And you are sure the difference isn't created by you taking the "fast track" in relation to the long method used by Martin?
Yes, absolutely sure. Even for a traditional swept spectrum analyzer, the required total sweep time for accurate amplitude measurements would be less than 4 milliseconds.
The formula for the sweep time is as follows:
SWT = k * Span / RBW²;
In our example: SWT = 3 * 1 GHz / (910 kHz)² = 3*10e9 / 828.1e9 = 3.623 ms;
K is a factor, depending on the filter characteristics, usually between 1 and 3 - and I’ve assumed the worst case here.
But our FFT on a DSO isn’t a swept analyzer, rather a realtime analyzer which requires much less time, let’s try to calculate this:
This is a 8k FFT, that means we have 4096 frequency bins, each of them ~244 kHz wide. Even more important, the resolution bandwidth (RBW) is even wider, ~910 kHz with the Flattop window, so we gain quite a safety margin if we just consider the frequency step instead of RBW.
With a 600 seconds sweep for 1 GHz, the signal stays within one frequency bin for 600 s * 244 kHz / 1 GHz = 146.4 ms – this is about 40 times longer than the entire sweep time required for a swept analyzer. In other words – it’s orders of magnitude slower than what is needed for accurate measurements.
I don’t know the exact speed of the FFT with these settings, but it certainly is less than 100 ms. That means, at 600 seconds sweep speed, we get at the very least one full FFT for the width of a single frequency bin. And this is exactly what we want to get a nice contiguous trace with only one single sweep.
I’ve also tried different sweep speeds and the net result has always been the same.
Or the gen quality?
That’s a valid question. Apart from the better matching because of the attenuator, I also had to raise the output level to +10 dBm. So maybe the output level accuracy suffers at the higher level, or has different flatness?
I’ve looked up my old measurements, which indicate an accuracy of +0.38 / -0.14 dBm at 0 dBm output and +0.34 / -0.15 dBm at +10 dBm for the frequency range from 100 kHz to 1 GHz (it would be vastly better if we stick to just 100 MHz). At 600 MHz, the amplitude error is about -0.1 dB for both level settings. So this is not the reason.
Thinking about the differences, there are only two things:
1. The attenuator, where I’ve used a high quality 18 GHz Narda part to ensure best accuracy.
2. The cable – now I needed SMA connectors so couldn’t use the Hyperflex 5 BNC-BCN cable from before.
I’ve grabbed the first random cable, which looked trustworthy – but a closer look now revealed that it’s just crappy RG58 C/U. So right now I’ve repeated that test with a proper Hyperflex 5 low loss cable, this time with SMA connectors and BNC adapters
SDG7102A_1000MHz_-10dBm-2
.
Yes, this makes quite some difference. The -3 dB bandwidth is only 570 MHz, but that is within the ballpark. Particularly the drop of only 1 dB at 500 MHz is a very welcome result.
The less than stellar flatness between 350 MHz and 450 MHz remains and is just a property of my instrument.