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

[Bob kb8tq]

Hi

> On Aug 26, 2022, at 1:08 AM, Carsten Andrich <carsten.andrich at tu-ilmenau.de> wrote:
> 
> Hi Bob,
> 
> On 24.08.22 21:20, Bob kb8tq wrote:
>>> [...] 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.
>> Given the dirt cheap nature of PCB’s these days, the idea making one application 
>> more expensive / complex is generally not a great approach. Let JLPCB do up 
>> enough of this or that and do a second board for a different application. Layout
>> wise, modern tools get things done very quickly.
> You're right in terms of component and (automated) production cost, but I'm also looking at it from an engineering cost perspective. Qualifying multiple different solutions for a similar* problem is costly. Hand-soldering the SMA connectors is already more expensive than the price difference between logic buffers and buffer opamps. Add the connectors on top and I'd doubt total savings on components would ever amortize the added engineering costs for producing a few dozen devices.
> 
> *) By similar problem I mean low-noise buffering of arbitrary wideband signals with 50 Ohm source impedance into both high impedance and 50 Ohm loads. By arbitrary wideband signal I refer to any kind of typical timing signal, whether it's a pulse (not necessarily only 1PPS) or periodic rectangle at frequencies up to a few hundred MHz.
> 

If this is a commercial product that is targeted at a market then yes, the qualification
process gets into it. For a hobby use sort of design, it’s a pretty quick process and
“part of the fun”. If this is indeed a commercial design?
> 
> 
>>> 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.
>> But your -175 dbc phase noise is long gone ….
> Unfortunately, yes. If phase noise is of most concern, then RF gain blocks are obviously a better choice than buffer opamps. The       HMC589 has 4 dB noise figure down to DC (allegedly) and even comes with phase noise specs [1]. However, I doubt these gain blocks are a suitable choice if 3.3V CMOS logic levels are required.
> 
> Regardless, the LTC6957-3 I plan on using to convert a 100 MHz sine to 3.3V CMOS is considerably worse than the suggested buffer opamps in terms of phase noise. Gotta spin off another thread on that ...
> 
> 
> 
>> A discrete buffer typically is the right answer in most cases for low phase noise. 
> Are you referring to the same logic buffers that have 1~4 ps jitter? How are these better than dedicated clock ICs with < 90 fs jitter specs and a noise floor above my suggested opamps?
> 

No, an amplifier made up of discrete transistors, resistors, capacitors and
the like.

> As always, I'd be grateful for references to exemplary components, datasheets, designs, and measurement results.
> 
Unfortunately that measurement data and the documentation that ran up to it is 
property of my various employers over the last 50 years of designing OCXO’s and 
GPSDO’s.

Bob

> Best regards,
> Carsten
> 
> [1] https://www.analog.com/media/en/technical-documentation/data-sheets/hmc589ast89e.pdf <https://www.analog.com/media/en/technical-documentation/data-sheets/hmc589ast89e.pdf>