Philips PM 6670 Operating Manual page 15

17
SPECIAL FUNCTIONS
Arming
Arming enables the counter to be prevented from
starting on unwanted signals. The external ARMING
input (input E on the rear panel) allows an additional
trigger condition. When input E is set to high logic
level ( > 2V), the counter is prevented from starting
a new measurement. However, the counter makes all
preparations for a measurement. When input E returns
to low (< 0.5V), the measurement will start with a
minimum of delay. The delay is approx O.Sps.
Note that arming cannot be used in the COUNT A
manual mode.
-m
/mp u { E —n '
^ le s
IMlt-
Burst to be
^ 10
« - »
iiil/il
lliilii
Fig. 3.13. Burst requirements for muitipie burst
frequency average.
A monostable flip-flop or a pulse generator with
external triggering (e.g. PM 5716) could be used,
see Fig. 3.14. It is also possible to measure a single
burst by means of the input E control.
External reset
Electrical reset, provides an equivalent function to
the front panel reset pushbutton. The counter is
reset when input E is set to high logic level (>2V).
A new measurement can be made when input E
has returned to low ( < 0.5V).
Single burst frequency measurements
The input synchronized counter is in general suitable
for burst frequency measurements. The frequency
measurement does not start until the burst has
arrived, because the opening of the main gate is
controlled by the input signal. However, there are
some restrictions:
• The set measuring time must be shorter than
the burst duration.
• The burst must contain at least 20 cycles.
• The minimum measuring time is 10ms.
Multiple burst frequency average
PM 6671 and PM 6672 are equipped with an exter
nal gate function, permitting the counter to make
burst measurements down to 500ns. The external
gate control signal should be connected to input E
(Set to FREQ AVERAGE) for controlling the multiple
burst frequency average function. The measure
ment is interrupted when input E is higher
than 2V. The external gate time can be down to 500ns.
The actual measuring time is the sum of all
individual gate openings made during the set
measuring time.
Note that the burst must contain at least 10 cycles
during the time input E is low and 10 cycles after
input E has returned high, as shown in Fig. 3.13.
Fig. 3.14. Example of a measurement set-up for
multiple burst frequency average.
The difference in propagation delay in the two inter
nal channels in the counter-on-a-chip is approx 1 5ns.
In FREQ AVERAGE mode with very short external
gate times, this delay will cause a measureable error
as the 15ns will be repeated for each external gate
pulse. However, it is possible to compensate for
this error. Measure a stable, continuous wave of
approx the same frequency in the normal mode
without external gate signal with an external gate
signal having the same number of samples and the
same gate duration as will finally be used (measured
value = F2). To compensate for the error obtained
in the frequency average mode, multiply the reading
with the factor K = F1/F2.
The total relative error for a multiple frequency
average measurement is:
Rel error = -(- ^ 100ns ± trigger errorgg ± trigger error
" 'EG~ tEGVN
± rel. time-base error
where tgQ = external gate duration
N = number of burst samples.