In the course of investigating the retrofit of a low noise RF pre-amp to an old 2m mobile, RT85, a random thought from nowhere made me stop and consider regulator noise.
My goto part for 3 volt regulators in microcontroller projects is the LN6206. Cheap and reasonably low quiescent current. However, there is no specification of the output noise voltage.
Since I had seen a TL431 on a PCB earlier in the day the device was foremost in my mind. I wouldn't need a pre-regulator to reduce the voltage to the LN6206 which is a 6V maximum input device. However, the TL431 datasheet I had also made no mention of noise.
After a bit of searching I became cynical of the claims the TL431 is low noise. I found just one objective assessment of noise which helped reassure me. Going from 50 nV/√Hz to noise in a 0-100kHz bandwidth and adjusting for gain from the 1.25V band gap voltage of the TL431 to 3V returned 38uV. Which is almost identical to the 40uV output noise voltage of the L78L33C.
I'm not sure that qualifies as low noise, since a LT3094 is specified as 2 nV/√Hz but costs over 50 times more! I note in the web page above that an additional capacitor could be expected to reduce the output noise voltage considerably. Back of the envelope suggest under 10uV.
Can we do better? It appears so. A simple one transistor circuit after the regulator should make a large difference.
Verification is the hardest part. I could not find a low noise amplifier circuit that I could re-create with my junk box. My attempts to build a low noise amplifier in a shielded box are yet to bear fruit.
My whole approach to electronics is to re-use previous work until the need for improvement arises. While trawling through the internet I saw numerous statements that low noise regulators are vital to oscillator performance. This is not something I had considered beyond decoupling supply lines. So it appears the need for improvement exists!
73's
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