[time-nuts] FE-5680A Mechanical Question

[Chris Albertson]

On Fri, Jan 13, 2012 at 8:37 AM, Jim Lux <jimlux at earthlink.net> wrote:
> On 1/13/12 8:15 AM, Chris Albertson wrote:
>>
>> On Fri, Jan 13, 2012 at 3:19 AM,<EWKehren at aol.com>  wrote:
>>>
>>> What we know is that you can set the Rb in 7 E-13. Dithering would allow
>>> even closer setting, the question is what rate will the Rb accept with
>>> out
>>> loosing lock or deterioration of the performance. Some one should explore
>>> that.  I am still waiting to se some aging. Taking the 10 MHz output and
>>> than
>>> use an  analog loop and something like a Morion OCXO and you have the
>>> best of
>>> all.
>>> Digitally controlling the Rb will cut the cost of the control loop in
>>> half.
>>>  For $10 in parts and a PC board for less than $10 using Shera like
>>> controller  can be realized. What is needed is some one able to do the
>>> PIC.
>>> If some one is interested and able, please contact me off list. A low
>>> cost
>>> GPS or a 1 pps output of a Tbolt be perfect source.
>>> Bert Kehren
>>
>>
>> Don't use a PIC for the prototype.  A desktop PC could work as well
>> and everyone here already has one.   Connect the FE5680 to the PC's
>> serial port and send commands to adjust it.  The PC also needs to be
>> able to read a voltage.   Many already have audio input with 24-bit
>> ADC chips.
>>
>
> But those audio inputs are almost always AC coupled.
>
> Bringing up a question: Does anyone know of a cheap (<$20ish) USB voltage
> sensor (16 bits or better, ideally)..  I can see one of those Atmel USB
> capable micros (like the one on the Arduino Uno) hooked to a dual slope or
> successive approximation ADC.
>
> There seem to be an amazing number of times that I want something like that.
>  The DATAQ $29 widget is only 10 bits, unfortunately.  A USB interface DMM
> would work nicely, but I haven't found one that's in the under $50 price

Many computer audio interfaces are AC coupled but not all of them.
But even with AC coupling what you can do is use the VCO portion of a
CD4046.  This will convert voltage to frequency in the audio range.
It's a cheap work around.   But really not because frequency in nearly
imune to noise and can be sent over long cables.

What I really want is a lower price GBIP interface.   I just bought an
HP5328A on eBay that has the option 21 DVM.  This meter can measure
volts to about 5 digits and send the data out the GPIB but getting
that into a computer is the hard part.

OK so I check on eBay.   Most are $300 but If you can find a computer
with an old ISA slot then there are working GBIB cards for about $20.
 The trick is finding an old PC.  These are AT class or early Pentium
type computers.   Most of these are now in landfills some place.
But now I will be looking.

I do software all day, every day.  From experience, I can say it is
MUCH more productive to develop code on a larger desktop computer.
Better tools are available.  Then move it down to the target computer
after you have it debugged.   The process is helped if the target
computer is like the desktop computer.

There is a lot we don't know about the FE5680, like how fast can you
move the phase of the PPS?  Does the FE5680 maintain lock when you
step the DDS?   how fast can you step the DDS.   All this will take
experiments.  best to do those on the big desktop machine where we
have tools to log data to disk, make plots and so on.

Again, if anyone makes PCBs PLEASE include a way to program the uP on
the card without need of extra hardware.   The firmware will get
upgraded and not everyone has a programmer.  There must be a way for
end users upgrade the firmware.
-- 

Chris Albertson
Redondo Beach, California