A spectrum analyzer is a receiver.. It gets tuned by an electronic sweep signal rather than by using a knob. The spectrum analyzer gets tuned from one part of the band to another part of the band (called sweep width) just the same as you would manually tune your receiver from say frequency A to B.
The spectrum analyzer has a narrow IF filter just like any receiver does. However most spectrum analyzers have many selectable bandwidth filters where your receiver may have only one or two. In a regular receiver the detected audio feeds an audio amplifier. In a spectrum analyzer the detector has a response down to DC and feeds the vertical amplifier of a scope. The stronger the signal the more output from the detector and the higher the deflection on the vertical scale of the scope. The electronic signal that tunes the analyzers frequency is a saw tooth signal. That same tuning signal is also fed to the horizontal deflection circuit of the scope and causes the trace to move across the scope at the same rate as the receiver in the spectrum analyzer gets tuned. This gives you a horizontal trace on the scope that is in sync with the frequency being tuned. If the analyzer happens to tune across a signal you get a vertical deflection on the scope at the point that corresponds to the frequency. This lets you see the amplitude of the signal and the frequency it is on. You can do exactly the same thing with a receiver by tuning it and watching the S meter. You can tell the frequency and how strong it is at that frequency. The shape of the signal that is seen on the spectrum analyzer is actually the shape of the band pass filter in the analyzer. A narrower filter gives what appears to be a narrower signal on the screen. Most analyzers automatically select the narrowest usable filter for a given frequency span and sweep speed. Better analyzers also let you select these independently. There is a trade off though. The wider the sweep the slower the sweep speed needs to be. Also the narrower the IF bandwidth the slower the sweep speed needs to be and or the narrower the sweep width. When adjusting those manually you will notice that too narrow a filter will distort the shape of the display peak and it will also start to shrink in amplitude. The same thing happens if you select too wide a sweep or too fast sweep speed. All three are dependent on the others. Any time you see the display starting to shrink in amplitude as you are adjusting you need to change one of the three back the other way so this does not happen. The cause is phase shift in the IF. Think about it like the signal does not have enough time in the IF band pass to deliver enough power to be detected fully. Widening the IF filter, slowing down the sweep speed or narrowing the sweep width all accomplishes the same thing. More time for the signal to be in the pass band of the IF filter. Noise floor is the maximum sensitivity of the spectrum analyzer. The weakest signal it can detect. With a wider IF filter it lets in more noise power and raises the noise floor. To see a weaker signal (lower the noise floor) you need to use a narrower IF filter. (video filters help some too) When using a narrower IF filter as explained above, the sweep speed needs to be slowed down and or the sweep width made less in order to be effective. The problem with using very narrow filters is it takes a long time to sweep across a range of frequencies and you may miss a signal that comes and goes (is not on all the time). With a narrow filter it may take several seconds before the next sweep comes by on the analyzer because it has to run so slow. With slow sweeps it is necessary to have a storage scope in the analyzer. Most modern analyzers have digital storage to constantly display what the analyzer has already swept by. Without storage the sweep would need to run very fast as it does on an ordinary oscilloscope or you would have a hard time seeing "the picture". One other difference in a (good) spectrum analyzer and a regular receiver is dynamic range. A spectrum analyzer needs to be able to see very weak signals and at the same time not overload on a very strong signal. This is a very difficult task and many analyzers fall short in this area. The IFR1200 type analyzers are not good in the dynamic range department nor are the Motorola analyzers in the service monitors. You can easily be fooled when using one as a spectrum analyzer if not careful. They do not have the features of selectable filters and sweep speed either. There is a world of difference between the analyzers in service monitors and a real spectrum analyzer. Someone asked about lowering the noise floor of an analyzer. A good preamp may help but you have to be careful that it does not generate IM products in the analyzer and give you false readings. You would subtract the gain of the preamp from the readings to give absolute signal levels. It would be best tried with a good signal generator to see how it responds and how its bandwidth is. That's the 10 cent tour on spectrum analyzers. 73 Gary K4FMX _____ From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Don KA9QJG Sent: Tuesday, April 24, 2007 2:31 PM To: Repeater-Builder@yahoogroups.com Subject: RE: [Repeater-Builder] quick question about spectrum analyzer noise floor Can Someone Please put the Spectrum Analyzer thread in Layman Terms I have a Motorola Serv Monitor R2001C With the Analyzer and a Icom R-7000 Communications Receiver with a AVCOM Spectrum Analyzer I Can see 10 Mhz at a Time and I know that it's nice to Find Signals. But I always thought that a Actual receiver IE Scanner running the right Software would actually find and see more Hits because it is actually a Receiver. I know for a fact I can hear a lot more on a Cheap Scanner then using a Service Monitor on the same antenna. What do I not understand here Thanks Don KA9QJG
