Now that is all fine if we are looking at a CW signal, but most of the time we are going
to be looking at modulated carriers. As I am about to show you the standard markers
are not that useful in that situation. Here I am generating a spread spectrum signal
at 2 GHz. I will just tune in, Center Frequency 2 GHz
and let us enter a Span of say 10 MHz. Now this signal here is in fact a WCDMA signal
at 2 GHz. Now if I put a marker on there – there is
the marker - you can see it is at 2 GHz, and the level is around -25 dBm.
Now that is fine. It is in fact truly measuring the power at
that spot frequency, but in the case of this modulated carrier, you can see that the power
is in fact spread across a whole range of frequencies, the channel bandwidth of a WCDMA
transmitter. So we are not actually measuring the total
power of this transmitter. Fortunately these analyzers have a very useful
feature built in for doing that. If I press the MEAS KEY,
and then CHANNEL POWER, and I will set the Integration Bandwidth to
4 MHz and the Span is still set at 10 MHz, what is now happening is that the analyzer
is integrating under the curve. In other words "counting the squares" adding
up the total amount of power within these green Channel Power markers.
And you can see in fact that whereas the single Marker 1 is measuring about -25 dBm you can
see here that the actual Channel Power is much greater than that it is about -11 dBm.
So you can see, it says Channel Power -11 dBm per 4 MHz, so in the 4 MHz bandwidth.
But it also gives us Power Spectral Density of -77 dBm, normalized to a 1 Hz bandwidth.
Now in this instance with the WCDMA signal I know that the channel bandwidth is about
4 MHz. Let us say there is modulation on a carrier
from a satellite or a WiFi signal or Bluetooth that I do not know how wide the modulation
bandwidth is if I press RETURN, and press OCCUPIED BANDWIDTH.
Now this is very similar to Channel Bandwidth in fact it looks almost the same, except you
will see now that the green markers are not fixed at a 4 MHz channel separation.
They are actually moving and adjusting in real time.
In fact what is happening here is the analyzer is automatically measuring within what bandwidth
99% of the power on the display is contained. That is varying slightly because of the noise
on the signal but you can see here that the Occupied Bandwidth is about 4.1 maybe 4.2
MHz. It is also measuring what is called the Bandwidth
Centroid. Let us say this signal is nominally transmitting
at 2 GHz - which we know it is - but let us say the modulation is not symmetrical about
the center frequency. This Bandwidth Centroid will tell you what
the actual center frequency is, in other words, the center point between the two green markers,
which may or may not be the same as the nominal center frequency of the transmitter.
A very common problem these days at transmitter sites is interference from one transmitter
into the receiver of an another radio that is co-located at the same site.
How do we go about measuring the power generated by this transmitter in the adjacent channels,
which one would think would be the most likely spot where this transmitter might be leaking
into and causing interference. If I press the RETURN soft key,
you can see there is another useful measurement here called Adjacent Channel Power (ACP).
Now here of course again we are going to need to set up the Channel Bandwidth, so if I say
that the main channel is 4 MHz wide, and maybe the adjacent channel I will also
set to 4 MHz wide, (but I do not have to)
and the Adjacent Channel Spacing I will set to say 10 MHz,
and we will just widen the span a bit I will press SPAN,
and let us go out to say 30 MHz. You will see now that as well as measuring
the Channel Power in our main channel, the intended frequency of transmission, we are
also measuring the Adjacent Channel Power in these two adjacent channels.
As well as measuring the power in the main channel here - it says Main Channel Power
about -11 dBm - we are also, in the table below measuring the Adjacent Channel Power
for both the lower and upper adjacent channels. You can see that the Channel Power the absolute
value in dBm for this adjacent channel here is -56 dBm.
And it also gives you a relative measurement in other words comparing the level here to
the level of the center channel and that is -45 dB.
Then we have got the same again for the upper channel.
In fact we do not just have to do it with two adjacent channels as you can see here
we can have up to 6 adjacent channels on either side of the main channel.
In real time the analyzer will tell us the absolute values and the relative values relative
to the main channel power. If you would like further information on these
products including; demonstration guides,
operating manuals, or application notes,
please visit: www.agilent.com/find/hsa