I need to measure the noise spectrum of a diode under certain bias. The noise frequency I am interested is in the range of several Hz to tens or hundreds of kHz. I am thinking of using a bias tee with fairly small low cut-off frequency. But the low cut-off frequencies of bias tees I am looking at are still a little high (for example, in http://www.picosecond.com/product/category.asp?pd_id=2 the minimum low cut-off frequency is 3.5kHz). I am also thinking of DIY such a bias tee. For example, the post I need to design a Bias-T circuit for a diode laser - help choosing component values discussed how to design a bias tee working around 10MHz with 25kHz bandwidth. However, in my situation, I at least need it to go from hundred Hz to tens of kHz.
My questions is : 1) is there a commercial product available somewhere else that fits my needs? 2) If there is no such commercial product, how to DIY?