Understanding why panadapter noise floor & S-meter are different, why the atten. makes no difference, and S-meter cal.
Posted: Tue Jun 13, 2017 1:55 am
Summary: the panadapter noise floor will NOT match the S-meter reading of noise power and there are good mathematical reasons for this.
The levels shown in the S-meter represent the total power in the selected RX passband, either in S-units, dBm, or both, depending on the style S-meter selected. This is in accordance with the ITU standards for S-meter measurements.
The levels shown in the panadapter represent the total power in the selected panadapter FFT bin width in dBm. There is no corresponding ITU standard.
Since noise power is broadband and relatively monotonic, at least over 10's to 100's of KHz, it scales perfectly with respect to noise measurement bandwidth. CW signals, which generally fit within both the selected passband and the FFT bin width, will measure the same in either case because of of their narrowband characteristic. SSB phone signals can be a bit confusing, but you simply have to remember that any RF power measurement must be referenced to the bandwidth it is measured in. If you measure the power of an SSB signal in a 2 or 3 Hz bandwidth you are only measuring the power in that 2 OR 3 Hz bandwidth. If you measure the power of an SSB signal in a 2 or 3 KHz bandwidth then you are measuring all of the power in that 2 OR 3 KHz bandwidth. These measurements are NOT the same, are NOT intended to be the same, and provide different views of the same information, each with its own usefulness. The first measurement (panadapter) allows one to understand how the total power is distributed within the 2 or 3 KHz total bandwidth of the SSB signal. The second measurement merely measures the total power for the entire signal (S-meter measurement).
Introduction:
Understanding the difference between what the panadapter is showing you and what the S-meter is showing you is a relatively new problem for hams but old hat to RF engineers.
In the RF engineering world the same kind of confusion often exists for new engineers when they try to make sense of measurements using a spectrum analyzer (the panadapter equivalent) and an RF power meter (sort of the equivalent of the S-meter).
Panadapter/Spectrum Analyzer Displayed Average Noise Level (DANL):
What you are seeing on the panadapter (spectrum analyzer) is known as the Displayed Average Noise Level (DANL). Some people call it Displayed Average Noise Floor. By either name it's the same thing.
Before discussing the DANL further, understand that these days there are two different kinds of spectrum analyzer measurement methods. Older analog spectrum analyzers used a swept superhet receiver with a filter after it. The bandwidth of that filter is the "Resolution Bandwidth". Narrow resolution bandwidths require longer sweep times because the local oscillator in the spectrum analyzer can only be moved in an accurate fashion so quickly and because the risetime of a signal is inversely proportional to its bandwidth. Newer, digital spectrum analyzers perform FFTs and construct the spectrum using math. They can make instantaneous snapshots of a band because the data is gathered in a very wide filter and nothing needs to be tuned or swept. The Bin Width of the FFT is analogous to the Resolution Bandwidth of the swept superhet.
Now obviously our modern SDR radios are essentially inexpensive digital spectrum analyzers using FFT methods. When comparing the DANL on any combination of spectrum analyzers or SDR radios the Bin Widths and/or Resolution Bandwidths MUST be matched in order to get the same results. The reason for this is because ALL RF power measurements MUST be referenced to some bandwidth. Clearly, there is more noise power in 10Hz of bandwidth than 1Hz, so this should make some intuitive sense. Assuming the noise is normal Gaussian noise, and that the Resolution Bandwidth/Bin Widths are all matched, any differences between measurement instruments or radios are going to be the result of differences in the noise figure of those instruments or radios.
The bottom line here is that the DANL is the noise power that exists within a single Resolution Bandwidth/Bin Width on the panadapter or spectrum analyzer.
S-Meter Noise Measurement:
Now looking at the S-meter, we can just consider it a single channel, non-swept, spectrum analyzer with an adjustable Resolution Bandwidth/Bin Width. In our case, that adjustment is the receive filter passband. Let's look at an example of how that compares to the panadapter.
If the panadapter Bin Width is 3Hz and the S-meter bandwidth is 3KHz, then the S-meter will be taking in 1000 times more noise energy than a single panadapter bin. A factor of 1000 equals 30dB (10log1000). So clearly the DANL will appear MUCH lower than the S-meter reading of the same noise condition. You can easily demonstrate this to yourself by fooling with the Bin Widths and receive passband settings to obtain any difference you want within the range of adjustment of the radio. Indeed, most RF engineers will measure and report broadband noise by referencing it to the bandwidth of a single Hz. For instance the thermal noise floor used in many engineering calculations is -174dB/Hz. That way any other RF engineer can calculate the total noise power in any bandwidth he wants.
To calculate the difference between two bandwidths in decibels, use the following formula: 10log(bandwidth 1 in Hz/bandwidth 2 in Hz). Example: 10log(3000Hz/3Hz) = 30dBHz.
Better example: your panadapter bin width is 2.93Hz and you see a DANL of -120dBm. Your receiver passband is set to 2.9KHz. 10log(2900/2.93) = 29.96dB. With just noise in the passband (no signals), the S-meter will display -90dBm (approx. S6).
Why The Differences in Noise Power Don't Match Differences in Signal Power:
So far so good, at least until somebody tries to compare noise power to signal power. Then they get all confused. But that's because they are comparing apples to oranges. Broadband noise is different than a narrowband signal. Consider a CW signal with an amplitude of -73dBm, which is equal to S9. IF the bandwidth of the CW signal fits ENTIRELY within a single FFT bin then it will appear on the panadapter with an amplitude of -73dBm. AND it will appear on the S-meter with an amplitude of S9 (actually a tiny bit more because there is noise power in there too, but the difference the noise power makes is negligible when compared to such a large signal). Now imagine if the CW signal is twice as wide as an FFT bin (unlikely, but please bear with me). In such a case the amplitude of the CW signal on the panadapter will appear to be -76dBm. That is 3dB, or a factor of 2 less, because now the power of the signal is spread over TWO panadapter bins. Meanwhile the entire CW signal still easily fits within the S-meter passband so the S-meter still reads S9. If the CW signal spread over 4 bins it would appear to be 6dB lower on the panadapter. If it spread over 10 bins it would appear to be 10dB lower. And so on and so forth. Take this to the limit and you can see how the wider a signal is compared to a Bin Width the lower it appears on the panadapter compared to the S-meter. And noise is infinitely wide!
Now obviously CW signals are not that wide. But SSB phone signals are! Since the energy is spread over the entire SSB signal bandwidth no individual peak will ever be as high as the S-meter reading, unless the operator whistles a pure tone and the bandwidth of that tone fits entirely within a single panadapter bin. Thus it is very, very difficult to make comparisons of SSB power levels on the panadapter vs. SSB S-meter readings. They are not so neat and clean as a CW or noise signal.
S-meter Accuracy:
Finally, do not put too much faith into the accuracy of the typical Kenwood/Yaesu/Icom S-meter where noise is concerned. They are, in fact, not that accurate. It would appear that the marketing departments at those companies have had the engineering departments arrange things so that the S-meters are highly non-linear below S9, such that they can claim to have "quiet" receivers. Those receivers are not any quieter than the receivers on our ANAN series radios, or Flex radios for that matter. But their S-meters are junk compared to our honest ones. So don't feel bad when your ANAN is telling you that you have S7 noise on the band and your trusty FTDX5000 is telling you S3. Guaranteed the ANAN is correct and the Yaesu or whatever is lying. You can prove this by using an expensive spectrum analyzer to measure the correct channel power. The channel bandwidth will either be that of the S-meter or of the panadapter bin width. When testing S-meter accuracy set the Resolution Bandwidth to be EQUAL to your receiver passband. When testing panadapter accuracy set the Resolution Bandwidth to be EQUAL to your FFT bin width.
See also: http://rfmw.em.keysight.com/spectrum-analyzer/
and
73!
Scott
The levels shown in the S-meter represent the total power in the selected RX passband, either in S-units, dBm, or both, depending on the style S-meter selected. This is in accordance with the ITU standards for S-meter measurements.
The levels shown in the panadapter represent the total power in the selected panadapter FFT bin width in dBm. There is no corresponding ITU standard.
Since noise power is broadband and relatively monotonic, at least over 10's to 100's of KHz, it scales perfectly with respect to noise measurement bandwidth. CW signals, which generally fit within both the selected passband and the FFT bin width, will measure the same in either case because of of their narrowband characteristic. SSB phone signals can be a bit confusing, but you simply have to remember that any RF power measurement must be referenced to the bandwidth it is measured in. If you measure the power of an SSB signal in a 2 or 3 Hz bandwidth you are only measuring the power in that 2 OR 3 Hz bandwidth. If you measure the power of an SSB signal in a 2 or 3 KHz bandwidth then you are measuring all of the power in that 2 OR 3 KHz bandwidth. These measurements are NOT the same, are NOT intended to be the same, and provide different views of the same information, each with its own usefulness. The first measurement (panadapter) allows one to understand how the total power is distributed within the 2 or 3 KHz total bandwidth of the SSB signal. The second measurement merely measures the total power for the entire signal (S-meter measurement).
Introduction:
Understanding the difference between what the panadapter is showing you and what the S-meter is showing you is a relatively new problem for hams but old hat to RF engineers.
In the RF engineering world the same kind of confusion often exists for new engineers when they try to make sense of measurements using a spectrum analyzer (the panadapter equivalent) and an RF power meter (sort of the equivalent of the S-meter).
Panadapter/Spectrum Analyzer Displayed Average Noise Level (DANL):
What you are seeing on the panadapter (spectrum analyzer) is known as the Displayed Average Noise Level (DANL). Some people call it Displayed Average Noise Floor. By either name it's the same thing.
Before discussing the DANL further, understand that these days there are two different kinds of spectrum analyzer measurement methods. Older analog spectrum analyzers used a swept superhet receiver with a filter after it. The bandwidth of that filter is the "Resolution Bandwidth". Narrow resolution bandwidths require longer sweep times because the local oscillator in the spectrum analyzer can only be moved in an accurate fashion so quickly and because the risetime of a signal is inversely proportional to its bandwidth. Newer, digital spectrum analyzers perform FFTs and construct the spectrum using math. They can make instantaneous snapshots of a band because the data is gathered in a very wide filter and nothing needs to be tuned or swept. The Bin Width of the FFT is analogous to the Resolution Bandwidth of the swept superhet.
Now obviously our modern SDR radios are essentially inexpensive digital spectrum analyzers using FFT methods. When comparing the DANL on any combination of spectrum analyzers or SDR radios the Bin Widths and/or Resolution Bandwidths MUST be matched in order to get the same results. The reason for this is because ALL RF power measurements MUST be referenced to some bandwidth. Clearly, there is more noise power in 10Hz of bandwidth than 1Hz, so this should make some intuitive sense. Assuming the noise is normal Gaussian noise, and that the Resolution Bandwidth/Bin Widths are all matched, any differences between measurement instruments or radios are going to be the result of differences in the noise figure of those instruments or radios.
The bottom line here is that the DANL is the noise power that exists within a single Resolution Bandwidth/Bin Width on the panadapter or spectrum analyzer.
S-Meter Noise Measurement:
Now looking at the S-meter, we can just consider it a single channel, non-swept, spectrum analyzer with an adjustable Resolution Bandwidth/Bin Width. In our case, that adjustment is the receive filter passband. Let's look at an example of how that compares to the panadapter.
If the panadapter Bin Width is 3Hz and the S-meter bandwidth is 3KHz, then the S-meter will be taking in 1000 times more noise energy than a single panadapter bin. A factor of 1000 equals 30dB (10log1000). So clearly the DANL will appear MUCH lower than the S-meter reading of the same noise condition. You can easily demonstrate this to yourself by fooling with the Bin Widths and receive passband settings to obtain any difference you want within the range of adjustment of the radio. Indeed, most RF engineers will measure and report broadband noise by referencing it to the bandwidth of a single Hz. For instance the thermal noise floor used in many engineering calculations is -174dB/Hz. That way any other RF engineer can calculate the total noise power in any bandwidth he wants.
To calculate the difference between two bandwidths in decibels, use the following formula: 10log(bandwidth 1 in Hz/bandwidth 2 in Hz). Example: 10log(3000Hz/3Hz) = 30dBHz.
Better example: your panadapter bin width is 2.93Hz and you see a DANL of -120dBm. Your receiver passband is set to 2.9KHz. 10log(2900/2.93) = 29.96dB. With just noise in the passband (no signals), the S-meter will display -90dBm (approx. S6).
Why The Differences in Noise Power Don't Match Differences in Signal Power:
So far so good, at least until somebody tries to compare noise power to signal power. Then they get all confused. But that's because they are comparing apples to oranges. Broadband noise is different than a narrowband signal. Consider a CW signal with an amplitude of -73dBm, which is equal to S9. IF the bandwidth of the CW signal fits ENTIRELY within a single FFT bin then it will appear on the panadapter with an amplitude of -73dBm. AND it will appear on the S-meter with an amplitude of S9 (actually a tiny bit more because there is noise power in there too, but the difference the noise power makes is negligible when compared to such a large signal). Now imagine if the CW signal is twice as wide as an FFT bin (unlikely, but please bear with me). In such a case the amplitude of the CW signal on the panadapter will appear to be -76dBm. That is 3dB, or a factor of 2 less, because now the power of the signal is spread over TWO panadapter bins. Meanwhile the entire CW signal still easily fits within the S-meter passband so the S-meter still reads S9. If the CW signal spread over 4 bins it would appear to be 6dB lower on the panadapter. If it spread over 10 bins it would appear to be 10dB lower. And so on and so forth. Take this to the limit and you can see how the wider a signal is compared to a Bin Width the lower it appears on the panadapter compared to the S-meter. And noise is infinitely wide!
Now obviously CW signals are not that wide. But SSB phone signals are! Since the energy is spread over the entire SSB signal bandwidth no individual peak will ever be as high as the S-meter reading, unless the operator whistles a pure tone and the bandwidth of that tone fits entirely within a single panadapter bin. Thus it is very, very difficult to make comparisons of SSB power levels on the panadapter vs. SSB S-meter readings. They are not so neat and clean as a CW or noise signal.
S-meter Accuracy:
Finally, do not put too much faith into the accuracy of the typical Kenwood/Yaesu/Icom S-meter where noise is concerned. They are, in fact, not that accurate. It would appear that the marketing departments at those companies have had the engineering departments arrange things so that the S-meters are highly non-linear below S9, such that they can claim to have "quiet" receivers. Those receivers are not any quieter than the receivers on our ANAN series radios, or Flex radios for that matter. But their S-meters are junk compared to our honest ones. So don't feel bad when your ANAN is telling you that you have S7 noise on the band and your trusty FTDX5000 is telling you S3. Guaranteed the ANAN is correct and the Yaesu or whatever is lying. You can prove this by using an expensive spectrum analyzer to measure the correct channel power. The channel bandwidth will either be that of the S-meter or of the panadapter bin width. When testing S-meter accuracy set the Resolution Bandwidth to be EQUAL to your receiver passband. When testing panadapter accuracy set the Resolution Bandwidth to be EQUAL to your FFT bin width.
See also: http://rfmw.em.keysight.com/spectrum-analyzer/
and
73!
Scott