[time-nuts] Spectrum Analyzer Specifications

Bob Camp kb8tq at n1k.org
Fri Mar 25 08:03:13 EDT 2016


Hi

If the objective is to come up with something that runs over a wide range
of power levels, a gate is not the best choice. The mixer it’s self will run
fine with one input fully driven and a fairly low signal on the other side. It
is not as sensitive in this case, but your normal calibration process still
gives you accurate data. 

If you want an amp that runs over the full range of the mixer, a gate that 
does well over 1 MHz to 100 MHz (or 10 to 200 with the SYPD-2) is a bit
tough to find. They are not as good an idea at 100 MHz as at 10 MHz. 

The next thing is that the amp’s gain needs to be quite low. That sounds simple,
just put in an attenuator. If you want good noise figure (you do), the attenuator
must go on the output of the amp. For 3 db of gain on your 23 db amp, you need
a 20 db attenuator. With +5 dbm into your amp, it’s producing a clean, 
undistorted, low noise +28 dbm ahead of your attenuator. Yes, those are
random numbers, they do illustrate what the problem is. 

The traditional solution was a common base amp with a transformer in 
the collector. It’s not ideal, but it is relatively cheap and simple. To get the
most out of the system, you want it to be as good as the mixer *with* the 
amps running. A “normal” common base will likely degrade the system
by several db at floor. It could degrade the close in by quite a bit more. 

Bob

> On Mar 24, 2016, at 10:05 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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