[time-nuts] V standards

[Alberto di Bene]

WarrenS wrote:
> Dick
> 
> Here is one way to add 2 LS  bits (or more) to any Dac no matter how it is made.
> This assumes that the Dac that you are adding these bits to is 0 to 5 volts Fullscale 
> and accurate enough to justify adding LS bits.
> 
> To add 2 LS bits to a 14 bit Dac.
> An easy ways to do it is NOT to use PWM.
> That way it needs no filtering and no PWM 
> and few software resources, maybe none. 
> The added 15th Bit wants to add  1 / 32,768 of 5 volts 
> The added bit #16 wants to add 1 / 65,536 of 5 volts 
   ----- snip -----

In a GPSDO I designed and built about 4 - 5 years ago, I obtained almost 20-bit resolution without using any DAC,
I used the PWM of the Atmel AVR AT90S8535, which has a resolution of 10 bits. Then I changed the limits on the voltage 
ramp that define the switch over points of the PWM, subdividing them into 1024 intervals, which means another 10 bits.
I used the DDA (Digital Dithering Algorithm) method to minimize the amplitude of the about 4 Hz ripple generated on the 
EFC voltage by this technique. A low pass filter with a theoretical attenuation of 120dB at 3.9 Hz was then used before 
applying the EFC voltage to the Isotemp OCXO 134 I used in the project.  It worked.  I still have the Atmel assembly 
code, should someone be interested.

73  Alberto  I2PHD
P.S. The square wave produced by the PWM was conditioned with an MC1503U so to make it independent of the supply voltage 
of the Atmel processor.