Spectrum Analyzer and Network Analyzer

These are very different animals a spctrum analyser looks at the RF (radio frequency) spectrum frequency versus amplitude information. See alo fourier analysis.

Spectrum analysers use the heterodyne priciple that is they mix a frequency derived from an internal oscillator which is intern driven by a saw wave oscillator so as to sweep between the upper and lower limit frequencies. The sawtooth controll voltage also drives the "X" display or timebase signal so the "X" axis corresponds to frequency. Various filters and further IF intermediate frequency stages narrow the signal so it can be expanded. The sweep speed can be varied to narrow the search area. Digital techniques allow for a very narrow frequency range to be exammined.

Were as a network analyser is an instrument that measures the incident, reflected power and transmitted power at a two portnetwork. Both scalar(magnitude only) and vector (magnitude and phase) measurements can be made.

Regards

Though the question seems to be about Communication-Tech but Network analyzer may aso include " Network Protocol Analyzer " for LAN & WAN etc etc

Spectrum analyzer is for RF and Network analyzer data signal, wavefunction it is another equipment, and the use I synchronize does not replace to network analyzer.

With all due respect, BrainWave's explanation of how a spectrum analyzer works is somewhat unfinished/incomplete, and in fact, the two instruments are very closely related as far as the functionality he describes.

Let me try to clarify a little further: As BrainWave explains, BOTH devices contain superheterodyne receivers, with the attendant swept oscillators, filters, mixers, etc.

For both instruments this results in a display of the radio frequency spectrum - starting & ending at user-defined frequencies, displayed on an X-Y grid, with X = frequency & Y= amplitude.

Dependent on many other factors, when you plug the output of some gizmo (receiver, antenna, amplifier, etc. etc.) into the input of the spectrum analyzer, you see the relative amplitude of signals at their respective frequencies displayed (these traces can be quite tricky & misleading, depending on how those signals change in the time domain, i.e. if there's slow or fast modulation on them, digital or otherwise...).

In other words, if it helps: a spectrum analyser is a device which allows you to view how signals look in the frequency domain, analogous to how an oscilloscope displays signals in the time domain. The speckie is performing the Fourier transform...

Many spectrum analysers also contain a 'tracking generator' output - this is simply a leveled output of the swept oscillator (mentioned by BrainWave): ah-HA! this output is at the same frequency as the superhet receiver! - so if you plug that tracking generator output into a device (e.g. filter) and plug the output of that device into the input of the speckie, you'll get a trace of the device's frequency response (but BE CAREFUL doing it with amplifiers! it's easy to blow up the speckie input ;-)

A network analyser is very closely related to a spectrum analyser (in fact with some external plumbing and careful choice of rf coupler, one can build a 'poor man's network analyser based on a spectrum analyser). The network analyser contains all of the spectrum analyser's innards plus some fancy plumbing & 'couplers'. Usually they have two ports: 1 and 2. You connect a Device Under Test and the NA allows you to measure, very accurately (much more so than using the spectrum analyser's tracking generator as mentioned above) the rf power reflected from both input and output ports of the DUT, along with it's transmission characteristics. These are called the 'S parameters' (S11 = power reflected from the input port, S22 = power reflected from the output port, S21 = transmission response from port 1=>2, S12 = transmission response in the reverse direction, port 2=>1).

To summarize: a spectrum analyser is used to look at & characterise SIGNALS...but using the built-in tracking generator, you can also get a good idea of the rf frequency response of a DEVICE...but again, ONLY in the transmission direction (i.e. a rather rough version of S21). A Network Analyser is used to characterise DEVICES, normally with the aim to understand how to imbed them in circuits or systems with other devices, especially to determine how to match them with their neighbors, to either 50 or 75 ohms. So if you're doing e.g EMC work or telecom or many other things, you often employ a spectrum analyzer to see how signals look in the frequency domain. But if you're going to build rf hardware (from 10 Mhz or so thru millimeter microwave), you need a network analyser, in order that the various devices you buy or build match each other (so as not to waste power or create instabilities).

Good sources of info: http://www.microwaves101.com/index.cfm
http://www.microwaves101.com/encyclopedia/spectrumanalyzer.cfm
http://www.microwaves101.com/encyclopedia/networkanalyzermeasurements.cfm#networkanalyzers

(it seems the whole back catalog of HP applications notes is listed here:
http://www.hpmemory.org/ressources/resrc_an_01.htm
but unfortunately, they don't have many on-line)
But Agilent seems to have lots of the 'good ol' stuff' still available:

http://www.home.agilent.com/agilent/facet.jspx?c=153297.i.1&to=80030.k.1&cc=US&lc=eng&sm=g

Hope this helps. in general it helps to have someone 'sit beside you' when attacking either of these instruments for the first time...they are quite complex and it's rather easy to measure wrong (but think it's right).

RF_guy

To be fair to me he did say simple. you give fuller more complex explanation. I could have expanded but felt it would be wasted. Sorry for the odd spelling error I was in a hurry. I worked for MI a long time ago they made the 2370/2371 frequency exstender. SA. circa. 1986 ish. Then 2380 all new singing and dancing model 1995 ish.

They also made the first mobile phone test set 2955 plus 2956? and RF power meter 2022? True rms up to 2Mhz. low frequency I know. Their microwave division was based at Stevenage Instruments St. Albans. Now gone. Don't know if they exist at all now. Us take over.

Model numbers a bit hazy.

Hi,

All the above are correct to a greater or lesser extent.

RF Spectrum Analyser: Measures Frequency against Power. It will display and measure the frequency and power of any signals present within the defined upper and lower frequency range. The majority of old SA's are 'swept' in that the local oscillator sweeps across a frequency band to down convert and detect the RF signals. More modern equipments like the Tektronix new RSA (real time SA) don't sweep as such but 'stare' at a given bandwidth and digitise everything within that window, capturing it all into memory in realtime! Essential for anyone working in digital RF Comms, Radar, SIGINT etc.

A good quality modern spec an will also be able to measure the digital demodulated signal and make all the signal quality measurements needed.

A Vector Network analyser measures 2 or 4 port power v frequency and phase. Representing the information in either phase plots, Smiths chart or Polar chart. They are very narrow resolution bandwidth devices, enabling very low noise floor and high dynamic range measurements. You would use them to examine component or functional devices and their phase, impedance, capacitance, resistance and phase -transmission and reflection- over frequency and time.

A Scaler Network Analyser is a very different device. It too measure transmission and reflection coefficients but is not frequency or phase sensitive. It uses a broadband detector (much like a power meter thermistor) that detects ALL the RF present.

On the upside, this means it's fast. you would use it for instance in tuning up a filter. On the downside, the broadband detectors mean that your dynamic range or sensitivity is much lower.

Hope this helps.

Jon

Hi,

from Guest/Jon: "RF Spectrum Analyser: Measures Frequency against Power."

IMHO I'd reverse that, to be clearer: A spectrum analyser measures power (or better: relative amplitude) against frequency.

"It will display and measure the frequency and power of any signals present"

- To which I would diplomatically add: *relative* power...despite precise-looking readouts, measuring actual power with a spectrum analyser is often tricky & misleading, for all sorts of reasons (covered in the microwave101 & agilent resources).

Lastly, to add some historical perspective: none of these instruments is 'essential' as the first radars pre-date all of them! ;-))
http://math.la.asu.edu/~kuang/LM/030902-Radar_History10.pdf
oscilloscopes were just beginning to appear (ostensibly applied to radar research!):

http://en.wikipedia.org/wiki/Oscilloscope

But true: we'd all be 'blind' without these instruments in today's development environment!!

-RF_guy