I got the idea from reading the specs that the Fluke is flat - which Fluke defines as 2% - to 20 KHz. But, I like your number better. 🙂
So, I'll spend some time tomorrow testing to see just what it really is.
Question: to compare the noise floor in the plots above to other FFT graphs, I need to take into account the FFT bandwidth, the windowing function, and any averaging that might have been done, right?
Longer version of the same question:
According to the REW help file, locking the generator frequencies to the length of the FFT window assures that all the generated signals are periodic in the FFT, so that no signal leakage into neighboring FFT bins occurs (a pure sine will show up at a single frequency in the FFT graph). This also means that you can use a rectangular (= no) window, which is nice for noise calibration, because the 'window factor' is 1.
But I think you might still want to use a non-rectangular FFT window, because distortion products (sums and differences of base frequencies and of harmonics) will not generally be periodic in the FFT length, right?
If you use a (non-uniform) window then you will have to correct the noise floor. I guess you could make two FFT transforms, one with your window function of choice optimized for finding non-frequency-aligned peaks, and the other with no window function (i.e. rectangular window). Or you could just take the noise integral computed by REW, divide it by the square root of the bandwidth, and draw the noise line (assuming it is a flat line, i.e. white noise) by hand.
So, what is the correction factor for the noise floor in the plots above by @CG ?
Longer version of the same question:
According to the REW help file, locking the generator frequencies to the length of the FFT window assures that all the generated signals are periodic in the FFT, so that no signal leakage into neighboring FFT bins occurs (a pure sine will show up at a single frequency in the FFT graph). This also means that you can use a rectangular (= no) window, which is nice for noise calibration, because the 'window factor' is 1.
But I think you might still want to use a non-rectangular FFT window, because distortion products (sums and differences of base frequencies and of harmonics) will not generally be periodic in the FFT length, right?
If you use a (non-uniform) window then you will have to correct the noise floor. I guess you could make two FFT transforms, one with your window function of choice optimized for finding non-frequency-aligned peaks, and the other with no window function (i.e. rectangular window). Or you could just take the noise integral computed by REW, divide it by the square root of the bandwidth, and draw the noise line (assuming it is a flat line, i.e. white noise) by hand.
So, what is the correction factor for the noise floor in the plots above by @CG ?
Should be interesting. I certified many of these meters, and they always performed much better than expected. Mind you, HF performance is very sensitive to the internal shield positions. I took a dead 87 and drilled the case to line up with the adjustment points. The HF readings still changed when I put the original case back on, but they were much, much closer to where I adjusted it to.
With that in mind, your meter may not be close to where it should be, or is capable of. With this type of accuracy, "closed case calibration" was the only real way to maintain the potential of these meters. Sadly these are manual, open case.
I can't tell you how frustrating it was to adjust one of these really close, only to button it up and it might be out of tolerance. That's why I made the drilled case. Not perfect, way better.
With that in mind, your meter may not be close to where it should be, or is capable of. With this type of accuracy, "closed case calibration" was the only real way to maintain the potential of these meters. Sadly these are manual, open case.
I can't tell you how frustrating it was to adjust one of these really close, only to button it up and it might be out of tolerance. That's why I made the drilled case. Not perfect, way better.
Hi Gruesome,
Many meters are far worse. In fact, you would be shocked by how many are not in tolerance brand new out of the box. Then there is the question of time, and holding calibration.
I only buy HP / Agilent / Keysight or Fluke handheld meters. Experiences in the cal lab.
Many meters are far worse. In fact, you would be shocked by how many are not in tolerance brand new out of the box. Then there is the question of time, and holding calibration.
I only buy HP / Agilent / Keysight or Fluke handheld meters. Experiences in the cal lab.