[time-nuts] Thunderbolt tuning DAC theory of operation

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On a later version, the Trimble/Nortel 45k, there are a few obvious HW 
differences (lousy Rx, bigger FPGA - XC5204, second Flash EEPROM).
The PWM is generated differentially (better CMR) in the FPGA (output on 
pins 12, 13) registered, synchronously with the squared OCXO output 
signal to reduce jitter, in the 74AC174 on D0, and D1 (input on pins 3, 
4), which is being supplied with a stabilized voltage from the LT1021-5 
reference through one section of the quad Op Amp LT1014, and a series 
transistor. Those operations (jitter minimization  and clean supply 
voltage) are crucial to the quality of the PWM signal.
While the operation is obviously by PWM modulating the 102.4us period 
(10MHz/2^10) signal by the 10MSBs, the processing of the 10LSBs is less so.
The PWM signal is dithered, by a ripple minimizing pattern, with a 
periodicity of 1024 pulses (104.8576ms = 1/10MHz/10^20), giving the DAC 
a full 20bit monotonic resolution.


On 11/2/2013 10:41 AM, Stewart Cobb wrote:
> While poking around the Thunderbolt to determine whether -5V could be
> used in  place of -12V, I discovered how the OCXO tuning DAC works.
> Apologies if this is old news, but I haven't seen it documented
> before.
>
> The 10MHz sine wave from the OCXO  is squared up and used to clock the
> Xilinx 5200 CPLD (U22) and a 74AC174 hex D flip-flop (U14).  Inside
> the CPLD (apparently) the 10 MHz clock is divided by 1024, giving a
> square wave with a period of 102.4 us (about 9.7 kHz).  The duty cycle
> of that square wave is modulated by the 10 MSBs of the commanded DAC
> value.  The LSBs are used to offset the falling edge of the square
> wave one clock cycle (100 ns) later, during a fraction of the 9.7 kHz
> square waves proportional to the LSBs value.  On a modern digital
> scope, you can zoom in on the falling edge of the square wave, set the
> display to "average", and see that the averaged height of that clock
> cycle is proportional to the DAC LSBs.  There appear to be at least 8
> LSBs, perhaps as many as 10, giving a total DAC resolution of 18 to 20
> bits.  (If the DAC value is averaged over one second, there are 10^7
> clock cycles which can be controlled, giving a theoretical maximum
> resolution of 23+ bits.  Trimble may have chosen a shorter averaging
> time and fewer bits.)
>
> The PWM square wave travels from pin 13 of the CPLD (U22) to pin 4,
> the D1 input of the 74AC174 (U14).  The flip-flops in this chip are
> also clocked by the squared-up 10 MHz from the OCXO.  The Q1 output,
> pin 5 of U14, goes to one side of R83 in the circuitry around the
> LT1014 op-amp.  The other five inputs and outputs of U14 are
> constantly high or low.  They may also be fed to the op-amp circuits,
> to help it handle the square wave in a purely ratiometric manner.
>
> The inputs and outputs of the Xilinx CPLD can be programmed for many
> different I/O standards.  Unfortunately, this makes their output pin
> drivers far from ideal.  The purpose of the 74AC174 is presumably to
> drive the analog circuitry with a input that is as close as possible
> to a mathematically ideal digital signal.  Outputs in the 74AC logic
> family can source or sink 24 mA and have relatively balanced raise and
> fall times.  This was probably the most ideal digital output available
> to the Thunderbolt's designers in the late '90s.
>
> This DAC implementation is guaranteed monotonic, an important
> consideration.  There is exactly one rising edge and one falling edge
> per cycle, so that any difference between rise and fall times will
> have a constant effect which can be tuned out.  Unlike a sigma-delta
> DAC, this PWM DAC produces strong spectral lines at multiples of the
> 9.7 kHz square wave frequency.  On the one hand, it is comparatively
> easy to design filters to remove a single frequency (and its
> harmonics).  On the other hand, this signal is strong enough that it
> may appear in phase noise plots anyway.
>
> If you want to view the 9.7 kHz square wave for yourself, it appears
> on a small square test point next to the silkscreen designator for
> C78, very close to the 6-pin power input jack.  This test point is
> part of the connection from the Xilinx CPLD to the hex D flip-flop.
> Probing it does not affect the OCXO tuning.
>
> Hope this helps.
>
> Cheers!
> --Stu
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