[time-nuts] Noise Floor

[kb8tq at n1k.org]

Hi

Gain without an input termination will very much depend on the source you are driving it from. With a 
"proper" 50 ohm source, the gain will be 6 db high without the input resistor. If it's got more gain than 
that, your source is not 50 ohms. There are a lot of OCXO's out there that are not "true" 50 ohm sources.

Input termination or not depends a *lot* on what you are going to use it for. In my case, I'm after a high
input impedance to low output impedance conversion. I need to deal with some gear that was made back
before 50 ohms was "how it's done".

Bob

-----Original Message-----
From: time-nuts <time-nuts-bounces at lists.febo.com> On Behalf Of Taka Kamiya via time-nuts
Sent: Wednesday, March 25, 2020 2:49 PM
To: Discussion of precise time and frequency measurement <time-nuts at lists.febo.com>
Cc: Taka Kamiya <tkamiya9 at yahoo.com>
Subject: Re: [time-nuts] Noise Floor

I've been playing around with Clifton amplifier as well.  Mine, input is terminated with 50 ohm register, and rest is unmodified, so it has 6dB gain.  I have a 10dB pad on input side.  I, too, noticed there will be a severe clipping with driving it too hard.  I plan to zero the gain and retest.  Without proper termination on input side, it showed phantom gain of 10dB.....?
I stayed conservative and output is 7dBm.  I with there was a little more room there....  Looks pretty clean spectral density wise.  (frequency is 10MHz)

--------------------------------------- 
(Mr.) Taka Kamiya
KB4EMF / ex JF2DKG
 

    On Wednesday, March 25, 2020, 2:00:08 PM EDT, kb8tq at n1k.org <kb8tq at n1k.org> wrote:  
 
 Hi

So, here we have a couple of plots of a Clifton Labs Z10000 amplifier board.  It's a part that several list members (including myself) 
have recommended using.  The board has the "stock" high input impedance and has been modified for 0 db gain (470 ohm resistor so
just over 0 db ...). There is a 6 db pad between the splitter and the amp to terminate things.  The run is fairly short so the data is a bit rough. 
For a better look, an overnight run would show a bit more detail. 

Phase noise looks pretty good. ADEV has some weird "stuff" going on. Time to start tearing the board apart to see what's wrong with it?
Maybe there's some noise in those resistors .... let's go !!!!

Well..... maybe not so much. If you go back and dig up the original plots in this thread, the ADEV is *essentially* at the "noise 
floor" of the measurement system. There are sure to be some wobbles in a shorter ADEV run so it's not a 1:1 sort of thing. 
Phase noise wise, everything from about 80 Hz and lower is at noise floor. The data out at 100 KHz offset suggests that for a low 
phase noise system, this board would degrade the typical 10811 by a slight bit "wideband". 

Since noise floor likely changes with things like drive level and frequency, to be 100% sure of the floor, one would need to repeat the original
test at this frequency and these levels. I'm not that ambitious 😊 What I have tells me pretty well when I've hit the limit. It keeps me from 
going crazy "debugging" a board that actually does not have a problem. (or if there's a problem, my quick test can't see it )

Some other Z10000 trivia:

If you drive this board so it has a couple db more output, it goes into clipping. When that happens .... yuck. Noise and ADEV both are massively
impacted. You very much do *not* want to overdrive this board. (This is also true of most amplifiers)  At ~14 dbm out at 5 MHz  you are in clipping. 
I would stick with 12 dbm or less (at 5 MHz ... who knows about other frequencies ...).  

Dropping gain to zero db with the resistors on the board is better than doing the same with a pad on the input. The wideband phase noise is a bit 
better when done with the resistors. Very much like the front end on a radio ....

Fun !!!

Bob



-----Original Message-----
From: time-nuts <time-nuts-bounces at lists.febo.com> On Behalf Of jimlux
Sent: Tuesday, March 24, 2020 7:31 PM
To: time-nuts at lists.febo.com
Subject: Re: [time-nuts] Noise Floor

On 3/24/20 2:17 PM, John Miles wrote:
>>> It would be interesting to know what ADC was used and if there's an
>>> SDR-board out there that uses the same ADC.
>>
>> Uh.. I remember John telling me what ADC it was, but I forgot, sorry.
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