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A Dilemma when evaluating a RF Power Sensor

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That assumption would require the noise amplitude to be totally consistent across that entire frequency range which I suspect that it is not.   However, it certainly does appear to be very wide.

As an experiment, you could look at the noise at various frequencies within the range of your analyzer - say every 100 MHz or so - to get an idea of how flat the noise is for at least the first 3 GHz.

You might want to look at the Agilent / Keysight application note 1303 "Spectrum Analyzer Measurements and Noise" - 5966-4008E.  There are several versions out there (e.g. 2002, 2009, 2012) as it has been periodically updated.   

I know I'm late here to reply but I checked the noise issue with my SM300 on a Rigol DSA815TG.

Without the SG connected, I got similar noise readings as @NY2KW with someting like -163dBm/Hz. Yet, with the SM300 connected, there wasn't any noticeable increase of the NF observable. I tried different power settings while the output was off (in case the output amplifier gets reconfigured) and also output some small signals which all didn't affect the noise level.

But if I operate the setup at slightly higher power settings (had to change input attenuation of the SA) and I change the output level of the SG with the output active across the PA enabling threshold (i.e. from -26dBm to -25dBm), I get an increase of noise level by almost 20dB (-157dBm/Hz -> -138dBm/Hz) which can be expected due to the additional gain block routed into the signal path.

If your SM300 always has some raised noise present at the output, it may well be possible that the output RF switch is defective (SkyWorks AW002R2-12) and it doesn't disconnect the PA properly.

Thats indeed a strange behavior.
Older synthesizers can have substantial  wideband noise, that's true.
However, I would not expect this noise to remain constant with output power setting but to decrease as the output attenuator steps are engaged.
Your unit may therefore have a weird step attenuator issue. As it seems to be a solid state attenuator, the switch ICs or their power supply might be degraded, damaged and/or noisy.

Did you zero the sensor before you calibrated the sensor, and again after? (probably you did, but you don't mention it)  with no signal (e.g. put a load on the power sensor) and hit zero, it should read lower than the min power value on the power sensor label.  After it sits for a bit you will see it climb up and eventually it will reach some larger value but usually I see this on the order of no more than 10-15 dB above the minimum power label. BUT, the first time you connect a sensor there are lots of different offsets that have to be removed.  Some of the residual offsets drift so the specs only apply immediately after zeroing (and as far as i know, there is no specified time frame for "immediately").

I have a very similar question, and similar weirdness, with a Boonton 4210 and its 4210-4A sensor (full-wave diode based; -60 to 10 dBm range).

- sensor terminated, after zeroing the meter : -70dBm. Ok, everything normal.
- hooked to an hp 4195a generator : readings make sense until about -40dBm;
- I tried hooking up 40dB of attenuators between the 4195 and the power sensor :  I never saw any number below about -50. Even with the 4195 off I read -51 !!

Maybe some residual DC from the 4195 ? But then, how am I still reading -51dbm with the 4195 off , with 40dB attenuation before hitting the sensor ?

Could it be some kind of noise coupled into the shield / chassis on everything in the lab ? As soon as I disconnect the cable from the 4195, readings start to drop towards -65  as expected..

So if anything below about -50dBm is so troublesome, how does one even verify / use one of these ?


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