[time-nuts] Distribution amp - Use a video amp unit ?

[Bruce Griffiths]

The LMH6702 is one of the few current feedback amplifiers that is stable 
with a relatively low value (237 ohm) feedback resistor.

Its estimated phase noise floor is around -171 dBc/Hz with +13dBm output 
(and input) in the noninverting 2x configuration with a 50 ohm load and 
a 50 ohm resistor in series with its output.
Thats about 3dBc/Hz less noisy than the measured 10MHz phase noise floor 
of a OPA653 in the same configuration with the same input and output 
signal levels.
For offsets below about 100Hz the phase noise of an OPA653 is 
indistinguishable from the noise of  measurement setup.

The effect of drafts and other air currents on the phase shift of RF 
transformers is also significant for offsets of around 1Hz or less.
Extra thermal mass (encapsulation) and draft shields (even a piece of 
paper) can work wonders.


Bruce

Joseph M Gwinn wrote:
> I have seen National LMH6702 current-feedback video amplifier chips in
> non-inverting amplifier configuration used to implement a wideband 10 MHz
> distribution amplifier.
>
> Joe
>
>
>
>
> From:	"Charles P. Steinmetz"<charles_steinmetz at lavabit.com>
> To:	Discussion of precise time and frequency measurement
>              <time-nuts at febo.com>
> Date:	03/27/2012 07:28 PM
> Subject:	Re: [time-nuts] Distribution amp - Use a video amp unit ?
> Sent by:	time-nuts-bounces at febo.com
>
>
>
> Bruce wrote:
>
>    
>> The above reverse isolation [~35 dB] is about 25dB lower than I would
>>      
> expect.
>
> D'oh!  Bruce is right -- I calculated the reverse isolation
> incorrectly.  I had only been expecting 40 dB, so I didn't question
> the result.  The breadboard actually measured nearly 63 dB.
>
>    
>> Stable operation at unity gain is necessary if a feedback capacitor is
>>      
> used.
>
> The Miller capacitance of the output transistors sees to that (with
> an even greater phase margin when a faster transistor is used for Q1).
>
>    
>> An LM329 has similar noise without the dissipation of the internal
>> heater in the LM399
>>      
> I know.  I just particularly like the 399, and have a pile of
> them.  I rationalize using it in this case by noting that the range
> of frequencies where phase noise of the DA is important includes sub-
> to low-Hz frequencies at which thermal effects could make the
> unheated 329 significantly noisier (though if you keep drafts off
> both of them, it might not be by a large amount).
>
> Best regards,
>
> Charles
>
>
>
>
>
>
>
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
>
>    
>
>
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.