many of today's bench top
oscilloscopes have built-in hardware
counter measurements
including keysight's infiniivision
x-series oscilloscopes
but there is a lot of confusion about
what the difference is
between a hardware counter frequency
measurement
versus a waveform frequency measurement
so what is the difference and which one
should we use
hi i'm johnny hancock product manager
for keysight's infiniivision
x-series oscilloscopes let's get started
and i'll show you
the difference between these different
types of frequency measurements
on oscilloscopes so here's an rf signal
this is a 13.56
megahertz carrier which is a pretty
common frequency
for applications such as rfid
or nfc stands for near field
communication
so let's measure the frequency a couple
different ways so i'm going to
select the frequency measurement that's
based
on the digitized waveform
and it's measuring right now it says
about 13.5
something it's flipping back and forth
what it's doing is measuring the period
of of just one cycle taking the
reciprocal
to come up with the uh measurement
results
and we can see the statistics now let's
let's do this measurement a different
way and see if we can get a higher
resolution measurement so what i'm going
to do
is go into the analyze menu and select
counter now the counter is a
hardware based counter measurement just
like a standalone
counter it makes measurements based on
trigger crossings not based on the
digitized waveform
so let's select it's already selected
here
a frequency measurement on channel one
and it measures
13.560
with no flipping digits so we have five
digits
of very stable resolution and what it's
doing
is it's capturing multiple trigger
crossings it's counting them
over a gate time in this case with five
digit resolution
it's got a gate time or it counts
trigger crossings for 100 milliseconds
now i can increase to six digits
now you see six very stable digits
seven digits it takes a little longer to
make the measurement but you could see
seven stable digits or i could go all
the way up to eight
digits the gate time is about 10 seconds
long
so you can see it makes very stable
measurements
relative to conventional oscilloscope
frequency measurements
so why not use the counter measurement
for
frequency measurements all the time
let's see
so here's an example where the hardware
counter is probably not going to work
for us so we we have
two bursts of signals there's a
analog burst here and then followed by a
digital burst and what if we want to
know
the frequency of each of those bursts
if i turn on the hardware counter let's
do that
it measures 750 kilohertz
and some digits flipping it's just
measuring the
average frequency of this signal
including all the dead time
at this point over 100 millisecond
windows so this is totally
inaccurate now let's do it the correct
way i'm going to
go into my measurement menu and i've got
frequency turned on
and i'm going to select gated by cursors
and
and so now it's going to perform
and let's move this over here so we can
see it more clearly
this digital burst it's measuring just
between the cursors
it's measuring a frequency of 1.25
megahertz a lot faster than what the
counter measured
which was incorrect now let's
let's measure the analog burst
kind of over there on the front porch
and we see that it measures about
5 megahertz so for continuous wave or cw
type
signals the hardware counter will always
provide
higher resolution and more accurate
frequency measurements
but for more complex non-cw type signals
especially when measurement gating is
required
the scope's standard frequency
measurement based on the digitized
waveform
is the one to choose if you think you
might have learned something
in this short video to make you more
proficient in performing oscilloscope
measurements
i invite you to view our other
infiniivision oscilloscope
measurement tips videos to learn more
about keysight's infiniivision x-series
oscilloscopes
go to the url listed on your screen
or contact one of keysight's authorized
distributors
thanks and semper fi