RBW and VBW

[flo]

Hi,

if i measure the signal with an open input at the SA44B with RBW at 30Hz and change the VBW to 10 or 3Hz the the signal or noise level measured with a marker increases about 4 to 5dBm compared to a VBW of 30Hz.

I always thought, that the noise is reduced with a lower VBW?

Can someone explain me this?

Thanks
Florian

RBW Shape: Flat Top
20kHz to 40kHz
Video units: Power
Detector: Average
RefLevel: -50dBm

• This topic was modified 7 years, 1 month ago by flo.

[Justin Crooks]

This has to do with the SA-series software-based image/spur rejection algorithm. Behind the scenes, it is injecting LO high side, making a sweep, then injecting low side, making a sweep, and picking the lower measurement. For this reason, the SA44B is not recommended for broadband measurements (such as broadband noise).

If you need to make a noise measurement with the image/spur rejection enabled, set your VBW to 1/10 the RBW. You will need to compensate for energy at the image frequencies if your noise bandwidth is >42 MHz. Subtract 3 dB from the measurement for these situations.

The alternative is a BB60C or an SM200A, which have excellent hardware-based image rejection and much lower spurious. These will provide good noise measurements without any tricks.

[flo]

Hi Justin,

thanks for your answer.

So turning off the image/spur rejection will not solve the problem?

And if I set the VBW to 1/10 of the RBW and measure below 42MHz (I measure from 20 to 40kHz or 60 to 100kHz) I dont have to substract 3dB from the measurement.

I will try it with the SA44B at first.

[Justin Crooks]

Flo,
The image response will be 21.4 MHz above the 20-100 kHz signal. If your configuration is measuring flicker noise or similar, and you have a low frequency amplifier with, say, less than 10 MHz of bandwidth, between your DUT and the SA44B, you would not subtract 3 dB, as the noise at 21.4 MHz would be negligible compared to the noise at 20-100 kHz. You could turn off spur rejection, deal with the spurs, and not worry about VBW settings. Or you could set VBW to 1/10 RBW with spur reject on, and still get a valid reading.

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