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Are there any NanoVNA products, of any version, that have an adjustable Port1 output level? My V2 unit's output is -10dBm and while that's great for working with passive networks, it's a problem for measuring preamps that can't handle that much signal. Yes, I can add a pad at the input of the preamp, but that will affect the SWR and other measurements. A way of reducing the Port1 output level by 0, 10, or 20dB would be very helpful but there doesn't seem to be any way to adjust the stimulus level, only its frequency range.

Perhaps this capability can be added to a future version.

On Fri, Sep 4, 2020 at 06:38 AM, Bob M. wrote:

.. Are there any NanoVNA products, of any version, that have an adjustable Port1 output level?
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Bob,
The current version NanoVNA-F as well as the NanoVNA/NanoVNA-H4 with DiSlord's firmware have a 'Power' menu for adjusting the output power. I haven't looked at the actual dBm increment so I don't know if the range would meet your needs.

If you did an SOLT calibration after an attenuator attached to Port1 wouldn't that correct for impedance measurements after the attenuator? To answer my own question I did a SOL calibration after a 6dB attenuator attached to Port1 and then measured a 75 and 100 ohm precision mismatch attached to the output of the attenuator. The measured values were 75 and 100 ohms as expected.

- Herb

It depends. The problem is that adding an attenuator at the test port will degrade the (raw) system directivity by _twice_ the attenuation value. So while in your case it works fine, if you were to repeat your experiment with a 20dB attenuator and tried to measure a DUT with 25dB return loss you may be less successful. The calibration will compensate for the attenuator, but you will find that the noise floor has increased by 40dB in the process.

The software of the SAA2 has an option to adjust the power. I have not tested, if it really works.

--
Klaus, DL5KV

On Fri, Sep 4, 2020 at 08:06 AM, @switchabl wrote:

It depends. The problem is that adding an attenuator at the test port will degrade the (raw) system directivity by _twice_ the attenuation value. So while in your case it works fine, if you were to repeat your experiment with a 20dB attenuator and tried to measure a DUT with 25dB return loss you may be less successful. The calibration will compensate for the attenuator, but you will find that the noise floor has increased by 40dB in the process.
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switchabl,
That makes sense to me. I only had the 6dB nearby so didn't check any higher values. I also was thinking the round trip through the attenuator would increase directivity by that amount. Sometimes logic doesn't work and its better to just look it up.

I guess if you are aware of the above and only need 20dB of attenuation before the amplifier; and expect less than 20dB of return loss it wouldn't hurt to see what kind of results you get. I have a set of precision mismatches so I usually have an idea before testing an actual device what to expect.

- Herb

On Fri, Sep 4, 2020 at 08:38 AM, Klaus W?rner wrote:

The software of the SAA2 has an option to adjust the power. I have not tested, if it really works.
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Klaus,
VNA-QT was ported from another open source device that allowed options that the SAA2 does not. Adjustable power is one of the features the other device allowed but is not applicable to the SAA2.

- Herb

Thanks. My NanoVNA is the S-A-A-2 version and there are no menu items that I've run across that deal with the output power level. It's not listed on the SAA2 Menu Map either. I guess not all V2 units are the same.

I can just calibrate the unit without the attenuator then put it at the preamp input when I want to measure the gain, which is about 14dB, so two 10dB pads will knock the input down enough that the preamp won't overload and the input to Port2 won't be overdriven.

Speaking of Port2, is there a maximum safe input level that could be applied to it, i.e. 0dBm or perhaps even higher, or is it not expecting anything higher than what would come out of Port1 (about -10dBm)?

If you just want to measure gain, it is totally fine to calibrate and measure with the attenuator at the port. The reflection measurement will probably be very noisy with 20dB attenuation and may not be too useful, but this won't impact the transmission measurement (at least with the calibration algorithm used in the S-A-A-2 firmware).

If you can get away with putting 10dB on port 1 and 10dB on port 2, you should be able to get a reasonable return loss measurement as well.