[time-nuts] Thunderbolt spurs on 10MHz output at 100Hz and 200Hz from signal.

Gerhard Hoffmann dk4xp at arcor.de
Wed Sep 21 23:17:06 UTC 2016


Am 22.09.2016 um 00:24 schrieb Bruce Griffiths:
> For the later, Bruce commented that an jFET input stage would
>> probably be more quiet. Gerhard Hoffman has designed a similar
I must insist in my second "n"     Hoffmann   :-)
>> system[3] that uses a couple of paralel low noise opamps instead
>> of a jfet stage and claims a noise floor of 220pV/sqrt(Hz). All of
>> these would need to be adapted to imporve thier frequency response
>> up to 10MHz (or actually a bit higher), but then the input stage
>> gets also a bit simpler as you don't need the huge capacitors anymore.

The ADA4898 has ft= 50 MHz, so that will be hard. It already takes some 
cascoding
to get to 2 MHz with the new FET amp.
>> Another issue is the requirement to trim the current sink for low output
>> offset. If one takes advantage of the fact that the PLL imposes a low
>> frequency cutoff to the PN measurements, the amplifier input   can be AC
>> coupled, allowing dc bias feedback to be applied to the input device gate.
>> Use a parallel dc coupled JFET input opamp for the PLL
>>
>> Using a single ended input stage is significantly quieter than using a
>> differential pair.
>>
>> Paralleled BF862's can be substituted for the input JFET.
>>
Hi, Bruce and Attila (and ...),

the JFET version of my preamp is making progress. It supports either upto 16
BF862 or 2 pairs of Interfet IF3602. Upper bandwidth limit will be 2 MHz if
it runs like the simulation. There is also a low gain DC output for the PLL.

< 
https://www.flickr.com/photos/137684711@N07/29193737144/in/dateposted-public/ 
 >

One is populated with BF862, the other with Interfet. The 3rd will be 
populated
according to the outcome of the first tests, so that I have 2 equal ones for
X correlation. There will be some leftover boards available, but not to 
put them
into the drawer.

Soldered them last weekend, debug them on the next :-)
I expect 170 pV/rt Hz at least for the Interfet version. That is 
probably pearls before
swine after a ring mixer, but then we are nuts.

Attila, you are just half an hour of driving away, at least during the 
weekend!

regards, Gerhard







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