FE-5680b 1pps to 10Mhz

[David Hess]

The example chip (in photo), MC74HC393, has status "OBSOLETE" in datasheets. Could you recommend some other Ripple Counter chip that would work as well with the "20MHz square wave point"? (also, how can we tell from the datasheet that the MC74HC393 can handle the 20MHz input on one of the "CLOCK" inputs?)

Other companies make the 74HC393 including NXP, TI, and On.  Any of them should work.

The datasheet will either list the minimum clock input toggle frequency or the minimum clock input pulse width.

It seems that user: h572 suggested to power the chip with the 3.3V output. The datasheet for the MC74HC393* guarantees operation with a CLOCK pulse width of 100nS @ 2Vcc and 20nS @ 4.5Vcc (Both at <=85 deg C). Real life interpolation of a 3.3Vcc better be <= 50nS (20 MHz)...

*) http://www.onsemi.com/pub/Collateral/MC74HC393-D.PDF

It would be way better to find a 5V source inside that Rubidium standard.

And then you have the recommended rise time of the CLOCK pulse being nothing less than 400nS, even with a Vcc of 6V.

I agree that at 3.0 or 3.3 volts, the 74HC393 and 74HCT393 would probably not be fast enough.  There are faster direct replacements for the them like the 74VHC393 and 74LV393 which would work and are specified for 3.3 volt operation.

For dividing 20 MHz down to 10 MHz lots of flip-flops could be used like the common '74 dual D flip-flop.  Pick the right logic family like AC and it will provide plenty of drive as well.

[h572]

I  suggested  power the chip with the 3.3V .used 74XX393 is reduce wiring?

[h572]

about the frequency.
in philips semiconductors datasheet ,power in 2.0V,25 degree the typ fmax is 30MHz.if in 3.3V power the fmax more then 30MHz.
what is "typ fmax" ,it means lots of this chip can reach parameter.
But strictly? power in 3.3V? the 74hc393 can not good work.
the best choose is 74vhc393 or 74lv393.

why did i choose 74hc393,because it is easy to buy and unexpensive.

[nixxon]

I  suggested  power the chip with the 3.3V .used 74XX393 is reduce wiring?

May I ask why you suggest connecting the 3.3V operating voltage also to the unused CLOCK input pin ("1CP") and the RESET ("1MR") pin?

[h572]

Connect  pin-1CP and pin-1MR to 3.3V is easy to connect.

[nixxon]

Connect  pin-1CP and pin-1MR to 3.3V is easy to connect.

Yes, it is easy to connect. But what are the benefits of connecting the unused half of the chip like suggested?

One more thing: How sure are you that the 20MHz signal is present in all (1pps-only) FE-5680B?

[David Hess]

what is "typ fmax" ,it means lots of this chip can reach parameter.

Typ fmax is the frequency that the parts will typical operate at but it is neither tested nor guaranteed.  Min fmax is the guaranteed minimum specification.  If you want higher than that, then you should grade them yourself for operation at the frequency of interest over temperature and voltage.

[jimon]

Today I've poked different test points & pads with oscilloscope (DS1054Z\hacked), and look what I've found !

• Point 1 that was mentioned in
  

  20MHz point
  

  Look how strange signal looks. Dunno why is that, I've poked other test points in that area - they are all digital or power.
  
• Point 2 is on other side of board, looks like DDS generator should be there, look how clean waveform is and how big V peak-to-peak. Poking around with this test point doesn't mess with PLL, on other hand if I poke around VCO that whole unit goes out from lock really fast (1-2 sec)

Any good ideas how to use this 60 MHz point ? I have AD9850 laying around.

PS. If I trigger on this 60 MHz and then scroll for couple of microseconds then the waveform start to look really jittery (on point 1 or 2), is it a issue with time domain oscillator in the scope ? I assume frequency standard is a standard for a reason  (Reposted this question here)

PS2. Is it safe to operate this unit without a case ? Rubidium 87 decays by beta emission, so maybe the case is there for a reason ? Also it's getting pretty hot without the case.

[Bryan]

Picked one up and it is testing out fine. I do have  question, does anyone know if the frequency can be tuned with a serial connection. Appears some of the 5680A models with option 02 can be tuned  with a serial connection. Once I have it outputting 10MHZ I would like to tweak it to match my GPSDO. The datasheet from the seller seems to suggest their is a serial connection on pins 14 and 15, but I can't seem to find a Max232 onboard so wondering if perhaps this model does not support this option.


 

[smgvbest]

This is not a topic I'm strong on so excuse a dumb question
is there any value in trying GPS Stabilize a Rubidium standard or is it a case diminishing returns?

[Bryan]

REMOVE SOME COMPENT

Does anyone know why the components need to be removed. Not sure what the purpose is if the 20Mhz point is there.

[andre.lubbock]

I've modified my FE-5680B to output 10MHz square wave. I've posted a couple of pics below but there's a full writeup here.

http://andrelubbock.blogspot.co.nz/2015/09/rubidium-frequency-standard.html

Essentially though I found a 20MHz which I brought out on the DB15 and divided it by 2 with a D-type flip flop. My standard also has 5V that I used to power the external circuitry.

[AnalysIR]

excellent write up - tnx for sharing