1/f noise with a passive filter.
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Posted by Jean-Pierre Coulon on August 8, 2008, 7:00 am
Please log in for more thread options I am making a simple lowpass LC filter at 6 MHz in a 50-ohm system with passive elements. I've made the inductor (L=4uH) by winding copper wire around a 1-Mohm, 2W carbon resistor. Since I want Q = 1 or so I don't care about the loss angle of the resistor material. But when I compare the phases of the input and output signals with a passive phase detector (diode-mixer like) and a spectrum analyzer above 5 KHz the noise floor is consistant with that of an attenuator in a 50-ohm system, but below, the noise spectrum climbs in 1/f. Could the material of my resistor-based cylindrical core explain this? Are other materials better than a big resistor at 6 MHz? Regards, Jean-Pierre Coulon (here "cacas.pam" is what others call "nospam") | |||||||||||||||||||||||||||||||||||||||||||
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Posted by MooseFET on August 8, 2008, 9:36 am
Please log in for more thread options wrote: > a 1-Mohm, 2W carbon resistor. Since I want Q =3D 1 or so I don't care abo=
ut the
> loss angle of the resistor material.
ive
> > But when I compare the phases of the input and output signals with a pass= > phase detector (diode-mixer like) and a spectrum analyzer above 5 KHz the
but
> noise floor is consistant with that of an attenuator in a 50-ohm system, = > below, the noise spectrum climbs in 1/f.
> > Could the material of my resistor-based cylindrical core explain this? > Are other materials better than a big resistor at 6 MHz? 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. Bad connections can sometimes rectify RF. You may be seeing the local rock and roll station as noise. 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. | |||||||||||||||||||||||||||||||||||||||||||
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Posted by Jean-Pierre Coulon on August 8, 2008, 11:17 am
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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 | |||||||||||||||||||||||||||||||||||||||||||
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Posted by Phil Hobbs on August 8, 2008, 2:42 pm
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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. Cheers, Phil Hobbs | |||||||||||||||||||||||||||||||||||||||||||
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Posted by John Larkin on August 8, 2008, 4:36 pm
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On Fri, 08 Aug 2008 14:42:38 -0400, Phil Hobbs >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 | |||||||||||||||||||||||||||||||||||||||||||
> passive elements. I've made the inductor (L=3D4uH) by winding copper wire=