[time-nuts] FE-5680A FAQ update: question about frequency synthesizer architecture

Javier Herrero jherrero at hvsistemas.es
Sat Jan 28 15:54:22 EST 2012

Another though... The FE5680A manual says that the most basic unit 
outputs 50.255+ MHz (1/136 the Rb oscillation frequency). I suppose that 
this is the "other" style of units, that uses a 50.255MHz crystal to 
excite the cavity. 50.255MHz/2^48 is 1.785416^-7Hz, the claimed 
resolution for these FE-5680A.

I'm starting to suspect that the claimed resolution of 1.7854^-7Hz is 
only for a serial protocol compatibility reason, and that really the 
effective tuning step for these Rbs is in fact around 6.81^-6Hz.


Javier, EA1CRB

El 28/01/2012 20:44, Javier Herrero escribió:
> I've checked now that they do not seem to be any activity on the DDS 
> SPI lines during normal operation. I've tried with several small 
> offsets through the serial port, and the square wave at FSELECT seems 
> to be same square, no appreciable duty cycle variation.
> Regards,
> Javier
> El 28/01/2012 20:32, ed breya escribió:
>> The lower bits can be pulse width modulated to get finer resolution, 
>> but it gets harder to maintain monotonicity. Eventually you run out 
>> of processing resolution, or the control signal falls into the noise 
>> floor, whether the system is digital, analog, or both.
>> Ed
>> Javier Herrero wrote:
>> Sat Jan 28 18:38:53 UTC 2012
>> It can be... but the resolution of the center frequency of two DDS
>> frequencies would be limited to half DDS LSB... so not enough. I will
>> try to monitor FSELECT and also to extract some SPI data if I found some
>> free time today or tomorrow :)
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Javier Herrero
Chief Technology Officer                  EMAIL: jherrero at hvsistemas.com
HV Sistemas S.L.                          PHONE:         +34 949 336 806
Los Charcones, 17                         FAX:           +34 949 336 792
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