[time-nuts] Spectrum Analyzer Specifications

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Mar 24 17:30:08 EDT 2016


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 <jra at febo.com> 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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