Given some pricy advice to make progress debugging the 1130 MRAM memory infrequent bit failures

1130 MRAM BOARD HAS BITS DROP ONCE IN MANY THOUSAND READS

The failure is rare. When I loop through memory, there are almost 278,000 reads done every second. With just 8K of memory implemented, this means the system has looped through every location almost 34 times in a single second. 

The symptom is a drop of bit 14, when it should have been set, at varying addresses in memory. It only happens after thousands of successful reads of the same word. When I put my Rigol oscilloscope probe on the pin where the pulse arrives from my board into the 1130 memory register, the error goes away. Instead, bit 10 will now fail at a even less frequent rate. Putting a second channel's probe on that bit leads to almost flawless operation. 

THE QUANTUM MECHANICAL NATURE OF THIS FAILURE

Therefore, when I put on the scope probe or a logic analyzer probe, the failure does not happen. It only happens if I am not observing. That means I can't see how it is going wrong, because I can't get the failure to occur while I am measuring.

ATTEMPTED MANY TIMES TO CREATE AN EQUIVALENT LOAD TO THE PROBE

I can't run the machine with my Rigol scope permanently attached. Further, it may be that after 20 minutes or five hours, another bit will fail to be set. Since the presence of the probe seems to cure the situation that causes the failure, I hoped I could develop an equivalent set of components that I could attach to the backplane pin in lieu of my scope probe. 

However, this is a very messy and complicated situation. Many inductances, capacitances and resistances are involved in the scope probe through to the input amplifier. I looked up the probe and the Rigol scope schematics, but those only show the discrete components. They don't show the wire and connection capacitance, inductance and resistance, but those values are also involved if you want a correct model to analyze the resultant equivalent circuit. 

ADVICE ON HOW TO BYPASS THE QUANTUM MEASUREMENT EFFECT

My friend CuriousMarc suggested that if I used an active FET probe, which has much lower loading than the passive probes, I can probably observe the failure. If I can see what is going wrong, I have a better chance of fixing it. 

I immediately went online to look for active FET probes for my Rigol DS1054Z scope. What I saw set me aback. Rigol is a very cost efficient brand of scopes, but the active FET probe choices ran from about $1,500 to well over $4,000 depending on bandwidth. 

Since I don't regularly have a need for that kind of probe, I would be spending that money just to measure this situation - a one time process. I can't even be sure that what I observe would definitively lead me to a solution, thus it is a gamble on top of a serious investment. 

LOOKING INTO DO IT YOURSELF ACTIVE FET PROBES

At this point, I am willing to consider building my own active FET probe. I did find a few projects done by competent engineers. This gets into serious RF magic territory, well beyond my usual engineering experiences. I picked one and am seriously considering the effort.

To control the characteristics of the PCB for this probe, I would have to shift from ordinary PCB material (FR4) to Rogers material, which would push the price for the bare board up to around $150. The design I found is centered on a MOSFET amplifier used in UHF analog television circuits - the BF998 dual gate enhanced mode N channel FET. This has dual gates and characteristics like extraordinarily low gate capacitance.

Of course, with the advent of digital television, the market for those devices has dried up. The chip is obsolete and not stocked by any of the reputable distributors. Of course there are many offers for the device on Amazon and eBay, all from operators in China, but the risks that I would be sold a lesser quality FET that was relabeled are just too high. If the gate capacitance and other specs aren't what the design expects, it isn't going to work correctly. 

I did find one location that has stock of the FET, is not in China and seems to have a good reputation. Assuming they didn't just by the stock from the scammers, but instead had a legitimate supply that came from NXP through a distributor, then this solves the availability problem.

I found an Instructable from someone who built the probe successfully.  They characterized it as less than $20 but that assumes free PCB construction and a substantial electronics lab to measure and adjust the probe. The values they determined for filters to make this work linearly are only for the board they built. Because - RF magic. 

Minute variations in the PCB, the components and even in my soldering will mean that when I build it, I may need to do the measurement and tuning process myself. Even the presence of a bit of solder on the traces beyond what is strictly needed to mount a component adds capacitance and inductance. Also, too little solder turns the joint into an unwanted capacitor. 

It isn't the most practical Instructable. There are only simplified schematic fragments. The gerber files show many components, about half of whose values are NOT specified. Instead of showing a full schematic, the person uploaded an equivalent analytic model in NI AWR Microwave software format - thus having elements that stand in for straight and right angle traces, as well as actual components. 

As one caveat, they mention that "you may need a membership in a laboratory, or acquire some people's affection who do (i.e. via cold beverages). Also, the 20$ price tag suggests you only need to buy the special components, not the standard things which lie around in an RF-lab anyway." They also assume I will be milling my own PCB from 512µm Rogers RO4003 w. 17µm copper dual-sided board material. 

I was able to take their gerber files and get them into KiCad so that I could produce an acceptable set of gerbers and drill files to send to a professional PCB fab house. I am working hard to identify the components they installed at all the open gaps on the PCB, so that I can purchase them. If I am successful in developing a complete bill of materials and can find the parts, I will then order the PCB and commit to the project. 

The component values which are totally custom are the ones needed to form a filter to compensate for a resonance around 800 MHz. He started with components such as 1.8 pf capacitors and 22 nH inductors, but suggests that I have a well stocked supply of different values to try. This might involve some iteration ordering parts from a supplier and stretch out the completion. 

The current snag in finding parts is the SMA connector he uses - it is a straight SMA female with a solder-on pin, but has a mounting flange with two holes. The flange is at right angles to the connector, but the inner conductor comes out straight. All I have found so far that have the flange at a right angle bring the inner conductor out at the right angle, which isn't going to work. If anyone recognizes this and knows where I can find it or at least how to describe it when searching, let me know. 

This probe produces a 50 ohm output impedance. My Rigol DS1054Z oscilloscope has a 1M input impedance. The solution is to add a 50 ohm terminator in a Tee connector at the input of the scope. I will have an SMA to BNC cable hooked to the tee where it is terminated, the other side of the tee connected to the scope input.