Frank wrote:
As a test, I sent the 10 MHz out from my Trimble Thunderbolt through one
TADD-2 Mini and to a Siglent digital scope. A screen shot from the scope
is attached.
1) Does this look like a decent waveform for TICC purposes? I verified
in advance with the scope that the Thunderbolt is putting out a clean
sine wave.
No, that is not a promising waveform for driving any sort of
edge-triggered counting circuitry. There is way too much ringing. Note
that there is only about a 1.25v margin from the bottom of the ringing
after the rising edge and the top of the ringing after the falling edge.
Any edge-triggered circuitry would have to reliably trigger within
this rather narrow window under all conditions, automatically. Not
impossible, but very far from optimum.
2) For this test, I just used 50-ohm coax between the Thunderbolt and
the TADD-2 Mini, and between the latter and the scope. I see the TADD-2
Mini instructions suggest avoiding a 50-ohm load, instead terminating
into a TTL or high-impedance load. Is there a recommendation for the
best cabling to use to connect the TADD-2 Minis to the TICC?
No idea why the T2-Mini recommends against using a termination that
matches the coax. The TAPR blurb on the T2-Mini says, "The T2-Mini has
a single low-impedance output that delivers greater than 3.5 volts into
a 50 ohm load." That is what you need to do -- use a 50 ohm termination
at the load end of the coax between the T2-Mini and the TICC to tame the
ringing.
Once you hook the T2-Mini up to the TICC, you can do that easily by
installing the jumper at the TICC input (JP3), to put 51 ohm resistor
R17 across the input connector.
Doing as you are now -- feeding the T2-Mini output through coax to the
input of your scope -- you will either need to switch the scope input
impedance from 1 Mohm to 50 ohms (if the scope has that feature --
sadly, many digiscopes do not), or use a 50 ohm feedthrough terminator
on the scope input.
Best regards,
Charles
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