If you are working on transmitters with analog modulation such as:
narrow-band FM for land mobile radio, wide-band FM for the broadcast FM band,
or the broadcast AM band with wideband AM modulation,
or even narrow-band AM for the air band, then you will definitely appreciate the benefits
of option AFM on the Agilent Handheld Spectrum analyzers.
Without option AFM we can of course tune in to a signal.
I have a 450 MHz carrier narrow-band FM modulated. I can type a Center Frequency of 450 MHz,
and enter a Span of 50 kHz, and you will see the typical FM modulation
pattern you would expect to see. But it is very difficult making accurate measurements
of this such as the deviation values or the actual center frequency.
That is where option AFM comes into its own. If I press the MODE button,
and press MODULATION ANALYSIS, and select FM,
We can now enter a carrier frequency of 450 MHz,
and I need to tell it what IF bandwidth we are using in fact if you do not know just
press it onto AUTO and the AUTO IF bandwidth feature on the analyzer will automatically
work out the optimum IF bandwidth for the measurement.
Here you can see the 1 kHz tone that I am modulating on this FM carrier from the signal
generator. We have an oscilloscope display on the top
of the screen. You can see here on the X-axis we have time
in milliseconds going from 0 to 10 ms and on the Y-axis we are measuring FM deviation
in kHz. Best of all, in the table at the bottom of
the screen, you can see it is telling us the carrier power,
the current value in dBm, the modulation rate 1.00 kHz - that is the
frequency of our test tone, and you can see the FM deviation 3.01 kHz.
It even measures the SINAD of the signal - currently 53 dB because we have got a nice strong carrier,
it even tells us the carrier frequency offset. That is the error from the nominal carrier
frequency of 450 MHz that the analyzer is measuring.
So you can see the carrier frequency of this transmitter is about a 120 Hz high.
If we were measuring a 25 W mobile radio transmitter we would obviously need an attenuator.
These analyzers can handle a maximum continuous input power of +30 dBm,
but for a 25 watt transmitter you would need an attenuator such as this.
You simply inset this between the transmitter and the RF input on the Analyzer.
If I press the MORE button, you can see that we can enter a value for
the external gain or in this case attenuation of the attenuator and still therefore obtain
calibrated readings on the display of true RF power.
We can do exactly the same with an amplitude modulated transmission as well.
If I change the modulation on the Signal Generator to AM,
we simply press MEASURE, select AM,
and you will see that the automatic IF bandwidth function has automatically worked out what
IF bandwidth the analyzer needs to measure this modulation rate.
The X-axis of the oscilloscope display still shows us the time in milliseconds, but on
the Y-axis it is now measuring AM depth in percentage.
If we look at the table, you can see the current, max and min values of carrier power,
modulation rate - still 1 kHz, the AM depth - approximately 50%,
the SINAD, and again the carrier offset - that is the
error in carrier frequency from what we have set, 450 MHz to what the transmitter is actually
generating. If you would like further information on these
products including; demonstration guides,
operating manuals, or application notes,
please visit: www.agilent.com/find/hsa