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S21 oddity while measuring an LNA


sdturne 2020/08/16 16:30

Hi,

Today, I wanted to look at this mystery Avantek branded LNA that I have sitting around. It's specified for 5-1000 MHz and is supposed to be nominally 15 dB gain over this frequency range. However, the NanoVNA V2 is acting like there is a big step in S21 gain at about 125 MHz (see screenshot of NanoVNA saver in attachments). I also included the Touchstone file (second attachment) exported from the software. I have a high quality Narda 6 dB pad on Port 1, and I have entered this into the sweep settings of the software.

It is interesting to note that this step in S21 gain apparently does not exist. There is no sudden step in gain in this area when the same amplifier is fed a CW signal from an RF signal generator and the output is observed on my 8566B.

Any idea what might be going on?

Thanks,

Sean

OwO 2020/08/17 11:48

It looks like ADC saturation, the gain of the amplifier looks to be
around 15dB so try a 15 or 20dB attenuator on port 2.

On 17/08/2020 07:30, sdturne@q.com wrote:

sdturne 2020/08/17 20:07

On Sun, Aug 16, 2020 at 08:48 PM, OwO wrote:

>
> looks like ADC saturation, the gain of the amplifier looks to be around
> 15dB so try a 15 or 20dB attenuator on port 2.

Good call; tried with a 20 dB pad and all is well.

Sean

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