[time-nuts] Re: DIY Low offset Phase Noise Analyzer (Erik, Kaashoek)

[Lux, Jim]

On 7/12/22 3:51 PM, ed breya via time-nuts wrote:
> I forgot to mention that you should also consider possible effects 
> from the RF present, on the LNA. This can be more significant than 
> SMPS frequencies getting where they don't belong, especially since the 
> RF is intentionally right at the opamp's input. Your LPF only reduces, 
> and does not eliminate, the 2F and harmonics, so there can be 
> significant RF present on the LNA circuit.
>
> A simplistic view is that the RF is far beyond the opamp's GBW or 
> closed loop gain and should have no response, but it's not at all 
> beyond upsetting or altering the operation. This can result in extra 
> DC offsets and noise due to RF rectification in the input circuits, 
> which only remain "linear" at frequencies where the output and 
> feedback can keep up with the input.
>
> This can be fixed if necessary, by adding extra RF filtering, 
> particularly some built to low-pass at a higher cutoff frequency well 
> above the analysis frequency, and well below the expected f and 2f.
>
> For instance, in your circuit it looks like L1 is 1 mH, with 100 nF 
> caps, which ideally cuts off quite low. However, 1 mH is a pretty big 
> choke, and will tend to have a lot of inter-winding capacitance (and 
> high resistance - don't forget to include it in noise), making it less 
> effective at the higher frequencies. Adding an LC section in front of 
> it, but set up for something in the MHz region, will give much greater 
> rejection of the f and 2f, due to having more appropriate smaller L 
> and C.
>
> Anyway, if it works fine as is, then no problem, but it's something to 
> be aware of if you get strange effects down the road.
>
> Ed


and a single LC is only a single pole, so the roll off isn't all that 
great in a dB/decade sense.