On 10/15/20 3:20 AM, Bob M. wrote:
I had a similar concern and determined that on my S-A-A-2 the output level is NOT adjustable from the touch screen menu system. I only have WinXP so I can't run the software that's available, which might let me change the output level from -10dBm.
I can add one or two 10dB SMA attenuators at the input of the preamp, but only for measuring the gain. I can leave attenuators in place when calibrating the THRU parameter and the LOGMAG screen will correctly display the actual preamp gain without me having to do the math.
Unfortunately I can't reduce the input level to the preamp when measuring return loss. If a pad is installed at the input of the preamp, the NanoVNA will end up measuring a combination of the pad's return loss and the preamp's input impedance. The particular preamp I'm testing seems to want a stimulus level of -30dBm; anything higher than about -21dBm causes the preamp's input return loss to become very poor, going from around 20dB to 8dB. The only way to get around this is to reduce the NanoVNA's output level, and that's not possible with the S-A-A-2 from its touch screen menu system.
Testing this particular preamp with a real HP VNA or a spectrum analyzer / tracking generator / return loss bridge produces a proper return loss value as long as the input level is at or below -21dBm. The preamp is designed to amplify very small signals in the microvolt range, not signals around 0.1V. Strong signals upset the bias of the preamp, which alters the input return loss and produces output compression.
It would be nice if the NanoVNAs had the ability (via the touch screen menu system) to set the CH0/Port1 signal level, in 5 or 10dB steps, from -30dBm to 0dBm, or even -40dBm to -10dBm, or even just a selection of -10dBm or -30dBm, or "normal" and "reduced/attenuated" for the less technical users.
That would require a step attenuator (hardware change), and would reduce the SNR (raise the uncertainty) of the measurement as well.
If you leave the pad on Port 1 (Ch0), and do the cal, why wouldn't it work? You're still measuring the S11 of the amplifier.
