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

[Dr Bruce Griffiths]

Stephan Sandenbergh wrote:
> Hi Bill,
>
> Judging by your question - that is probably something that I should do?
>
> Considering that you'll attenuate the 5dBm signal to about 0dBm and then
> amplifying it back up to about 10dBm. My gut tells me that by attenuating
> the signal before amplifying it will raise the noise floor. (I wouldn't want
> to ruin my -165dB noise floor)  However, I must confess that I'm new to RF
> components and sinusoidal signals, so I might be argumenting completely
> wrong.
>
> I guess I should do the math on that one.
>
> Regards,
>
> Stephan.
>
>
> On 2/1/07, Bill Hawkins <bill at iaxs.net> wrote:
>   
>> Have you investigated the concept of an attenuator, to use
>> before the RF gain block?
>>
>> Bill Hawkins
>>
>>     
Stephan

I presume your OCXO phase noise floor is actually -165dBc/Hz.
Which corresponds to -160dBm/Hz in the 5dBm output.
An amplifier with a 0dB noise figure has an input phase noise floor of 
around -174dBm/Hz
So you can degrade the noise floor by 10dB or more by using an 
attenuator and an amplifier with a more realistic noise figure of a few 
dB without appreciably degrading the OCXO phase noise floor.
For hints and phase noise figures for some microcircuit amplifiers see:
http://www.wenzel.com/documents/hints.htm


For example a Minicircuits MAV11SM has a 1dB compression output of 
+17.5dBm and a gain of 10.5dB with a noise figure of 3.5dB so you would 
need much input attenuation to achieve the required output level. This 
amplifier wont appreciably degrade the OCXO phase noise floor. low pass 
filtering the output will reduce its harmonic content.
Its input phase noise floor with no input attenuator and a 5dBm input 
will be about -175.5dBc/Hz or about 10dBc/Hz better than your OCXO.

Bruce