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1/f noise with a passive filter.

  • Thread starter Jean-Pierre Coulon
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J

Jean-Pierre Coulon

Jan 1, 1970
0
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")
 
M

MooseFET

Jan 1, 1970
0
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 wirearound
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?

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.
 
J

Jean-Pierre Coulon

Jan 1, 1970
0
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
 
B

Ben Jackson

Jan 1, 1970
0
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.

Am I missing something? X should be in milliohms at 5kHz.
 
J

Jean-Pierre Coulon

Jan 1, 1970
0
I'm assuming that the two signals into the mixer are in phase
quadrature. In that case, the splitter / 2 unequal delay paths / mixer
form a 'delay-line discriminator'. As such, it will detect FM noise
from the 6 MHz source. To measure really low phase noise of
the filter-under-test will require low FM noise from the source --
probably a synthesizer with very low phase noise close-in.
In other words, you could be measuring the FM noise of your source.

To test this you could --
Redo the test with a really low noise souce, such as HP 8662.
Redo with your present source, but replace the filter with a coax
delay line of delay equal to the filter delay.

Both are done.
Also, you say the mixer is "(diode-mixer like)". What does that mean?

It's a Minicircuit MCL RPD-1, called "phase detector". The internal topology
is the same a that of a mixer. Simply the output impedance is 500 ohms, due
to unusual transformer ratios. This provides a higher max output DC level than
a mixer.
(I would be very surprised to see any phase noise from a passive
L - C filter at 5 kHz offset. At 1 to 10 Hz maybe, due to various
mechanical effects, but not at 5 kHz.)

What material would be more professional than a 1-meg resistor to provide a
core with a relative permeability of 1? Are there commercial cores for this?

Jean-Pierre Coulon (here "cacas.pam" is what others call "nospam")
 
M

MooseFET

Jan 1, 1970
0
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 wirearound
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.

Another thought:

Do you have a high Q resonant peak in the system at a harmonic of the
working frequency? This can lead to there being a lot of the harmonic
in the signal. This messes with the ability to measure phase
correctly.
 
J

JosephKK

Jan 1, 1970
0
Both are done.


It's a Minicircuit MCL RPD-1, called "phase detector". The internal topology
is the same a that of a mixer. Simply the output impedance is 500 ohms, due
to unusual transformer ratios. This provides a higher max output DC level than
a mixer.

I expect that the diodes in your mixer exhibit some 1/f noise and the
front end of your spectrum analyzer also does. See the manuals.
What material would be more professional than a 1-meg resistor to provide a
core with a relative permeability of 1? Are there commercial cores for this?

Jean-Pierre Coulon (here "cacas.pam" is what others call "nospam")

Try wood, plastic, glass or ceramic in solid rod or tubular form of
some convenient cross section.
 
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