[time-nuts] PDIP package 100 MHz decade dividers

[Bob kb8tq]

Hi

If you want to get the 74AC74 (or better the 74LVC74) running it’s best:

1) Power it off of 5.50V

2) Transform the drive signal so that it presents 5.5 to 5.8V p-p to the
gate input.

3) Bias the AC signal so that it swings from about 0 to 5.5V

Bob

> On Jul 19, 2020, at 8:18 AM, Magnus Danielson <magnus at rubidium.se> wrote:
> 
> Hi Rick,
> 
> On 2020-07-19 04:30, Richard (Rick) Karlquist wrote:
>> 
>> Yes, the simple pair of resistors doesn't work well on TTL.
>> That was probably done by a production engineer, who was
>> working above his pay grade.  We can't all be Len Cutler.
> Sounds very reasonable. It's a crude hack and works OKish, but to fix it
> not much would be needed to be done.
>> 
>> OTOH, that circuit works well with 74ACXX circuits.  We did that
>> at 80 MHz in the 5071A and it worked well.
> 
> OK, that can be useful to know. I should probably do some tests then.
> Maybe replacing the 7474 with a 74AC74 will make it perform better. It's
> simple and straight-forward enough, but I would like to measure and
> compare to know. Maybe someone already done that. The problem is to
> lower the added noise enough, and slew-rate limited signals into a
> trigger point is a know source, then the gate itself can contribute
> naturally.
> 
> Anyway, thanks for that little insight.
> 
>> 
>> Back in the 70's when I worked for Zeta Labs, my boss taught
>> me to use a 1 transistor buffer that had a pull up resistor
>> on the collector and a resistor from the collector to the
>> base and a resistor from the base to ground.  We used it
>> in all our designs and it worked well, despite being
>> ridiculously simple.  My boss was really smart.
>> 
>> That is what they should have used in the 5065.
> 
> Sure. It's not very hard to do. The 5065A synthesizer input have the
> same challenge, so it has higher noise because of that, but if one has a
> 00105 oscillator that will cover that up anyway. The 5065A synthesizer
> isn't very quite for sure, and just using a 3325B is replacement I
> dropped the ADEV floor by over a factor of 2 on a 00105 based 5065A,
> because the 3325B lock-up filtered much of the 00105 noise out and did
> not have as terrible sidebands and input treatment. That said, the 3325B
> isn't particularly "clean" but sufficiently clean to achieve that
> improvement. Sometimes "sufficently" is the key word, and applying it
> one can get cheap fixes that moves things out of the critical path for
> performance. Sometimes all it takes is a 2N3904 or two with a few
> resistors and caps. Figuring out where the key performance limiters are
> and address those sufficiently well may achieve most of the gain at times.
> 
> Cheers,
> Magnus
> 
> 
> 
> 
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