[time-nuts] Spectrum Analyzer Specifications

Tom Holmes tholmes at woh.rr.com
Thu Mar 24 18:45:35 EDT 2016


Bob...

This is not related to your comment below but is still related to the thread..

Most SA's will generate noticeable internal distortion products at about -20 dBm to the first mixer. Internal or external attenuator will knock down the much louder 10 MHz from the Oscillator under test to that level. The old reliable "add 10 dB of attenuation" test and look for commensurate changes in distortion products vs the fundamental is essential for this. If you add 10 dB and the trash changes by 20 or more, then they are in the SA.

Most close-in stuff won’t be affected, but as has been mentioned, the phase noise of the SA will likely be the limiting factor for what you will see unless you are testing a pretty bad synthesizer or using a current generation SA. With the modern boxes you can see more but it still won’t be clean enough to evaluate many XO's. You'd use the XO to find the PN of the SA!

Tom Holmes, N8ZM


-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Bob Camp
Sent: Thursday, March 24, 2016 4:55 PM
To: Discussion of precise time and frequency measurement <time-nuts at febo.com>
Subject: Re: [time-nuts] Spectrum Analyzer Specifications

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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