HOW ABOUT COVERING (MONITORING) FM & AERO BANDS

  • 4
  • Idea
  • Updated 11 months ago
  • Not Planned
  • (Edited)
I would like to see coverage of the FM and AERO bands - even if they had to be excluded from pre-selection for whatever reason.  Both provide information and entertainment when one's preferred ham bands are dead, and in general.

In addition, emergency interoperation with aviation should probably not be excluded.

Alex/KM5YT
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Alex - KM5YT

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Posted 4 years ago

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Photo of Steve - N5AC

Steve - N5AC, VP Engineering / CTO

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Official Response
In a direct sampled system such as the FLEX-6000, the spectrum will be broken into a series of Nyquist zones related to the sampling frequency.  Each zone is the width of the sampling frequency, fs, divided by two.  In the FLEX-6000 the sampling frequency is 245.76MHz and so the spectrum is broken into a series of zones that are 122.88MHz wide as such:

Zone 1: 0 - 122.88MHz
Zone 2: 122.88MHz - 245.76MHz
Zone 3: 245.76MHz - 368.64MHz

and so on...

Frequencies from each zone alias into the other zones, wrapped about the Nyquist frequency if a filter has not been employed to prevent it.  For example, A signal at 98.9MHz will alias to 146.86MHz (122.88 - 98.9 = 146.86 - 146.86).  To prevent this, filters are used to select a single Nyquist zone to pass to the data converter (ADC).  The filters must reduce the signal to a level where the signal will not be present at all when aliased so that it is not visible in the other band.  So we have to filter the broadcast FM band to a level so low that it will never appear when we look at the 2m band.  To achieve this with strong FM transmitters, we use both a band-notch filter on the FM broadcast band and a bandpass filter on the 2m band.  The result is that the FM band is long gone in the FLEX-6000.

If you'd like to understand all this in more detail, here are a few references:

Analog Devices: What the Nyquist Criterion Means to Your Sampled Data System Design 
Texas Instruments: Intermediate Frequency (IF) Sampling Receiver Concepts 
Wikipedia: Nyquist-Shannon Sampling Theorem