Before we start making measurements of modulated carriers from transmitters, let us start of
by making a few simple measurements from this MXG signal generator I have here, which is
generating a 2.4 GHz CW in other words, unmodulated RF signal.
If we know what frequency the signal is we can simply press FREQUENCY, and enter in this
case 2.4 GHz. That tunes the signal to the center of the
screen. I can then press the SPAN button,
and using the down arrow key I can zoom in on the signal,
and I can press the AMPLITUDE button, and change the Reference Level again using
the arrow keys to bring the signal to the top of the graticule,
and then I can press the MARKER button, and I can read the amplitude value of the
signal: approximately -20 dBm. Now if we do not know the frequency of the
signal there is another way we can do this so I will press the green PRESET button to
preset the instrument again. This time I am going to press SHIFT,
MARKER PEAK, that will put a marker on the highest frequency
signal on the display. I can then press the MARKER button,
and press MARKER 2, CENTER FREQUENCY,
so that has now tuned that 2.4 GHz signal to the center of the screen (you can see here
it says 2.404 GHz) It is roughly in the center of the screen
and I did not need to know what the frequency of it was.
Once again we can press the SPAN button, and use the ARROW KEYS to zoom in.
We can then more accurately place the marker on the top of that signal now we have zoomed
in so: SHIFT,
PEAK, and again we can go MARKER,
MARKER TO, CENTER FREQUENCY.
There are a couple of other useful marker features we ought to take a look at while
we are doing this. If I press the MARKER button again,
you will see first of all we have got up to 6 markers that we can have turned on at the
same time. Currently the marker mode is set to Normal
for marker #1. If I press the MODE button,
you will see we can have a DELTA marker, this can be quite useful, so if I press DELTA,
instead of the marker saying 2.4 GHz, I will press the button now.
It now says that marker 1 or the Delta marker #1 is now 0 Hz and 0 dB.
If I turn the knob you will see a second or Delta marker comes away from the main marker
and we can see now that for example that at a 4.7 MHz offset, the difference in amplitude
is -42 dB. That is very helpful for measuring relative
signal levels between say a main carrier signal and one of its harmonics.
Or in this instance a carrier and the noise floor.
Talking of noise floor if I press the MARKER button again,
and press MARKER FUNCTION, you will see I can select a NOISE MARKER.
Now, you will know that if you are measuring carrier-to-noise ratio it is no good just
saying the carrier-to-noise ratio is 50 dB, because the noise floor can be different according
to Boltzmann’s law depending on which Resolution Bandwidth setting I have selected.
I can set that noise floor to almost any value I want.
Normally when expressing noise values, or carrier-to-noise ratios, we express the noise
floor measurement in a normalized bandwidth typically to 1 Hz.
aving pressed the noise function marker, you can now see that the carrier-to-noise ratio,
the difference between the two is about a 122 dBm/Hz.
In other words it is normalized the level of this noise floor down to, as if we were
using a 1 Hz resolution bandwidth filter. You will also see there are other marker functions
for things like Band Power and Demodulation. One I like very much is the Counter Marker.
If I just go back to Normal first of all, and if I just press the MODE from DELTA back
to NORMAL, I will go SHIFT PEAK,
We are now measuring our 2.4 GHz signal again, and I will press the MARKER key,
and go FUNCTION, and turn on COUNTER.
This has now turned on a very accurate 1 Hz resolution frequency counter that is built
into the Spectrum Analyzer. You can now see that with the Counter active
the marker is now called C #1 instead of M #1 and it is measuring the frequency as being
2.400000714 GHz. Unlike a stand-alone frequency counter instrument
that can only measure one frequency at a time, this can be used to measure any frequency
within a spectrum of carriers. Just line the marker up with whichever signal
you wish and it will apply the frequency counter to that carrier.