[time-nuts] Re: RCB-F9T Adapter PCB with USB and 50 Ohm Timepulse SMA Connectors

[Carsten Andrich]

Hi Bruce,

On 24.08.22 10:26, Bruce Griffiths via time-nuts wrote:
> I don't see any explicit jitter specs on the BUF602 datasheet either.
> Have you measured it?
> How consistent is it from part to part?

Jitter isn't typically specified for analog amplifiers. At least the 
ones I worked with never specified jitter. I'm not an expert on analog 
RF components, but – like I wrote before – I'd assume that the opamp's 
internal noise to be the only relevant source of additive jitter. Please 
let me know if that assumption is wrong.

So far, I haven't measured anything. I've relied on the datasheet values 
and the BUF602 seems to be the most promising (hand solderable) solution 
for the job.


> The jitter of most CMOS families has been measured.
> For example 74HC buffers typically have ~4ps jitter at room temperature.

Could you kindly point me to such measurements? I'm curious.


> Even this is much lower than the jitter of typical FPGAs when the effect of cross coupling from other clock domains such as internal oscillators etc are taken into account.
> Faster logic families such as 74AC buffers (~ 1ps) have even lower jitter.
> CMOS devices have a typical propagation delay tempco of around 0.4% of the delay per/C. FPGA gates have similar delay tempcos.
> Neither the delay tempco nor the jitter is typically specified on CMOS device datasheets nor are they on the BUF602 datasheet so you have to measure them or leverage the results obtained by others.

These numbers (1~4 ps) are excellent in comparison to the ZED-F9T's 
timepulse jitter. I estimate about 100 ps jitter when integrating John 
Ackermann's phase noise measurement [1] from 1 Hz to 100 kHz. However, 
my use case is not limited to the RCB-F9T adapter. I'm looking for a 
generic solution that will also perform well with low noise OCXOs.

I'm taking a decent 100 MHz OCXO with the following phase noise as an 
example: -100 dBc/Hz @ 10 Hz, -130 dBc/Hz @ 100 Hz, -160 dBc/Hz @ 1 kHz, 
-170 dBc/Hz @ 10 kHz, 175 dBc/Hz @ >100 kHz.
That integrates to 18 fs from 12 kHz to 20 MHz (range is used for all 
subsequent jitter values). The LMK1C110x weighs in at 8 fs typ. (20 fs 
max.) for f = 156.25 MHz [2] and the LTC6957-3 has 90 fs typ. for f = 
100 MHz [3].

The BUF602's input voltage noise of 4.8 nV/√Hz equals -153 dBm/Hz into 
50 Ohm. Better, non hand solderable opamps exists, e.g., BUF802 with 2.3 
nV/√Hz (-160 dBm/Hz) and AD8000 with 1.6 nV/√Hz (-163 dBm/Hz). Depending 
on the OCXO's output level, the opamps' additive noise is barely 
measurable over the OCXO's phase noise. Assuming +7 dBm OCXO output, the 
AD8000 has a -170 dBc/Hz noise floor. Hence, I'd expect negligible 
additive jitter.

With 1~4 ps jitter, the logic buffers are significantly worse. Also in 
terms of temperature coefficient, the opamps are better off, because 
their overall propagation delay is smaller. The BUF602 datasheet reports 
300~600 ps group delay [4, Fig. 12].

Best regards,
Carsten

[1] 
https://hamsci.org/sites/default/files/publications/2020_TAPR_DCC/N8UR_GPS_Evaluation_August2020.pdf#page=29
[2] https://www.ti.com/lit/ds/symlink/lmk1c1103.pdf#page=6
[3] 
https://www.analog.com/media/en/technical-documentation/data-sheets/6957fb.pdf#page=9
[4] https://www.ti.com/lit/ds/symlink/buf602.pdf#page=9