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NOISE SOURCE CIRCUITS FOR MICROWAVE

R

RealInfo

Jan 1, 1970
0
Hi all

How broadband noise sources are built from the circuit point of view?

I want to build some of my own so I ask about the circuits involved .

Thanks

Elico
 
J

Jeroen Belleman

Jan 1, 1970
0
Hi all

How broadband noise sources are built from the circuit point of view?

I want to build some of my own so I ask about the circuits involved .

Depends. If they make a lot of noise (~30dB ENR) they are usually
noise diodes. If they make only a little bit of noise, they may be
temperature controlled termination resistors. If they make very
little noise, they may be cooled terminations.

For quick and dirty noise sources, I've used RF transistor BE
junctions in reverse breakdown. I got a spectrum that was flat out
to about 1GHz. I suppose it's possible go higher, but that was
good enough for me. I also have a calibrated HP346B somewhere,
and a Dewar with an SMA terminator that I fill with LN2 when
I use it.

Jeroen Belleman
 
R

RobertMacy

Jan 1, 1970
0
Depends. If they make a lot of noise (~30dB ENR) they are usually
noise diodes. If they make only a little bit of noise, they may be
temperature controlled termination resistors. If they make very
little noise, they may be cooled terminations.

For quick and dirty noise sources, I've used RF transistor BE
junctions in reverse breakdown. I got a spectrum that was flat out
to about 1GHz. I suppose it's possible go higher, but that was
good enough for me. I also have a calibrated HP346B somewhere,
and a Dewar with an SMA terminator that I fill with LN2 when
I use it.

Jeroen Belleman

Breaking down the BE junction of a 2N3904 with a series 1Meg resistor from
12Vdc used to yield something like 1 Vpp of very flat noise over the audio
range. Cheap noise source for electronic games, sounds like applause in
the background. However, did NOT know the technique would get something
flat out to 1GHz! What transistor?
 
J

Jeroen Belleman

Jan 1, 1970
0
Breaking down the BE junction of a 2N3904 with a series 1Meg resistor
from 12Vdc used to yield something like 1 Vpp of very flat noise over
the audio range. Cheap noise source for electronic games, sounds like
applause in the background. However, did NOT know the technique would
get something flat out to 1GHz! What transistor?

I used a 2n918 --this was over 15 years ago-- with 20kOhm to +18V
and the load impedance must be low: 50 Ohms, since we're doing RF.
Bare ENR is in the 30dB ballpark. For serious measurements it's good
practice to sacrifice a bit of noise in exchange for good reverse
matching by adding a 12 to 15dB attenuator.

The upper cutoff is set by the impedance at the emitter node,
basically the BE junction capacitance in parallel with the load
impedance. I never tried to find the impedance of a BE junction
in reverse breakdown. I should take a look, some day.

As I learned how to make lower noise amplifiers, I 'upgraded'
to quieter noise sources. These days, I measure noise by
switching back and forth between a termination resistor at
room temperature and another in LN2 at 77K. The bandwidth
over which those are flat should extend to many GHz. (Albeit
surely not as far as Boltzmann and Planck would say.)

Jeroen Belleman
 
On 2013-09-17 14:31, RobertMacy wrote: > On Tue, 17 Sep 2013 02:32:39 -0700, Jeroen Belleman > <[email protected]> wrote: > >> On 2013-09-17 11:03, RealInfo wrote: >>> Hi all >>> >>> How broadband noise sources are builtfrom the circuit point of view? >>>

you can use a small "grain of wheat" incansescent light bulb.

Mark
 
R

RobertMacy

Jan 1, 1970
0
I used a 2n918 --this was over 15 years ago-- with 20kOhm to +18V
and the load impedance must be low: 50 Ohms, since we're doing RF.
Bare ENR is in the 30dB ballpark. For serious measurements it's good
practice to sacrifice a bit of noise in exchange for good reverse
matching by adding a 12 to 15dB attenuator.

The upper cutoff is set by the impedance at the emitter node,
basically the BE junction capacitance in parallel with the load
impedance. I never tried to find the impedance of a BE junction
in reverse breakdown. I should take a look, some day.

As I learned how to make lower noise amplifiers, I 'upgraded'
to quieter noise sources. These days, I measure noise by
switching back and forth between a termination resistor at
room temperature and another in LN2 at 77K. The bandwidth
over which those are flat should extend to many GHz. (Albeit
surely not as far as Boltzmann and Planck would say.)

Jeroen Belleman

That noise generator was circa 1974, went into coin-operated games, where
you had 0-3 months to design and product 'lived' for maybe 1 year, before
a new one was required.

If it's any help towards modeling the impedance of the 'reverse' biased
junction; the impedance of a 'forward' biased junction is a modified
Bessel Function of the first kind. I worked it out long hand circa 1968.
....took two weeks. The effort was to show that there was little
improvement in performance by biasing the RF detection diode more than
slightly ON. The RF Detection diode went into HP's Ku-band RF Source, the
12-18GHz RF Supply with Automatic Leveling (ALC circuit). The diode
detector was part of the control loop so had to analyze it a bit. Whew!
Those were the days, eh?
 
W

whit3rd

Jan 1, 1970
0
On Tue, 17 Sep 2013 02:03:13 -0700 (PDT), RealInfo
Drivel: Back in the dark ages, I shoved a small fluorescent tube into
a section of waveguide as a noise source. The tube ran on DC to avoid
conducted radiation from the 117VAC power lines.

Yep, that's how high-frequency noise is often generated; the negative-resistance
characteristic of a lightly conducting discharge tube stuffed inside a waveguide.
It's often nice to have a solid-state source, but gassy tubes win this one time!
 
J

Jeroen Belleman

Jan 1, 1970
0
AFAIK, noise diodes are purpose-built zener diodes that are then
calibrated.

I've used a regular ol' 6.8V zener as the source for a noise bridge for
amateur-radio antenna tuning and receiver work. It works great, as long
as it doesn't need to be well calibrated.

I've also wondered about how well comparing an LN-2 resistor with a room-
temperature one would work. It sounds as if you feel it works well.

Indeed I do. For any resistor just sitting there, the noise is
known from first principles. With two resistors at known temperatures,
I know the ratio of noise powers. I put that through my amplifier
and measure the ratio at the output with a spectrum analyzer. Solving
a pair of equations then gets me the noise of the amplifier.

It's called the 'Y method'. It doesn't require accurate knowledge of
the amplifier's gain, and the spectrum analyzer doesn't need to be
calibrated for bandwidth or absolute accuracy, as long as its scaling
factor is good. No worries about using peak, average or rms detection
either. It all drops out.

To get reasonably accurate results, the amplifier's noise has to be
in the same ballpark as the source's noise levels, and its gain must
be comfortably larger than the spectrum analyzer's noise figure.
That's usually easy.

Jeroen Belleman
 
How broadband noise sources are built from the circuit point of view?

How many GHz do you need ?
How flat does the spectrum be ?
How large amplitude do you need ?

How about a 50 ohm termination resistor followed by a few MAR/ERA
style MMICs ? If the first amplifier has, say, 3 dB noise figure, then
half of the noise is from the resistor and half from the first
amplifier.

This circuit should be good depending on amplifier types and frequency
compensation.

Since the noise from the resistor as well as the gain of the amplifier
is temperature dependent, these should in a constant temperature
enclosure. This enclosure should also be well protected against
external RF ingress.
 
G

George Herold

Jan 1, 1970
0
Indeed I do. For any resistor just sitting there, the noise is
known from first principles. With two resistors at known temperatures,
I know the ratio of noise powers. I put that through my amplifier
and measure the ratio at the output with a spectrum analyzer. Solving
a pair of equations then gets me the noise of the amplifier.

It's called the 'Y method'. It doesn't require accurate knowledge of
the amplifier's gain, and the spectrum analyzer doesn't need to be
calibrated for bandwidth or absolute accuracy, as long as its scaling
factor is good. No worries about using peak, average or rms detection
either. It all drops out.

Jeroen, How do you deal with the cable (capacitance) that goes down into the LN2?

50 ohm coax and resistor?
Do you put equal lengths of coax on each 'arm'?

Thanks,
George H.
 
J

Jeroen Belleman

Jan 1, 1970
0
Jeroen, How do you deal with the cable (capacitance) that goes down into the LN2?

50 ohm coax and resistor?
Do you put equal lengths of coax on each 'arm'?

Thanks,
George H.

Hi George,

Cable capacitance is not an issue. This is 50 Ohms throughout:
Matched source, cable and load.

Something that may be an issue, is cable *loss*. It takes a
bit of getting used to, but that actually *increases* the amount
of noise that comes out of the cold source. It makes the amplifier
under test look noisier than it really is. I use the shortest
possible length of semi-rigid coax to minimize that problem.

Jeroen Belleman
 
A

Anthony Stewart

Jan 1, 1970
0
The HP346B uses an audio range clock into a current limiter and to drive an Avalanche Diode to yield an excess noise ratio of 15 dB ENR. It is AC coupled before the output impedance matching to 50 Ohms output and is powered by a switched 24 Vdc supply.
 
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