[time-nuts] Frequency counter recommendation

jimlux jimlux at earthlink.net
Fri Dec 17 09:29:31 EST 2010


Interesting discussion..
comments interspersed

Chris Albertson wrote:
> 
> 
> Jumping ahead to design.  No one wants a serial RS232 interface. they
> don't even make computers with RS232 ports much any more.  Those guys
> that designed equipment that forced people to load costom USB drivers
> just did not think.  There is no need for that.  What you do is make
> you project appear to be some "standard" USB class and then the OS
> (Linux, Windows or Mac OSX) will already have a driver.   That VNA
> should have presented itself as a serial port.  And then the software
> could read from a serial port.  But of course there would be not
> physical RS232 device.
> 
> If you have to select an interface I'd rather have any wireless type.
> WiFi or Bluetooth.

wireless interface and RF test equipment is a bad combination.  If 
you're trying to measure small scale performance (e.g. timing at 1E-10 
levels), small amounts of RF leaking in/out causes problems.  This is 
one of the things that separates good test equipment from great 
equipment.  It's hard to get better than 100dB isolation from packaging, 
and if you're looking for things at the -150dBm level, something at 0dBm 
is huge.

> 
> But if you are building a modular system you do NOT want to pick one.
> You just make a project standard to use (say) I2C, SPI, "two wire" ir
> whatever.  Then the counter module is controlled by i2c and if you
> want to connect it to a computer you build the USB module but if you
> want a stand alone no-computer instrument you build the "front panel"
> that has LED numbers.
> That is the entire ont is "modular", you avoid this kinds of decisions
> and allow for easy upgrade as technology changes.


IR/fiber optic interfaces are very intriguing.  Too bad that the plastic 
fiber stuff costs more than conventional wires/connectors.

> 
> Other questions to resolve are "how many slices to cut the pie into".
> I would argue for "very small" single funtions bulding blocks so we
> don't have the HPSDR problem of years of time to design each one.

Against that: every connection causes potential troubles.  A better 
solution for the generalized case is to put multiple functions together, 
but don't necessarily connect them all. Think of the old IF strip chips. 
Oscillator, amplifiers, variable gain stages, detectors, all on the same 
chip but the ins and outs brought out to pins.  I don't think you want 
to bring them out to connectors, but, rather, provide a way to do 
interconnections, etc.



> 
> Selecting a physical chassis to use willl take time.  I like the idea
> of using a disk enclosure because then you can buy a 1U or 8U rack or
> an old PC chassis, If you modual looks like a disk there are plenty of
> things it can fit into.

"old pc chassis" is a very limited life item in a particular 
configuration.  do you mean my old IBM PC?  Or an AT? or a tower case? 
or a midsize case?
When you say disk drive size, do you mean "5 1/4" floppy/CD-ROM/DVD" or 
something else.

Packaging is going to be critical for high performance.  Look at boxes 
from COMPAC, for instance.

> 
>>From my experience, for something like this to take off one person has
> to take ownership of the project and run with it, make the web site,
> write some golas and build "something" that works.  Only then do other
> jump in and help.
> 
> It really would be good to have a Time and Frequency Instrumentation
> Project as currently the state of the art seem to be that you simply
> buy something from a Chinese eBay reseller.  This is hardly what I'd
> cal "innovation."

> 
> 





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