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Why do we use ramp generator (sawtooth generator) with local oscillator in a spectrum analyser? I have searched in Google and I didn't find any answer that made me understand the concept. Thanks !

Sri Harsha
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    sorry, but you *claim* "we" use a ramp generator. I don't think so. You will need to add a source from where you take that claim. Yes, a Spectrum Analyzer "sweeps" frequencies, but there's many ways to do that, and if I had a guess, Spectrum Analyzers have not been feeding voltage-controlled oscillators with a sawtooth in a long, long time. – Marcus Müller Mar 22 '17 at 17:03
  • The ramp-sweep of the LO is a time-honored implementation. Consider a narrow-band fixed-frequency amplifier, which outputs diode-detected amplitude and log(amplitude), either chosable to cause vertical deflection of an electron beam. The ramp causes horizontal deflection. Ahead of the amplifier, we have a 30-60MHz voltage-controlled oscillator, that VCO driving a mixer; the other input to mixer is the DC-30Mhz input. The mixer's output of 1.2MHz (signal) and 58.8MHz (from the VCO) is 60Mhz. Mixer output of 9.9MHz and 50.1Mhz is 60MHz. Our narrow-band amplifier needs to be 60MHz. – analogsystemsrf Mar 22 '17 at 17:22
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    By the way, Sri, hint: if your research shows no sign of an implementation actually being like it is in the slides, textbook or other teaching material you might be reading, that teaching material might be simplifying or simply outdated. – Marcus Müller Mar 22 '17 at 17:28
  • Traditional Spectrum Analyzers use a linear sweep over a 2:1 max range starting from the above the highest f capacity of the instrument in order to up convert to an IF above the spectrum. This is become you cannot make a VCO sweep so many decades or in linear manner. Then 2nd and 3rd IF's refine the BW. DSO's on the other hand use FFT's with linear or log sweep of the results. Now SA's are an intelligent hybrid of sweep, multi-IF and FFT – Tony Stewart EE75 Mar 22 '17 at 17:37

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See Spectrum Analyzer Basics App Note 150 from Keysight or Agilent AN 150 page 9 for the classic spectrum analyzer block diagram with a ramp. As others have commented, this is no longer the preferred implementation of a Spectrum Analyzer, but the concept is the same. Some analyzers still feed a sweep ramp out of the rear panel corresponding to what this voltage would be, if it were done that way.

The ramp is fed into a Voltage Controlled Oscillator. The swept voltage is converted into a swept frequency.

I don't speak for Keysight, even though I work there.

Tom Anderson
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