Design question: Possible simplification (ans'd)

Thread Starter

metermannd

Joined Oct 25, 2020
189
Now that I'm out of the pickle I was in regarding the tank circuit, I noticed one other thing that I may be able to do with this board's design, and want to see if my thoughts are in the right place.

There are two supply voltages coming into the board (5V and one of the two 12V supplies), and it occurred to me that there are only two nodes on the 12V bus on this board. It appears that I may be able to make a couple slight changes to the circuit that would allow me to eliminate the 12V bus from this board, simplifying the overall setup.

I've attached the schematic of this panel and added dashed lines around the sections in question. (Existing / Proposed groups 1 and 2)

First, there is a 2.5V supply derived on-board for use with the comparators. Right now, that 2.5V supply is derived by way of a dropping resistor and zener diode. My question is, isn't a 'bare' zener diode like this liable to generate noise?

I'm seriously considering taking that out and replacing it with a pair of 10K resistors acting as a voltage divider off the 5V bus instead. I want to make sure I'm not missing out on some benefit of using a zener versus a simple voltage divider (I have a schematic of a related board that was designed a few years after this one and it has the 2.5V derived by voltage divider).

The second question has to do with a different voltage divider... again, what would the benefit be in tapping the 12V supply for this purpose?

I did the math and it appears that I can get approximately the same voltage and current with the indicated substitute resistor, fed from the 5V bus. Could it be that particular resistor value was not available in a 1/4W axial resistor package back in the early '80s and they took this route?

If my guesses are right, this would enable me to remove one of the two 12V supplies from the power board.
 

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wayneh

Joined Sep 9, 2010
17,152
I see no problem with your #2 proposal. A voltage reference (like TL431) would be another option.

The #1 proposal will only work if the current on the 2.5V bus is constant, and it's probably not. I'd keep looking.
 

AnalogKid

Joined Aug 1, 2013
9,248
Proposal #1 will not work s shown. 3) All electronic components have / produce noise; that's why we have bypass / filter / decoupling capacitors. 2) Depending on the exact components used, two resistors might generate more noise than one zener diode. 1) Your proposal changes drastically the source impedance of the 2.5 V. Before it was near zero ohms. After it is 5000 ohms. This means that for every 1 uA of change in the 2.5 V circuit load, the 2.5 V value will change by 5 mV. I suggest you stay with the zener and put a 0.1 uF ceramic cap across it.

Also, R39 can be adjusted to reduce the zener current; 29 mA is high. A safe starting point is to calculate the current limiting resistor based on the diode's data sheet test current; that is 20 mA for this diode.

It looks like the Mon1 input stage has a highpass filter of 4500 Hz and a lowpass filter of 1400 Hz. We don't know what the circuit is supposed to do, but that seems off. Also, it requires an opamp with a gain-bandwidth product of over 10 million - around 1 million for the circuit requirement, and an additional 20 dB to close the feedback loop with some negative feedback. These numbers are based on a max signal frequency of 4500 Hz.

The Mon2 input stage comparators (U1A, U1D) do not have any pullup resistors or circuits on their outputs. As these are open-collector outputs, this probably is an issue.

The signal through R25 is providing positive feedback to U1B and negative feedback to U1C. Why?
 

Thread Starter

metermannd

Joined Oct 25, 2020
189
My main goal is to remove the need for the 12V supply at this point.

I appreciate the feedback on the 2.5V supply, and agree that I should keep the zener AND add a 0.1uF cap. Changing the zener supply from 12V to 5V reduces the required voltage drop from 9.6V to 2.6V, and picking 18mA to stay below the maximum, I arrive at 140 ohms for the dropping resistor value, and it seems that a 1/4W package should be sufficient (the original 330 ohm resistor was 1/2W, given the >200mA draw it handled).

As for MON1 circuit, are you talking about U3B in the corner? The output is piped directly into a 6821 PIA on the logic board. MON2 likewise is an output from the same PIA.

You are correct to notice the missing pullup resistors and I'm not sure why. On the 'load terminal' style of this board, R5 (330K) is the pull-up for U1A, and R6 (100K) is the pull-up for U1D. I'm not sure why they are clipped out on the 'configuration terminal' style of the board (which is the design I'm poking and prodding at).

Regarding your confusion with the signal flow at R25 / U1B / U1C, I looked at the original board, and sure enough, I made a mistake transcribing the circuit. I got pins 10 and 11 swapped. Thank you for catching that. Corrected on both my master and project schematics.
 
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Thread Starter

metermannd

Joined Oct 25, 2020
189
From what I understand, the Sig_In strip at the top amplifies a very low-amplitude signal (12.5kHz at as low as 1.5mV p-p) to where it can be converted into a binary data stream at the output comparator, and that too is fed into the PIA previously mentioned.
 

AnalogKid

Joined Aug 1, 2013
9,248
The problem is that R54-C38 form a single-pole lowpass filter, with a corner frequency of 1.45 kHz. At 12.5 kHz the gain reduction is over 18 dB. That is, below 720 Hz the gain is over 46 dB (we'll leave C39 out of this for now) (and assume the signal source impedance is zero ohms), and at 12.5 kHz the gain is under 28 dB.

Separate from that - the D16 - D19 diode bridge is interesting. Is that supposed to be some kind of logarithmic compressor / clipper?

ak
 
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Thread Starter

metermannd

Joined Oct 25, 2020
189
I'm going to start a separate thread on the receiver section as I actually have schematics of four different receiver sections that all accomplish the same thing and want your thoughts on them. And all do feature that bridge in the signal path.

One last bit before I mark this thread 'solved'... what's your take on the MON1 stage? In its present configuration, do you think it accomplishes anything? The fully populated version is attached.
 

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RPLaJeunesse

Joined Jul 29, 2018
164
To keep your 2.5V source stable don't even think zener, but rather use a shunt regulator. The TL431 is a good choice here, but make sure the tempco you choose is adequate. Many other 3- and 2-terminal shunt regulators exist at 2.495 and 2.500 volts, you have lots of choices that might work. FWIW powering a 2.5V shunt regulator of any sort from 12V (compared to 5V) causes it to see a higher source impedance, (assuming the nominal current is kept the same). Higher source resistance against same shunt resistance gives better attenuation of the supply noise and ripple.
 

Thread Starter

metermannd

Joined Oct 25, 2020
189
To keep your 2.5V source stable don't even think zener, but rather use a shunt regulator....
It's interesting all the options available for such a purpose, especially when several different versions of the receiver circuitry shown in this thread I posted worked just fine with a simple voltage divider, all of which were in production for over a decade apiece, while the one with a zener was only in production for a few years.
 

RPLaJeunesse

Joined Jul 29, 2018
164
It is often all about signal to noise ratio, inband vs. out-of-band. Moderately stabilize the supply, use enough filtering on the divider, and the noise that remains is out of band - until the cap fails from age. But if you are digitizing the result down to DC, then even a slow shift is inband. Now you have to do more filtering in the digital realm to cancel out the DC bias there. Tradeoffs...
 

Thread Starter

metermannd

Joined Oct 25, 2020
189
In the end, I decided to go with a backplane arrangement for power distribution. As the 12V bus passes this board on the way to the display board, I decided it was just as well to restore the 12V arrangement for the 2.5V Zener and voltage reference sections.
 
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