Re: 1/f noise with a passive filter.

On Fri, 08 Aug 2008 18:55:58 -0400, Phil Hobbs
<pcdhSpamMeSenseless@xxxxxxxxxxxxxxxxxx> wrote:

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

Our new Aeroflex spectrum analyzer is rated 9KHz to 3 GHz. Actually it
goes down to zero frequency, but they rate it at 9K because below
there the noise explodes.

John


.

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