Re: 1/f noise with a passive filter.

John Larkin wrote:
On Fri, 08 Aug 2008 14:42:38 -0400, Phil Hobbs
<pcdhSpamMeSenseless@xxxxxxxxxxxx> wrote:

Jean-Pierre Coulon wrote:
On Fri, 8 Aug 2008, MooseFET wrote:

If your resistor isn't a carbon based one, there is no good
explanation in the resistor for your increased noise. There can be
explanations in the joints between parts if they are not soldered. In
such cases, it us usually worth checking that your test setup doesn't
report the same noise from just a 50 ohm resistor or a long coax with
a 50 ohm load.
Done.

Bad connections can sometimes rectify RF. You may be seeing the local
rock and roll station as noise.
All this is in an RF box.

Carbon based resistors can have two problems. One is that they can be
slightly nonlinear the other is that they can make noise when current
flows through them.
I've already seen 1/f noise with carbon resistors with a DC flow in them, but that was in low frequency. Here I am demodulating at 6 MHz, so this
potential noise will be put around 6 MHz (if it goes through the mixer), and I'm measuring low frequencies.

Jean-Pierre Coulon

Carbon resistors exhibit 1/f conductivity fluctuations, and i=v/g. With DC excitation, conductivity fluctuations turn into 1/f noise, and with AC excitation, they turn into 1/f sidebands, which seems to be what you're seeing in your phase detector. Carbon and thick-film resistors both show this behaviour.



But he's wound 4 uH worth of L around a 1M resistor, and he's seeing
the 1/f corner at 5 KHz.

I'm thinking it's an instrumentation issue.

John


Hmm, I missed the 1M number, it's true--I was thinking Q=1. The inductive reactance is around 150 ohms, about 10**4 times below R. It would depend how far down the noise floor of the phase measurement actually is.

It could still be the resistor, but it might also be LO noise or even AM noise. It would be worth cranking up the amplitude to make the mixer clip hard, and then looking at the AM noise again.

Cheers,

Phil Hobbs
.

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