Pierce ACMOS Oscillator Stability

Why aren't you using 'HCU04 ??

"Linearizing" resistor ?? No. It's DC feedback for "Q-point" control.

...Jim Thompson

I don't think the 74HC04 has hysteresis. I think it has three gain stages in series, so when DC biased with a single resistor , has enough phase shift at the frequency where the gain is 1 to exhibit positive feedback. I would try splitting the bias resistor into two resistors and filtering the center node to ground, with a largish capacitor, to keep the high frequency through this path low. You might also have to divide the voltage into or out of the crystal with a capacitive divider, to lower the total loop gain at resonance (and at all other frequencies) to suppress the spurious oscillation.

See figure 4 of:

John,

Your are right about the multistage aspect, there are three inverting stages in each inverter circuit. The AC04 is not intended to exhibit hysteresis (as does the AC14) however I have experimentally demonstrated that static (DC) hysteresis on the order of 100mV does exist in the 3-5V supply range. This must be due to some weak internal positive feedback (possibly leakage). Originally I thought it might be due to low voltage operation in the subthreshold region of the N and P FETS but is does not diminish with increasing supply voltage. Probably some sort of substrate leakage effect. I'm going to try some temperature testing to establish a temperature effect. The unwanted oscillation I have expereinced is clearly not due to delay/phase shift combined with sufficient loop gain. It is a true relaxation oscillation that is supported by the hysteresis of the device.

Thank you for your thoughts...

Carefully observe the power supply. It's probably bouncing.

...Jim Thompson

Jim,

The HCU04 would be a good choice due to its single stage architecture however the final design is intended to be in a hi-rel space application and that will require a radiation hardened device like the JM38510R75701SDA (i.e. the radiation rardened version of the 54AC04). Unfortunately, there is no domestic source for radiation hardened HCU04's so the AC04 is all we have. The feedback resistor that establishes quasi-linear operation and allows for oscillator start-up has been refered to as a linearizing resistor by personnel in the oscillator business. It has two purposes, during startup it provides for quasilinear operation so low level linear feedback loop operation can occur (note that outside of the quasi linear region, the gain of the inverter as well as the composit loop gain is zero). Operation in the quasilinear region supports the growth of the intended oscillation. Once oscillation has saturated the output (i.e. the limiting aspect of the circuit), the resistor serves to provide approximate DC bias to the input of the inverter. Note that these are two distinct and different modes of circuit operation (startup and operating modes).

Thank you for your thoughts.....

Sounds like too much book-learning and not enough hands-on. Why is it you have the relaxation mode oscillation? Start-up my ass ;-)

...Jim Thompson

Please don't be offended but you are seriously blowing it out of your arse. It won't matter if your IC is RAD hard when it's in space and 'not' oscillating. Before you try and bullshit your way through a bunch of rubbish do yourself a favor and do a web search on crystal oscillators, in particular have a look at what your crystal manufacturer has to say about things.

You are using a digital IC in a linear manner and you are designing a feedback loop to be nominally unstable at or close to the crystals fundamental frequency. The only 'logic' IC's designed to work in this application are unbuffered inverters with a single P/N channel stage. This is because they will behave and can be designed to work in a 'predictable' as a gain stage.

The only reason you may have seen it attempted with 'standard' gates is because some asshole of a digital johnny didn't understand what U in the part number meant and just saw the symbol and saw he had a spare inverter knocking about the place so he used that.

Three months down the line all the product came back because the process varied and the oscillators started not to. What is the UPS charge for spacecraft recovery because you will be one seriously sick bunny if you try it and it does not work. While you're at it do a bit of reasearch into feedback loops and loop stability because, at the moment you are dribbling about like some sort of loony.

If you want to make it RAD hard tear a piece off your tinfoil hat and make a little house for it. I really dislike 'reasonable people'.

DNA

Hello Jim,

Yes. And hopefully there is a full ground plane. Else GND of the little AC04 could be bouncing as well.

Bwahahahaha ;-)

Genome/DNA, I've decided to host a SED reunion just so I can meet you ;-)

...Jim Thompson

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

Thanks, but I am sure you would be severely disappointed and I'd be picked up for vagrancy long before I got close.

In the meantime I'm looking closer at these things and they are not trivial. What got up my nose was the blah de blah.....

Anyway, closest I can get to some concept of Mr Rousty's 'hysteretic' problem is the two internal nodes (if there are three stages) are going ape s*it. The relatively lazy oscillation outside the IC biases them up and down and they end up demodulating the signal, as the operating point and whatever varies, to produce the DC offsets within the IC.

Something like that anyway. When you mention looking for the supply rails 'bouncing' I think you might find a sniff of current will show some really high frequency stuff going on.......

That's my story anyway.

DNA

A very likely scenario.

...Jim Thompson