[time-nuts] Spectrum Analyzer Specifications

Bob Camp kb8tq at n1k.org
Thu Mar 24 18:48:05 EDT 2016


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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