1/f corner is well below 100 Hz. Look at the noise voltage plots of
that audio guy I cited.

My results for the BF862 were the same shape, absolutely somewhat worse
in amplitude because I wanted a differential input and less FETs in parallel.

Most of my BF862 had abt. 12 mA IDss, btw.

The 300 pF Cin of a single  IF3602 could seriously detune the input low pass
and the 200 pF feedback capacitance in a stage with substantial voltage
gain would destroy the bandwidth unless cascoding is provided.

I think, I'll test some Analog Devices ADA9848-2 in parallel. It's hard to beat
that combination of noise, 1/f, bandwidth, offset stability and price.

Such a preamp can be used as an add-on to a scope or FFT-Analyzer, too,
to characterize power supplies, references or oscillator bias circuits.
It's fun to enter 60 dB probe gain into a scope channel menu
and still see usable traces  with uV/div scale factors.
( with a low pass, of course)

There are noise nuts, too!  ;-)

Gerhard

dk4xp@arcor.de wrote:

The IF3602 is suboptimal with respect to flicker noise the IF9030 is
superior.
Its capacitances are similar to those of the 2SK369 however it has
somewhat lower flicker noise (see attachment).
At low frequencies the following opamp (in the Wenzel and similar
circuits ) ensures that the voltage swing at the FET drain isnt large
and the effect on circuit bandwidth is somewhat smaller than a naive
analysis would indicate.
Cascoding done correctly, has the advantage of improving the input stage
PSRR as long as the noninverting opamp input is bypassed to the drain
supply of the cascoding FET.
Using a series string of LEDs connected to the drain supply and driven
by a high output impedance to bias the opamp noninverting inputcurrent
sink ensures that the PSRR is maintained down to very low frequencies.
For a sound card preamp used in a phase noise measurement setup the low
pass filter capacitance is usually 10nF or more so a 300pF input
capacitance has little effect on the filter cutoff.

If the desired bandwidth only extends down to 100mHz or so then a low
ESR supercap can be effective in ensuring that the input stage doesnt
have excessive dc drift.

Bruce

Oooops, ADA4898-2

http://www.analog.com/en/amplifiers-and-comparators/operational-amplifiers-op-amps/ada4898-2/products/product.html

sorry for the confusion, Charles!

Gerhard

Hi

If you start with a mixer that runs 500 mV / radian (an RPD-1 at the typical 8 mV / degree + 10%) then -180 below that would be 0.5 nV. Since noise it coherent close in, the DSB  to SSB process nets you 1 nV out when you have -180 dbc noise.

On Aug 22, 2010, at 10:51 AM, Bruce Griffiths wrote:

So everything above (and an AD797 and likely an OP-37) will do better than 2 nV / Hz into 1K Hz. That would let you check oscillators in the "below -170", but not below -180 range. You might or might not find such an oscillator in your junk box. They certainly do exist.

On the plot above, both devices would let you do the same thing at 10 Hz. Now you are into the range of "highly unlikely to find". At reasonable frequencies, -135 is doing pretty well at 10 Hz. Bragging rights start at about -140. "Highly unlikely" cuts in much past that 10 Hz offset. I'm not talking about a one of a kind piece of magic at NIST, but about what's in your junk box.

The 2SK369 is still holding  "ok for -170"  at 1 Hz. Even for one of a kind magic, that's pretty crazy. Unlikely to find (and really tough to measure) cuts in at about -120 at 1 Hz.

At 0.1 Hz offset, you will need to run an instrument bandwidth below 0.01 Hz to get anything close to a good approximation to the noise. Most lab analyzers run > 100X t to get enough data. That puts you out around 10,000 seconds for the run. That's a crazy long time. DC coupled offsets are likely to nuke that run just about every time.

I'm by no means knocking the idea of having a good preamp. I'm only trying to point out that the numbers above are way past what a reasonable person might need to sort through their basement oscillator collection. 50 db is a lot of margin.

Bob

Bob Camp wrote:

With a capacitive IF port termination the mixer sensitivity increases
somewhat.
It increases more when using something like an HP10514 or 10534 than
with an RPD-1.
Such a termination isnt particularly useful for offsets much above
100kHz or so.
If one terminates all mixer ports in 50 ohms as some insist is the best
method, then the mixer phase sensitivity is much lower, in which case a
somewhat lower noise preamp may be required.

The posted plot does show (together with the noise plot for the HPS5.1
preamp) that the 2SK369 and the IF9030 have much lower flicker noise
than the BF862.

For an AC coupled sound card based spectrum analyser dc frequencies much
below 2Hz or so are of little interest.
Being able to calibrate the preamp + sound card frequency response using
the thermal noise of a resistor is convenient.
This is more difficult to achieve with a bipolar input stage as the
amplifier input current noise is significant.

Bruce

Hi

I've always calibrated my phase noise setups to the phase slope of the mixer
I'm using. It does involve switching gains, but it's a direct system
calibration. Beat note is 360 degrees, so this chunk is x degrees and you
got y mv over that chunk. Check the slope on the other side of the beat note
to make sure it's the same. Do some math and you have a radian to volt
transfer function.

If you are sorting junk box OCXO's it's a pretty good way to do it. The only
added steps are an independent measurement of the switched gain / gain
flatness and a short circuit input check to estimate the noise floor. Both
are an initial setup / one time only sort of thing with most amps.

Bob

-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Bruce Griffiths
Sent: Tuesday, August 24, 2010 3:25 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Phase noise measurement (was - no subject)

Bob Camp wrote:

< CHOP >
....
Being able to calibrate the preamp + sound card frequency response using
the thermal noise of a resistor is convenient.
This is more difficult to achieve with a bipolar input stage as the
amplifier input current noise is significant.

Bruce

time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Having a simple method of determining the preamp frequency response can
be a useful diagnostic tool during development, particularly if one uses
componets like super capacitors in the amplifier signal path.

If one doesnt have a suitable offset source handy the mixer ports can be
driven in near quadrature by the same signal and the dc output as a
function of the relative phase shift between the 2 mixer inputs can be used.

However neither method calibrates the phase noise frequency response of
the system.
Adding RF noise to one of the mixer inputs can be used to measure the
frequency response of the system.
If the RF noise source is uncalibrated but stable then it can be used to
measure the relative frequency response.
The results of a dc (or beat frequency) measurement of the gain can then
be used to correct the results to obtain a calibrated frequency response.

If one is using a capacitive or other non conventional mixer IF port
termination, then knowing the relative frequency response can be vital.

Bruce

Bob Camp wrote:

Hi

For a one time frequency response check, a directional coupler and a signal generator do a pretty good job of creating a useful test tone.

Bob

On Aug 24, 2010, at 4:03 PM, Bruce Griffiths bruce.griffiths@xtra.co.nz wrote:

Von:    Bob Camp lists@rtty.us

Adding the two sidebands may double the power, but not the voltage?

73, Gerhard

Hi

If the sidebands are un-correlated noise then they add as power (3db). If they are correlated they add as voltage (6db). Noisy modulation processes produce correlated sidebands.

Close in noise likely comes from a modulation process. Far removed noise is unlikely to be a result of modulation.

Bob

On Aug 24, 2010, at 4:24 PM, dk4xp@arcor.de wrote: