Philips PM 6672 - Page 12

14
The low-pass filter
The built-in 50kHz low-pass filter is used for improved
triggering on noisy LF-signals. The filter characteristic
is shown in Fig. 3.6. It is also possible to use this
filter for signals with frequencies above 50kHz, but
at a reduced sensitivity.
SO -
A t ienuQ i ion
in d B
AO -
30-
20-
JO-
ErcCfucncy
in MHz
1 1 1
0.02 0.05 0.1 0.2
1 1
0.5
\
2
Fig. 3.6. The low—pass filter reduces noise and inter
ference.
Input C
PM 6672 has a special RF input, called input C.
Note that not more than 1 2Vrivis's allowed at input C
and that the input sensitivity is adjusted automatically.
MEASURING MODES
Frequency A, Period A and RPM
PM 6670...72 perform a frequency and period
measurement as given in the definitions;
Frequency
Period =
Number of cycles
Time
Time
Number of cycles
The counter:
1 .Measures the effective measuring time.
2.Counts the number of input cycles during the
measuring time.
3.Computes the number of cycles per second
(frequency) or time units per cycle (period).
The measurement is synchronized with the input
signal. This is called the input synchronized or
reciprocal method.
In the input synchronized mode, both the opening
and closure of the main gate is synchronized with
the input signal, so that only completed input cycles
are counted. This means that a±1 input cycle error
is avoided. During the gate time, the counter also
totalizes the number of 100ns crystal clock cycles;
see Fig. 3.7.
fOOns
X- TAL
CLOCK
TRiGGCR
OUTPUT
"MEASURE'
S yNCHRON/EC
GATE
SIGNAL
CO UN TED
INPU T
CYCLES
TIME COUNTS
[Nurnber of
/OOns
clockpu (ses )
COUNTED INPUT CYCLES
rRcguENcv
TIM£ COUNTS ' 10-''s
■ COUNTCD INPUT CYCLES
Fig. 3.7. input synchronized mode.
The resolution in the input synchronized mode, is
caused by truncation of the clock pulses, which
results in a ± 1 clock pulse error (100ns). The
resolution of the measurement thus only depends
on the measuring time, e.g. the resolution for Is
measuring time is 10"' (lOOns/ls) independent of
frequency.
In conventional counters, the gate time is synchron
ized with the clock signal. The first and last input
cycle, can therefore be truncated, causing a ±1 cycle
error. This results in a good resolution for high
frequency measurements, but a poor resolution for
low frequency measurements (±1/frequency for Is
measuring time).
A RPM measurement is in fact a normal frequency mea
surement, but the microcomputer multiplies the fre
quency for 60 before displaying the result.
Measuring time
The measuring time can be varied in 33 steps per
decade between 10ms and 96s. Set measuring time
can be displayed by pushing the measuring time
rotary knob. This resets the counter and a new
measurement will start. The counter continues to
totalize input cycles until the set measuring time
has elapsed and the synchronization conditions are
met. Hence, the effective measuring time (also called
gate time) is longer than the set measuring time.