[Abstract]

For measuring the LED intensity noise, the large DC voltage spoils the dynamic range of the FFT analyzer. The AC coupling could be usable to overcome the situation, so the difference between the AC and DC coupling inputs of 35670A's are measured. We should include the fit function for the calibration of measured data with the AC couping input.

[Measurements]

Measuring an input signal (or noise) with Ch1 and 2 of 35670A simultaneously and make it calculate the ratio Ch2/Ch1. A signal input is divided by a T-shape stuff and input into each channel.

• (a) Freq. response of Ch2/CH1 are measured with a random noise input created by the 35670A itself.
• (b) Swept sine mode to measure the same thing.

For the both cases, Ch2 is set to AC float and Ch1 to DC float. (I've tried other combination for just a confirmation.)

[Results]

The results of (a) and (b) are consistent, and  the fit transfer function is approximated by

f(x) = (i*x/x0)/(1+i*x/x0)

and

 x0 -> 0.485(5),

where I just assume f(x) -> 1 as x -> infinity.

You can use this function for the calbration when you use this 35670A with the AC coupling input (CH2).

Note: I'd like to avoid SR560s for the OSEM screening setup, as it appears to have large noise comparing to what I want to measure in the low frequency region, and each SR560 looks to have different noise level one by one... maybe our SR560s are getting old since they had been bought in 1998; one of them even sometimes has a jump in the DC offset during the measurement.