ns resolution is improved by a
factor of I/iTN, where N is the number of time
intervals being averaged.
N =
Measuring time
Pulse repetition time
When using time interval average, the number of
leading edges of the clock pulses occurring in each
individual "time window" are totalized. Figure 3.9.
illustrates a rise-time measurement.