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panadapter signal level units

Mike - K6QY
Mike - K6QY Member ✭✭
edited December 2019 in FLEX-6000 Signature Series
New to Flex Radio, so this question might be something that's general knowledge for y'all. However, the Flex literature does not speak to this question at all...

Power spectra have units of (power)/(measurement bandwidth). The Flex panadapter on my 6400, however, is calibrated only in terms of power. So... are the true values of the spectral densities displayed to be interpreted as the "dBm level" indicated, divided by the bandwidth I've selected within the chosen operating mode (e.g. USB/LSB/CW/...)? Or are they calibrated in terms of a fixed bandwidth already and just not labeled correctly on the panadapter's y-axis?

On the other hand, the standard value for spectral densities in the measurement community is typically in dBm/Hz (although some folks in the amateur community seem to prefer dBm/MHz).

Which is it for the Flex signature series?

-thanks,
Mike / KB2TO 

Answers

  • VK7WH Winston
    VK7WH Winston Member ✭✭
    edited April 2019
    Mike, there are others better qualified than me to comment, however I believe Flex use a bandwidth of 500Hz for calibration. Steve, FlexRadio VP of Engineering wrote an excellent paper on this subject. I will try and locate it for you. Welcome to Flex. I’m sure you will enjoy the ride. Winston
  • Mike - K6QY
    Mike - K6QY Member ✭✭
    edited December 2019
    Winston - you're right on. I did find a comment from Steve in the Flex community archives. He said precisely that... so I'm now good! (But I think they should explicitly label the y-axis that way!) 

  • Mike - K6QY
    Mike - K6QY Member ✭✭
    edited December 2019
    FYI... Answering my own question... here is that post from >1Y originating from Steve at Flex, which VK7WH mentioned:


    The panadapter simply measures the signal in a given bandwidth and draws what it hears. If you look at any given pixel, it represents a certain amount of bandwidth. We call this the "bin size" of the FFT that is used to produce the display. If you cut the bin size into two pieces, the amount of noise in each piece goes down by half (3dB). In PowerSDR, the bin size is generally fixed for any given setup and does not change when you zoom. This is why the resolution gets worse as you zoom in on PowerSDR -- you begin to show one bin with multiple pixels. But for SmartSDR, we knew we wanted to have a larger range of zoom and this method was no longer acceptable. So we vary the bin size across a 1000:1 range. So the noise in each bin also varies. 1000:1 is a change of 30dB so from min zoom to max zoom, the noise in a bin will lower by 30dB and you see this change in the panadapter as you zoom in and out. 

    When people talk about noise floor in ham radio they are generally talking about the noise level with a 500Hz bandwidth. When the panadapter is zoomed in to the max level, the bin size today is about 5.8Hz. This is a 19dB difference in noise from where a ham would say the noise floor is to what you can see on the panadapter. This means that the panadapter can see 19dB below what most hams would call the noise floor. Your ear and brain are also able to hear below the noise floor in 500Hz because of how they work. But there are limits to how well you can hear. If you've ever worked JT65 or another long-term integrating mode, you have noticed that your computer can copy signals that you cannot hear. 

    So if you ask another ham "where is your noise floor on 80 meters" and he says "S5," what has he told you? Well with most hams, you don't know because you don't know the answer to these questions: 

    1. What bandwidth are you using to measure the signal? 
    2. Is your S-meter calibrated? 

    An S5 signal corresponds to -97dBm. And if he's getting this on sideband set to, say 2.8kHz bandwidth then the actual noise floor in 500Hz would be -97 - 10*log(2800/500) = -104dBm. There's nothing magical about 500Hz, it just happens to be the convention for measuring noise in the ham radio world. In SmartSDR, if you set the passband filter to 500Hz, the S-meter in the slice will show you the 500Hz noise floor. 

    If you start at maximum zoom and begin zooming out, you can see a point where the noise reading of the panadapter equals this number. What do you think this point is? ... if you've been following along, you will realize that this is the point where the FFT bin size is 500Hz. To get a rough idea if this is right, you could measure the width of your panadapter window and divide the amount of frequency displayed by this number. It should be in the 250-1000Hz range. The answer will not be exact because we do not continuously vary the FFT bin size -- we adjust it in steps and don't tell you where the steps are or what size they are. We do what's right for what you are viewing. 

    I know that was a long-winded answer, but I hope it provides some insight into noise and how it changes what you see and hear. 
  • Mike - K6QY
    Mike - K6QY Member ✭✭
    edited April 2019
    And I must add one clarification to what Steve said... in the fifth paragraph, the units should actually be -104dBm/500Hz = -131dBm/Hz

    Suggestion to Flex: I would like Flex radio to output the y-axis in dBm/Hz, just like all of my lab's HP/Agilent/Keysight spectrum & network analyzers. Hey, why not... all the requisite goodies are already in the Flex hardware & software! I think it's high time for hams to move away from ambiguous, situation-dependent metrics like "S"!!
  • Craig Williams
    Craig Williams Member ✭✭
    edited April 2019
    2 my distractor, see, this is a useful conversation for everyone in the community. VS whining about the ANF, forever.

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