"Max Input" / "Ref Offset" parameters understanding for EMI precompliance

Hi,

I’m a happy owner of a brand new BB60C, which I use for EMI precompliance.
I’m not very familiar with spectrum analysers, but still the setup and use of Spike software
was quite straighforward. My only concerns are the two parameters “Max Input”, and “Ref Offset”.
I would like to understand the link between these parameters and the pre-amp/attenuator of the BB60C.
Indeed I’m confused while trying to read absolute values, because a slight modification of these two parameters
change the reading a lot.
I used a signal generator (sine wave, 500kHz, 20mV_RMS, 50 Ohm output impedance) in order to confirm my understanding,
and found that the correct reading (86dBuV, if I’m right) was obtained at “Max Input”=”115dBuV”, and “Ref Offset” set to zero.
Could you explain how to set these parameters in order to get correct absolute readings?

Kind regards,

Vincent

Hi Vincent,

Max input directly controls the sensitivity of the receiver. For the best sensitivity max input should be set to ~5 dB above the maximum possible input level. The receiver can clip signals at or above the max input level. The software will warn you with a signal is clipped.

Ref offset is a convenience control which adjusts the sensitivity of the receiver to account for external attenuation/gain. IE, if you attenuate the input signal by 10dB (pad), setting the ref offset to +10dB will account for this and adjust the receiver sensitivity accordingly.

Let me know if you have additional questions.
Regards,
Andrew

Hi,

Thank you for your quick response. I fully understand your response, and it corresponds to what I understood from the user manual.

But still my concern is: if I have a 80 dBuV (rms) sine input signal, how can I get the right absolute value displayed on the reticule? I’m sorry it may seem ridiculous, but I expect to read the correct value, whatever the gain and attenuator settings if they are taken into account by display process (like an oscilloscope when you change the time or voltage scale).

regards,

Vincent

Vincent,

When the ref offset is zero, the CW peak you measure in the software should be equal to what is on the input port. It sounds like this is the case for you based on your first message? Changing the max input should not affect the CW peak reading (assuming you only use values above the input level). You can contact me directly at aj@signalhound.com if you have further questions.

Regards,
Andrew

Dear Sir,

I face a similar situation as Vincent when trying to do measurements on a wide band noise source (Radiated emissions measurement and calibration). When changing the “Max Input” Setting, sometimes, the curves is shifted towards upper or sometimes lower measured values, or its shape is modified.

This makes me confused on the right level to choose. Why would it be so dependant on the max hold value, provided we stay away from Saturation ????

Another clue I use is the “IF Overload” message appearing on the top left corner of the screen. I was guessing that to have the maximum dynamic range, I should set the “Max input” parameter, just above the level where we have “IF overload”. The message then disapears and I was expecting the measurement to be OK. Nevertheless, if I increase further the “max hold” value, the curve is still changing a lot !. I have tried to respect your recommendation of having the highest signal about 5 dB below max input level, but is this the same for wideband noise source ?

I would need to have a clear explanation on the relative “hardware” and “software” effects of settings like “Attenuation Factor”, “Max Input” and “Ref Offset”.

For example, the Ref Offset has an influence on IF Overload, while the attenuation factor does not, it is just a software compensation for input attenuation.

See in annexe pictures measurement results for “Max input” parameter set between 70dBµV and 100 dBµV.

Please advice,

Thanks you in advance and best regards,

Yves Borlez

Please find enclosed the related pictures.

Best regards,

Yves Borlez

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(Yves has contacted us through email)

– Andrew

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