panadapter signal level units

Mike - K6QY

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 

VK7WH Winston

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

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

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

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"!!

Steve-N5AC

Mike, great question and good data collection on your part.  So a couple of things I should add.  We alter the bandwidth of the panadapter bin based on your zoom level.  We currently do not report this value so you really don't know what it is.  If you zoom in and can record the bandwidth on the screen, however, you can probably calculate it.  A 6700 zoomed all the way in has a bandwidth of 1.4648 Hz per pixel.  The 63/64/6500 have a minimum bandwidth of 5.8594 Hz/pixel.  Off the top of my head, I don't remember where it is in the 6600, but it's likely 1.4648Hz/pixel.  As you zoom out, each time the panadapter noise jumps up by 3dB, this will be a doubling in the bin size.  

So given this, lets say you have zoomed out three levels on a FLEX-6400 from max zoom, and your noise floor appears to be at -115dBm.  Your noise would be -115 dBm - 10log(5.8594 * 2 ^ 3) = -131.7 dBm/Hz.

As an engineer, this is a useful number for me as well, but I believe you're just about the first person to ask for this data from the radio.  I, personally, would like to have a checkbox somewhere that would add this data to the screen.  Maybe we'll do something like this in the future, but for now I hope this will help you calculate the noise.  FYI, we're probably accurate to around 1-2dB for the most part.  In an expensive piece of test equipment that uses direct sampling, great care is taken to ensure no FFT scalloping which is simply a non-concern for us.  Because this is very expensive to implement (minimal scalloping), we just don't bother because it's not what the device is made for and it makes little sense to burden it with this kind of cost.  This is one reason why COTS SA's are expensive.

Also, another convenient way to measure the noise floor is to use the slice receiver.  You could set it to a width of 1kHz and then use the s-meter to measure the noise (obviously in a part of the band with no signals).  The s-meter can be clicked to switch the reading to dBm.  After you've done this, you can subtract 30dB to arrive at the proper dBm/Hz value since 10log(1000) = 30.  This is generally what we do in the lab as it's more certain that we know what we're measuring and the zoom-in/out method is subject to more human error.  For those doing this for the first time to understand how this works, you can try changing the slice receiver bandwidth to many different values and re-run this calculation to verify that you understand how it works.  The calculation is (dBm reading from slice receiver) - 10log(bandwidth of slice receiver in Hz).  Incidentally, if you get a number less than -174dBm, you're doing it wrong ;-)

Craig Williams

2 my distractor, see, this is a useful conversation for everyone in the community. VS whining about the ANF, forever.