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

[Dr Bruce Griffiths]

Stephan Sandenbergh wrote:
>> Hi All,
>>
>>  
>>
>> 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
>>   
>>     
Stephan

A differentially driven common base pair with transformer coupled input 
and output is another option with lower distortion than a single ended 
input common base amplifier.

Another option is a common base noiseless feedback amplifier using 
transformer feedback.
However these have a similar input output isolation to a transformer, 
they are in effect a"transformer" with power gain.
If the lack of input output isolation is is acceptable then these have 
the lowest noise of any RF feedback amplifier.
You will need to wind your own feedback transformer.

A cascode amplifier with transformer output could also be used.

A high speed opamp could be used however it will be noisier than a 
common base or cascode amplifier.
1/f noise isnt important as long as there is no upconversion to 
frequencies close to the carrier.
Feedback minimises this effect.
AC coupling the output with a suitable time constant will virtually 
eliminate any 1/f noise on the output.
Using a bandpass filter will filter out harmonics and even more of the 
output noise.

Bruce