[time-nuts] Re: Build a 3 hat timestanp counter (hans-georg at lehnard.de)

[Magnus Danielson]

Hi,

It looks as if you have a higher noise floor with the TLV3501. I see two 
effects, both higher slope (usually but not always due to gaussian 
noise) and then also a higher systematic noise. The later could be from 
power-suppy for instance, but any form of RF and LF frequency pickup.

The actual noise of the TLV3501 I just fail to spot in the datasheet on 
a quick look, but also the bandwidth.

I assume that the trigger point is good, that is that you trigger on the 
highest slew-rate point of the curve. For a sine that would be at the 
through-zero point, but for square-wave it is actually closer to the 
previous level for actual signals. A DC blocker and keeping the trigger 
close to zero usually suffice. Then just not loosing amplitude going in, 
as amplitude convert to slew-rate.

You might benefit of doing spectrum analysis on the data to locate RF 
frequencies and track them down in the analog domain.

Consider using amplification stages to increase slew-rate before hitting 
an input.

I remember once a design where the hardware guys had an ECL "comparator" 
setup so it in one state gave a solid signal but the other acted as a 
linear amplifier of all the noise on the board. While it may seem like 
adding hysteresis would cure it, it will only cure it for the non-timing 
parts (which will be the amplitude part) of the signal where as the 
timing part would still be affected. Also, hysteresis shifts the trigger 
point to one which has somewhat less ideal slew-rate for timing 
purposes. As always the timing/phase and amplitude parts of the signal 
is on orthogonal parts.

Cheers,
Magnus

On 2022-06-06 15:19, Hans-Georg Lehnard via time-nuts wrote:
> Hi,
> I tested the TLV3501 with the HP E1740A TIA and there is a visible
> difference. First test an OCXO on reference and directly on the input.
> Second test OCXO via the TLV32501 on the input.
>
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