Electronics > RF, Microwave, Ham Radio

Does professional spectrum analyzer such as keysight,RS has `aliasing` problem?

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TheUnnamedNewbie:

--- Quote from: tggzzz on May 28, 2022, 09:37:03 am ---

A standard question I used to ask interviewees is "There is an audio signal on a 10MHz carrier. What is the minimum sampling frequency". The answer isn't 20MS/s. The point is to be careful of the definition of "signal".

--- End quote ---

Lots of research going into undersampling ADCs, that have S/H that can go to a few times the sample frequency for digital RF/SDR type applications.

Joel_Dunsmore:
The term of art used for the vernacular "aliasing" is "image-response" when you refer to spectrum analyzers.  And you want your SA to reject images, thus the figure of merit is often called "image rejection".  High performance SAs use two methods of image rejection: 1) up-convert, filter, down-convert (used from DC to mid 3.6-8 GHz range depending on SA).  2) Filter and direct down-converter, usually with a swept tuned YIG filter.  The up/down has the advantage of being very stable, but exposes the front end mixer to all signals in the input range and thus the input converter can be saturated by a nearby signal that is not being measured (e.g. a 900 MHz cell phone transmitter could saturate the front end when looking at a tiny 2.4 GHz wifi signal).  The swept-tuned mode used in microwave bands has the disadvantage that it can be more unstable and has limited bandwidth for real-time acquisitions (typically 40 MHz).  But typically you can bypass the preselector to get wider bandwidths at the expense of being sensitive to images. 

Images come about because the first converter is tuned to an LO freq (say 20 GHz) to measure a signal offset (typically above) to the first converter, mixing it to the IF (e.g. 322 MHz IF).  So when you set the SA center frequency at 20.322 GHz, the LO is at 20 GHz, and IF comes out of the front end mixer at 322 MHz and is digitized.  But if you ad a signal at 19.678 MHz, it would also covert to 322 MHz and show up as a signal and we call this an image; the process is quite similar to alias signals in an ADC.  So we rely on the YIG filter to remove the image.

Or, some versions of SA don't have any image protection, and use digital image rejection to move the LO about, process what shows up in the IF, and sorts out real from image signals.  Turns out, if you do this well you can get very good results; we do it well in the Keysight PNA (option S9309xx).

P.S. This is all discussed, in excruciating details, in Chapter 8 of second edition of my book ( www.tinyurl.com/JoelsMicrowaveBook )

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