[time-nuts] NIST frequency doubler

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Jul 10 00:19:39 UTC 2007


SAIDJACK at aol.com wrote:
>  Hi Bruce,
>   
>  
> nice circuit. Many questions:
>  
> I wonder how well it works to get a 5MHz source up to 10MHz?
>  
> Also, would you have recommendations on the transformer part numbers?  
> (MiniCircuits, MaCom, etc)? I think the transformers are probably key  to getting 
> good phase noise (preventing them from saturating, crosstalk,  etc).
>  
> Do you have a suggestion on who makes the best (lowest-noise) JFets?
>  
> Where could we find the original paper's PDF?
>  
> Lastly, how much fundamental attenuation would you expect from such a  
> circuit?
>  
> Thanks!
> Said
>   
Said

Embedded in:

1992 IEEE FREQUENCY CONTROL SYMPOSIUM
ULTRA-HIGH STABILITY SYNTHESIZER FOR DIODE LASER PUMPED RUBIDIUM
John P. Lowe, F. L. Walls, and R. E. Drullinger
Time and Frequency Division
National Institute of Standards and Technology
325 Broadway
Boulder, CO 80303

There is little more than the image I extracted.
This circuit had been developed somewhat earlier and as far as I have 
been able to tell hasn't appeared anywhere else.

It has also been used for a 5MHz to 10MHz doubler. (Wenzels 5- 20MHz 
JFET doublers are supposed to be derivatives of a NIST design)
There is no fundamental reason that it cant be used at even lower 
frequencies, provided suitable transformers can be built/obtained.

The real question is perhaps who make JFETS (especially matched ones) 
anymore.
JFET matched pairs:
Interfet
http://www.interfet.com/
Linear systems
http://www.linearsystems.com

Other JFETs:
Vishay/Siliconix
http://www.vishay.com/fets-small-signal/SSFsgnchjampP/
Onsemiconductor:
http://www.onsemi.com/PowerSolutions/parametrics.do?id=806
Fairchild semiconductor
http://www.fairchildsemi.com/sitesearch/fsc.jsp?command=eq&attr1=AAAFamily&attr2=Junction+FET+%28JFET%29 
<http://www.fairchildsemi.com/sitesearch/fsc.jsp?command=eq&attr1=AAAFamily&attr2=Junction+FET+%28JFET%29>


Various other suppliers
Hitachi
Sony
etc.

Noise specifications are similar for all manufacturers, however to 
achieve the stated phase noise performance testing and selection of 
JFETS may be required.
At lower frequencies higher capacitance JFETS should be usable.
a pair of J310s can be substituted for the U431 but some matching of the 
JFETS is desirable
The fundamental attenuation depends on the matching of the 2 FETs and 
the input transformer.
I would expect 20dB suppression of the fundamental to be easy and 
perhaps 40dB if the circuit is modified slightly to allow compensation 
for residual JFET mismatch.

Transformers:
There should not be any great problem with input transformer saturation 
as the dc current flowing in the secondary due to FET mismatch will be 
small.
The output transformer has dc current flowing in the primary (~20mA for 
28mA peak JFET currents) this doesn't appear to exceed the specified 
limits for most Minicircuits transformers, however these limits aren't 
well specified in the datasheets (it does make a difference if it is 
flowing in the secondary or primary of a transformer with a turns ratio 
of other than 1:1, Minicircuits do not specify which winding the dc 
limit refers to). NIST regularly drive the primary of Minicircuits 2:1 
stepdown (200:50 ohm) transformers without difficulties due to 
transformer saturation.

The best turns ratio for the input transformer is best determined by a 
combination of simulation and testing to achieve the desired JFET peak 
drain currents.

Bruce






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