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I have a step attenuator with a series of switches (1-20 dB assorted values) that can be combined for (in theory) up to 80 dB total attenuation. Measuring the S21 individually, the 1,2,3,6,10,and 20dB switches yield S21 results on the nanoVNA that are very close to the stated values, testing over a 3-30 MHz frequency range. However, when I combine assorted switch values that sum to greater than 40 dB, the S21 results more or less max out at about -43 dB. On the other hand, the nanoVNA does displays S21 values in the -80 dB range with the step attenuator disconnected.

This makes we wonder whether the -43 dB maximum S21 result is a property of the step attenuator itself (internal leakage?), or is it simply the true dynamic range with my specific calibration, cables, connectors etc? Even 40 dB of dynamic range is plenty for my uses, and it seems like I find a new way to use this amazing device every day, but I am curious to learn more about what the dynamic range is.

Hi Bruce.

Interesting. Well, I have measured HF filters whose center frequency is less than 30 MHz and readily saw skirt responses that were 80 dB below the pass band response. Now I made sure I took the 0 dB reference line to the very top of the display range. So with 8 divisions at 10 dB/division I would see the 80 dB attenuation point plus some noise.

Now the attenuator certainly could be limited in its true attenuation range if not properly constructed. Not easy to build 100 dB attenuators! Take a decent HP step attenuator, there not cheap when new! Could be leakage.

As a check, I'll try the same simple experiment at my end.

Alan

I just did the experiment here with my hp 355D 100 dB attenuator/ 10 dB steps.

Attenuation is spot on and the VNA follows it just fine all the way down to 80 dB.
No surprises at HF.

If you have a COM receiver, certainly with sensitivity of better than -80 dBm and a sig gen,
try that combo and see if you have a defective step attenuator.

QSL. Alan

Bruce

What you are seeing is leakage through your attenuator. Attenuators intended for HF work and below, especially those with a network of mechanical switches and resistors, often display this behavior. The higher the frequency the more the leakage around the attenuator.

Measurement of the Nanovna dynamic range is simply a matter of reading from the screen. It is defined as the difference between the level shown on the screen when port 0 is connected directly to port 1, and the level of noise shown in the screen with nothing connected to port 1. This would be measured using Logmag format and looking at CH1.

The dyhamic range decreases with frequency due to the output level from port 0 decreasing as well.

WA8TOD

All,

Thanks for the excellent information - so it does sound like step attenuator leakage is the culprit....which is not at all surprising given that it is a very cheap one ($40) that I purchased on Ebay earlier this year. I used it for some low power (-10 dBm to 10 dbM final RF output) WSPR mode experiments, typically at settings below 30 dB of attenuation, using an oscilloscope to verify the actual RF output power.

It's great to know that the dynamic range of the nanoVNA in the HF zone is as large as the S21 displayed values.


73,
Bruce