[time-nuts] Stepping up the output of an OCXO

[Dr Bruce Griffiths]

Stephan Sandenbergh wrote:
> Hi All,
>
>  
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> Say for instance you have a 5dBm clean 10MHz sinusoid (such as that provided
> by an OCXO). Now you want to run this signal to a device that will take a
> minimum of 10dBm and maximum of 15dBm as input. How does one amplify the
> 5dBm?
>
>  
>
> The first thing that sprang to mind is a step-up RF transformer. However, it
> is impossible to step-up because the following device has an input impedance
> of 50ohm. To step it up you'll need a higher input impedance. A step-up
> transformer will also mess with the matching.
>
>  
>
> After Googling a bit I found that another option might be LC-resonance. This
> seems like a good idea since resonance is per definition a filter-like
> effect. 
>
>  
>
> The easiest, of course, would be if there exists some integrated solution. I
> browsed through the list of RF gain blocks on the Mini-Circuits site - these
> little guys seem perfect for the job. However, the typical situation is that
> it amplifies by at least 10dBs while the maximum output of it sits at around
> 13dBm. 5dBm will thus more than saturate the poor thing.
>
>  
>
> My final idea is a high speed opamp. I suspect this would add nasty 1/f
> noise. Probably the same with a simple RF transistor amplifier. 
>
>  
>
> Do you guys have any better ideas?  Your thoughts on this are much
> appreciated.
>
>  
>
> Kind regards,
>
>  
>
> Stephan Sandenbergh
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>   
Stephen

The optimum method depends on the load characteristics.
If you want to drive a CMOS logic gate then using a series tuned LC tank 
to step up the voltage will be satisfactory see:
http://www.wenzel.com/documents/waveform.html
Or for a version that allows the effect of tuning the tank to be 
monitored without significantly loading the stepped up output see

Silicon Chip February 2007 p71.

If you need to drive a 50 ohm load, consider using a common base 
amplifier with an input and output stepdown transformers.
Suitable transformers are readily availble from minicircuits or you can 
wind your own.

Use a 2:1 turns ratio (4:1 impedance ratio) input step down transformer 
and a 4:1 turns ratio (16:1 impedance ratio) stepdown output transformer 
with a shunt 800 ohm impedance matching  resistor connected across the 
primary windings. Use a resistor (~8.2 ohm) in series with the emitter 
to set the RF input impedance at 50 ohms as seen from the primary of the 
input transformer. A common base amplifier has the advantage of having 
high reverse isolation independent of source and load impedances.
If you want circuit schematics just ask, but when you do so I need to 
know the OCXO frequency.

A bandpass filter can always be used to cleanup the amplifier output if 
required.

If you must use one of the minicircuits amplifiers just place an 
attenauator between the OCXO output and the amplifier input.

Bruce