Bruce,
The anti-correlated thermal noise can however create big havoc to the
measurement. That catastrophy has yet to sink in to some. Effectively,
some is overstating their noise performance as a consequence.
However, it depends on the power-splitter being used.
Cheers,
Magnus
On 03/25/2016 02:31 AM, Bruce Griffiths wrote:
Actually if you split the signal and then amplify each output independently
then the PN performance of the RF amps is not too critical in that cross
correlation averages the amplifier PN down as well as that of the mixers.I've
done this with the Timepod using quite noisy amps as nothing else was
immediately to hand.It just takes a little longer but works very well. I
measured the output PN noise of an LTC6957 evaluation board this way using a
couple of minicircuits HELA10s which are fairly noisy at 10MHz.
If you don't need a PN floor below -180dBc/Hz then there are simple inexpensive
one transistor (plus another for biasing) circuits that will achieve a few dB
of gain with a PN noise floor well below -170dBc/Hz.The only real limitation is
due to the presence of anti correlated thermal noise at the splitter outputs.
Bruce
On Friday, 25 March 2016 2:06 PM, Bob Camp <[email protected]> wrote:
Hi
That’s another good point about the need to work out a target device. Both
of the possible target devices I mentioned have enough channels to do at least a
dual channel measurement. That would add another mixer and a pair of power
splitters along with another amp chain.
The other part of that news is the RF drive power required goes up. You can do
a pretty good job
of saturating an RPD-1 with +7 dbm. Most (but not all) OCXO’s and other gizmos
will
provide that without any amplifiers involved. Adding a 3 db splitter gets you
into the 10 dbm
range. That is getting fairly close to the limit for a lot of devices.
You can add an amp. The ones that work without impacting the phase noise of a
high quality
OCXO cost as much as the audio cards or USB devices. Cost wise, I’d keep that
sort
of thing off the main board.
So what is the second channel worth?
The basic single channel design will get you into the -173 to -176 dbc / Hz
range on a fairly high
power pair of OCXO’s. The cross correlation “stuff” will get you past that
point. Is that worth taking
the BOM (without board and power supply) up to $80 or so? Consider that with
the board and
power supply, it likely is over $100.
Would I do it as an accessory to a Janus or QA401? Maybe. You would need to
pick one or
the other. In the case of the Janus, there are more software issues and some
noise floor testing.
The QA401 is mighty expensive. The original idea was to use the sound card you
already have ….
Bob
On Mar 24, 2016, at 5:30 PM, Bruce Griffiths <[email protected]> wrote:
If the software implements acquisition of cross power spectra
Then one could implement a near state of the art cross correlation PN test set
based on this.
With a suitable preamp the sound card could also be used for power supply and
reference noise measurement.
Bruce
On Thursday, March 24, 2016 04:54:47 PM Bob Camp wrote:
Hi
The board is pretty non-critical. It’s 90% audio. The biggest hassle is a
power supply. You would *like* a fairly high voltage, at least if you are
driving a spectrum analyzer. That may not be quite the case with a sound
card. It depends a *lot* which one you are running. Something like a QA401:
https://www.quantasylum.com/content/Products/QA401.aspx
Would make a good target device. It’s based on the AKM 5397 So might some
Janus boards. They are based on the earlier(?) AKM 5394. The QA401 has the
advantage of a nice box and full USB isolation (ground loops are a pain)..
It also has drivers and all the OS hooks. The Janus is a bit more “DIY”
with no drivers or interface (let alone isolation). The Janus is < 1/4 the
price.
The high voltage (+/- 18V linear regulated) supply approach makes a lot of
sense with the QA401. It probably does not make as much sense with the
Janus. Switching regulators of any sort are something I would strongly
recommend against in a system like this that is trying to measure noise
floor at audio ….
The schematic changes a bit depending on what the target is. I can draw it
up if there is a consensus on the target. One example: If the “sound card”
is DC coupled, you can use it to indicate (and check) quadrature. If it’s
an AC device, you need some sort of isolated output for another indicator.
Bob
On Mar 24, 2016, at 1:31 PM, John Ackermann N8UR <[email protected]> wrote:
I'd be willing to assist with board layout if someone wanted to make this
into a real project (e.g., fully developed schematic). TAPR might be
talked into supplying at least bare boards; we'd have to get a sense of
demand before committing to a full kit or assembled unit.
John
----
On 3/24/2016 9:04 AM, jimlux wrote:
More like $40 in parts, without a board, etc.
The RPD-1 is $20.70
LT1678/LT1679 is a nice low noise opamp that does rail to rail and is
about $5
etc
On 3/24/16 4:42 AM, Bob Camp wrote:
Hi
Pretty simple:
Double balanced mixer, RPD-1 is one option, there are others.
Fairly simple L/C lowpass between the mixer and an op-amp.
20 db positive (non-inverting) op-amp amplifier string after the mixer
Output of the string goes to the sound card. Use a good (dual / quad)
audio op amp
Quadrature amp picks off the output of the first op amp stage, switch
and resistors to set gain, pot to set op point.
====
So what you have is an old style quadrature phase noise amp and “PLL”.
More or less a very junior version
of the 3048 test box. Like any setup of this sort, you check two
similar oscillators. They run in quadrature and
you do a few “measure this with switch in position A” sort of things
to set things up each time.
Nothing exotic.
Bob
Any documentation on this $40 phase noise test set?
Rick N6RK
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