OT: MC12080 prescaler chip weirdness


Michael Dunn
 

A Fluke counter I’ve been fixing up and adding homebuilt options to is missing the prescaler option. The vast majority of current chips seem to be binary, so, short of also dividing the reference clock, I decided to use a decade divider, an NOS MC12080 ÷10/20/40/80 chip.

Funny thing is, instead of dividing by 10, it’s dividing by 2! The chip internals consist of a fixed ÷5, followed by a programmable ÷2/4/8/16 stage. It’s like the ÷5 isn’t there, or is bypassed.

After some searching, I found a few other people bemoaning this exact same behaviour. So…at least I’m not crazy. Anyone here experienced something similar?

Gotta wonder what’s going on. Is there some sort of flaw in the chip design that leads to this failure mode? Was it a custom version? Hmm…

md


Tom Lee
 

That is a very fussy chip. The most common error is to drive the control signals with TTL level signals. That won't work reliably. They have to be very close to the rails. I don't remember the specs any longer, but IIRC you have to be within a diode drop of the positive supply, and the signals have to be clean. And, of course, the input signal has to be clean as well.

HTH.

Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 3/20/2021 10:46, Michael Dunn wrote:
A Fluke counter I’ve been fixing up and adding homebuilt options to is missing the prescaler option. The vast majority of current chips seem to be binary, so, short of also dividing the reference clock, I decided to use a decade divider, an NOS MC12080 ÷10/20/40/80 chip.

Funny thing is, instead of dividing by 10, it’s dividing by 2! The chip internals consist of a fixed ÷5, followed by a programmable ÷2/4/8/16 stage. It’s like the ÷5 isn’t there, or is bypassed.

After some searching, I found a few other people bemoaning this exact same behaviour. So…at least I’m not crazy. Anyone here experienced something similar?

Gotta wonder what’s going on. Is there some sort of flaw in the chip design that leads to this failure mode? Was it a custom version? Hmm…

md








Ed Breya
 

Is this a deal where you're just adding the 12080 to an existing board in the counter, or are you building the whole prescaler from scratch? Also, what frequencies and input levels are you using to test it with? The 12080 should work just fine if you apply it according to the datasheet, and have the right RF/HF environment around it. RF prescalers like this one must be AC coupled, and have a minimum cutoff frequency depending on the coupling cap size, and need sufficient amplitude and edge speeds to get proper operation. If the signal input isn't right, you can get strange results, and it can self-oscillate when the signal is not present or too small. Odd things can happen if you overdrive it too. Also, don't overlook the bypass caps - besides good supply bypassing, one is needed on the unused differential input.

Ed


Ed Breya
 

Like Tom said, don't drive the control signals with direct logic. Some prescaler types have control interface that's directly compatible with TTL/CMOS, but I don't think the 12080 is. It appears to be pure ECL, so you never pull any inputs solidly down to Vee - they can go to Vcc for high, or left open for low, presuming they have built-in pulldown Rs. If the modulus is to be fixed, just hard wire according to the instructions. For logic-controlled modulus, you can pull high at low impedance, but only pull low via a high resistance, like tens of k ohms. Built in type pulldowns in ECL are typically 50-100 k. For the 12080, it looks like it would be simple to use PNP Qs to pull high to Vcc, and just put them in cutoff to let the ECL inputs pull low.

Ed


tekscopegroup@...
 

Perhaps this item might be of use, or at least a good guide as there is a schematic posted as well. No relation to listing.

https://www.ebay.com/itm/153648250450


Michael Dunn
 
Edited

Thx for the replies, tho, believe it or not, I *can* read and follow a datasheet, usually... ;-}

https://www.ebay.com/itm/153648250450
Looks like a good board...

Admittedly, I'm not using a great layout...it's built on a little "Surfboard"(?) thingy. Vcc was kinda noisy at first, but that disappeared once I tacked a second decoupler as close to the pins as possible. Still divides by 2.

I'm heartened by one of the posts I read, where he'd tried a couple chips from Chinese sellers, which divided by 2, then one from a UK source, which worked. I've ordered a couple from Greece(!), so I'm sure they'll be good...???


Michael Dunn
 

Maybe I'll try pre-baking the next one also. I've read claims that it's unnecessary for hand soldering, but...


Michael Dunn
 

Follow-up on the wonky prescaler chip: Sure enough, the Euro-sourced MC12080 works fine...divides by 10 as nature intended. Why the China-sourced part divided by 2 is a mystery that may never be solved.

p.s., I'm also gonna add a switch to one of the control pins, so I can selectively divide by 20, taking the frequency range to 1.6 GHz, tho of course necessitating doubling the reading...


Harvey White
 

If you can find the counting gate, if you can multiply that by 2, then your readings can double.  You just have to be careful that this is the gate going to the display only.

Harvey

On 4/14/2021 6:19 PM, Michael Dunn wrote:
Follow-up on the wonky prescaler chip: Sure enough, the Euro-sourced MC12080 works fine...divides by 10 as nature intended. Why the China-sourced part divided by 2 is a mystery that may never be solved.

p.s., I'm also gonna add a switch to one of the control pins, so I can selectively divide by 20, taking the frequency range to 1.6 GHz, tho of course necessitating doubling the reading...






Tom Lee
 

Glad to hear that you kept at it and found that the problem really was with the chip. It’s a fairly widespread problem, sadly. We bought a large batch of DSO138 scope kits for our students, and they all showed significant crossover distortion. Turned out that the TL084 op-amps were in fact relabeled LM324s. It’s amazing to me that the economics are such that saving a few cents per unit is sufficient incentive for this fraud. I’m sure that the margins on that prescaler were a lot bigger!

—Cheers,
Tom

Sent from my iThing, so please forgive typos and brevity.

On Apr 14, 2021, at 3:20 PM, Michael Dunn <md@cantares.on.ca> wrote:

Follow-up on the wonky prescaler chip: Sure enough, the Euro-sourced MC12080 works fine...divides by 10 as nature intended. Why the China-sourced part divided by 2 is a mystery that may never be solved.

p.s., I'm also gonna add a switch to one of the control pins, so I can selectively divide by 20, taking the frequency range to 1.6 GHz, tho of course necessitating doubling the reading...






Michael Dunn
 

Why the China-sourced part divided by 2 is a mystery that may never be solved.
Hmm, I may have solved the mystery after all!?!

If you're so inclined, have a look at the MC12093 datasheet...

If that's it, the remaining question is: Accidentally mislabelled at the factory, or mislabelled by an unscrupulous reseller. Given the parts look pristine, and came in plastic cut tape, I'd guess the former. I'm not saying it couldn't be counterfeiting, but that would make for a pretty unlikely and strange story...