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In the radar literature, particularly in older references, the term "video signal", "downrange video", or just "video" is used to describe the radar signal at a certain point in the processing chain. Also, I have come across the related term "video frequencies".

In older radars with analog processing, I interpret the "video signal" to mean the voltage as a function of time at the output of an envelope detector after downconversion from the carrier frequency.

In modern radars, particularly coherent pulse-Doppler systems, the term "video signal" seems to be equivalent to the general signal processing term "complex baseband signal" or "complex envelope", in that it contains an in-phase and quadrature component. Although sometimes I see plots of the "video signal" where the magnitude of the baseband signal is plotted versus time, which seems roughly equivalent to the envelope-detected signal of an analog radar.

I interpret the phrase "video frequencies" to simply mean the signal has been downconverted to be centered at 0 Hz.

Are these interpretations of the term "video" correct, and is there any other nuance I should be aware of?

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  • $\begingroup$ In addition to Dan's accurate answer below, I'll just say that I believe your interpretations are correct in the contexts you mentioned. $\endgroup$ – Jason R Mar 21 '17 at 13:47
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You also see this on Spectrum Analyzers; "video bandwidth" for the same reason: the video is the output voltage from the power detector used to drive the vertical on a display (when the horizontal is the sweep), or in the case of the polar displays used to drive the radius where the sweep drives the angle. Since it is used for the display signal it is properly called the "video" signal. As I introduced, this is the same on a spectrum analyzer where we can adjust both "resolution bandwidth" and "video bandwidth": Resolution Bandwidth is the bandwidth of the effective bandpass filter prior to the power detector, while Video Bandwidth is the bandwidth of the effective low pass filter after the power detector. The first filter will reduce the total power incident on the detector when the input is spread as a spectral density, thus reducing the voltage out of the detector, while the second filter (video filter) will only smooth (average) the measured noise.

Here is a perfect picture showing what I describe in terms of the Spectrum Analyzer, credit Erik Diez of Agilent (now Keysight). http://electronicdesign.com/test-amp-measurement/fundamentals-spectrum-analysis

enter image description here

So for the case of radar, at least the original analog systems, the video output was a similar power detector used to drive one axis (either magnitude or as was common radius on a polar plot for a rotating system) of the radar output display.

enter image description here

(image credit: http://www.linuxwolfpack.com/swapshop.php)

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  • $\begingroup$ In modern radar systems, where both transmission and reception are quite sophisticated (polarization, chirps, different pulse combinations, etc.); /video signal/ refers to the un processed received signal. i.e. before removing spurious targets, fixed targets, etc. It is mostly used by technicians testing or troubleshooting the system. $\endgroup$ – Juancho Mar 21 '17 at 17:56
  • $\begingroup$ Yes. Without detracting from your comment which appears to be another answer (if that is what you intended you should post it there), the point of my response is that the origins of the words "video signal" are from legacy systems. $\endgroup$ – Dan Boschen Mar 21 '17 at 18:28
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In addition to Dan Boschen's answer:

There's also a quick and dirty "practical" reason that one would use the term "video" and is especially true if working on older analog radar systems. People will tend to refer to the signal as "video" once the RF signal is processed by some analog or digital circuit therefore making it a "non-RF" signal.

An example would be a legacy system that would use tuned vacuum tube RF pre-amplifiers to produce the "video". One would then consider the pre-amplifier the point of transition from RF to "video". The actual point of transition tends to be different to different people as well, such as considering it "video" after envelope detection.

Keep in mind that this type of jargon is used a lot by technicians and others who are quickly trying to distinguish which signal they are talking about. It's definitely a legacy term.

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