[time-nuts] Spectrum Analyzer Specifications

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Mar 25 00:25:13 EDT 2016


One issue with CMOS gates is that a really quiet power supply is needed or the PN contribution of the gate is severely degraded. Fortunately this is an easy problem to solve.On solution is to use an independent noisy supply for each gate.
Bruce

 

    On Friday, 25 March 2016 5:01 PM, Bob Stewart <bob at evoria.net> wrote:
 

 Hi Bob,

Could you use a gate instead of an amp?

Bob
--------------------------------------------
On Thu, 3/24/16, Bob Camp <kb8tq at n1k.org> wrote:

 Subject: Re: [time-nuts] Spectrum Analyzer Specifications
 To: "Discussion of precise time and frequency measurement" <time-nuts at febo..com>
 Date: Thursday, March 24, 2016, 5:48 PM
 
 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 <bruce.griffiths at xtra.co.nz>
 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 <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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