[time-nuts] Frequency counter recommendation
bownes at gmail.com
Sun Dec 19 20:59:54 UTC 2010
That's kinda my point about using rs232. Serial to USB, serial to Ethernet adaptors will be available for a good long while.
Also why I like the idea of a standalone instrument that also has a pc interface of some sort. Or a slot for a pc interface of whatever source I want.
So how does one build the core counter with a start, a stop, and maybe a 10 MHz reference input with sub 5ps resolution and accuracy?
On Dec 19, 2010, at 3:50 PM, Bob Camp <lists at rtty.us> wrote:
> The gotcha with caps is that they don't block RF very well. Of course if you go high enough in frequency, not much can be blocked anyway.
> Opto isolated RS-232 is just a pair of cheap optos, a couple of diodes and a few resistors and caps. It costs next to nothing and it will work just fine for any speed you are likely to need in a counter. Spend $4 on a USB to serial adapter and throw it away when the next generation of driver signing (or what ever) renders it obsolete. Replace it with one that likely costs 50 cents then. I won't go into the possibility that it might cost half a Euro or $50 ....
> On Dec 19, 2010, at 12:01 PM, Poul-Henning Kamp wrote:
>> In message <4D0E3080.1060005 at earthlink.net>, jimlux writes:
>>> Bob Camp wrote:
>>> There are some nice solutions, though, using USB directly, which has the
>>> advantage of providing a few watts of power. Isolated DC/DC converters
>>> are readily available. you might be able to do transformer isolation of
>>> the two data wires.
>> First: you want to use capacitors, not transformers.
>> Second: it is surprisingly tricky to get to work in practice, I have
>> only ever seen one device that managed to do so, and comply with
>> the USB standard, and the designer swore to never do it again.
>> People generally default to a small USB-enabled microcontroller
>> with an optocoupler to the real stuff.
>> Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
>> phk at FreeBSD.ORG | TCP/IP since RFC 956
>> FreeBSD committer | BSD since 4.3-tahoe
>> Never attribute to malice what can adequately be explained by incompetence.
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