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SN74HCU04: TUNING FORK OSCILLATOR UNSTABLE

KEVIN BARRETT
Prodigy 40 points
Part Number: SN74HCU04

Hi,

I use the SN74hcu04  in a basic 60khz tuning fork oscillator circuit. I have sold this product for decades without issue. Suddenly, this past year, I have encountered over 25% of production where the oscillator is unstable. Through substitution, I have determined that it is the device that is at fault. Increasing the circuit input capacitor from 22pF to 27 pF restores oscillator stability.  Has the Mfg process changed in some way that would change the device input capacitance or gain?

I am about to run a new production order, and need to have some confidence that changing the capacitor to 27pF is going to solve this problem.

the circuit runs on a regulated 5V line powered by a 9V battery.

thanks for any help or guidance  you can offer.

Kevin

KAB Electro-Acoustics

Website http://www.kabusa.com

Phone (908) 754-1479

Fax (908) 222-3442

  • 0
    TI__Guru* 96862 points

    Hi Kevin,

    Which package of the SN74HCU04 are you using?

  • 0 in reply to Emrys Maier
    Prodigy 40 points

    DIP 14 pin through hole design. I believe the issue is related to the input capacitance. This is known to increase when the inputs are operated linearly, ie VDD/2. I suspect some of these parts are running lower Ci than normal. But I was hoping to find out if there were any changes in processing that may have caused this to happen.

  • 0 in reply to KEVIN BARRETT
    TI__Guru* 96862 points

    Hey Kevin,

    It looks like the lead frames changed a couple of years ago to be the new "single sided roughened" version - this could account for a minor shift in parasitic capacitance at the inputs. It's also normal to see some minor process variations that could impact this type of thing, especially over a long time period. I don't see any changes to the process though -- not surprising considering the age of the part.

    Currently there are two manufacturing sites that produce this device, so you will likely see slight variations between the two. You should see the country of origin printed on the shipping label and they will be different.

    Depending on your volume requirements, I would recommend testing out each set of parts and choose the right capacitor before going to full assembly with those.

  • 0 in reply to Emrys Maier
    Prodigy 40 points

    I think sampling will be a difficult ask for the people who do my assembly. What I will do is increase the capacitance to 27pF. I did use that value back in the 90's but changed to 22pF after doing a stability study. But that was with the  M74HCU04 by ST Microcircuits.  I will also mention that if I remove the chip, install a socket and re install the chip in the socket, the circuit works. So it would seem that the capacitance is right on the edge. I worked in CMOS with RCA Somerville NJ in the 80's when these parts were being developed. Thanks for your help.