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" precision " ballasting resistors.
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Zero999:

--- Quote from: mrpackethead on December 31, 2018, 11:08:29 pm ---
--- Quote from: The Soulman on December 31, 2018, 08:52:33 pm ---Did you take in account the temperature stability of the resistors?
I'd use large resistors (normal tolerance) for minimum temperature rise and find a way of trimming after assembly.
Possibly with trimmer pot ( +couple resistors to define the range) or parallel mounted high value resistors chosen
strategically so that one or more can be clipped away after assembly and measuring.
Either way measuring and manual labor after assembly is mandatory.

Poor design?  :P

--- End quote ---

Multiple resistors is not really an option.   Each side of the pair will be part of a differential signal path that is carrying a signal clocking at 125Mhz.   I'd hate to think what having 10's of resistors in parrallel/series might do to that signal path.      Provided the resistors in the pair behave in a similar way, having them shift in value a bit over temp is not super critical.

The idea of using PCB traces was innovative. :-) though not practical.

--- End quote ---
Parallel will increase the parasitic capacitance, but reduce the inductance. Series will increase the inductance, but reduce the capacitance.

Series and parallel should theoretically keep the parasitic inductance and capacitance the same as one resistor, but in reality it won't work out because the circuit will be physically larger it will have more L and C.

125MHz is not that difficult, as long as you don't use wire-wound resistors, which will be highly inductive.
mrpackethead:
watching daves videos was interesting;

Lets assume that; ( all to be prooved, but based on Daves info )

(a)  The distribution of resistors follows a normal distribution.
(b)  Resistors of -/+1% tolerance   have a standard deviation of 0.125 

The mean value is 1.5R

What i'd like to know is the probability that two given resistors are more than 0.2% apart in value.   my statistics is too rusty to do this.   







spec:

--- Quote from: mrpackethead on December 31, 2018, 11:54:42 am ---Im considering a design idea at the moment, which requires a pair of closely matched resistors.   The pair could fall in the range 1.45 - 1.55 Ohms, will need to be rated at 0.5W,  Ideally there will be less then 0.1% difference in them.

I've had a hunt for such resistors and while they do exisit on a datasheet they have very long lead times, and are basically product unobtainable.

If i bought a reel of 1% parts, would it be insanity to consider measuring them, sorting them into 'range' bins, and then getting closer pairs?      They would still need to be placed on the pcb as pairs, and that is quite a lot of work, since they wont' be on a tape any longer ( though they coudl be loaded into custom trays and picked up, its still quite a lot of work.   And yes, i potentially have to place thousands and thousands of them.

--- End quote ---
Hi mrpackethead

In view of the high frequency, and quantities involved, yes it would be madness. Just buy the correct components. :)
mvs:

--- Quote from: mrpackethead on January 01, 2019, 10:06:50 am ---watching daves videos was interesting;

Lets assume that; ( all to be prooved, but based on Daves info )

(a)  The distribution of resistors follows a normal distribution.
(b)  Resistors of -/+1% tolerance   have a standard deviation of 0.125 

The mean value is 1.5R

What i'd like to know is the probability that two given resistors are more than 0.2% apart in value.   my statistics is too rusty to do this.

--- End quote ---
I have played a bit with Dave's data (400 1% resistors) in excel. Only around 63% possible resistor pairs are within +/- 0.2% tolerance. This might be acceptable if only one resistor pair is used per board, but not in case of 48 pairs.
You need 0.5^(1/48)=98,6% pairs in spec to produce half of the boards w/o rework.
mrpackethead:

--- Quote from: mvs on January 01, 2019, 06:11:21 pm ---
--- Quote from: mrpackethead on January 01, 2019, 10:06:50 am ---watching daves videos was interesting;

Lets assume that; ( all to be prooved, but based on Daves info )

(a)  The distribution of resistors follows a normal distribution.
(b)  Resistors of -/+1% tolerance   have a standard deviation of 0.125 

The mean value is 1.5R

What i'd like to know is the probability that two given resistors are more than 0.2% apart in value.   my statistics is too rusty to do this.

--- End quote ---
I have played a bit with Dave's data (400 1% resistors) in excel. Only around 63% possible resistor pairs are within +/- 0.2% tolerance. This might be acceptable if only one resistor pair is used per board, but not in case of 48 pairs.
You need 0.5^(1/48)=98,6% pairs in spec to produce half of the boards w/o rework.

--- End quote ---

I only need each pair ( not all the 48 pairs ) to be matched.    ( sorry if that was not clear ).   

I wrote a bit of python to programatically work out the statistical likelyhood of picking two resistors at random that were closely enough matched.   on the reel of 1% parts, there is about 55% chance that the two parts would be within a match.  Thats not really good enough, as nearly half of the random pairs would not match. 

Given that obtaining the .1% parts wont' really be possible, sorting them is the last option.  Grrh.

Parts will arrive on a 8mm reel.    I'm thinking i'll use a PNP machine to 'unload' the reel onto a custom made tray, that has cut outs the size of the resistor..  It should take about 10 minutes to unload it into the tray.
I'll make a little test tool, that has a pair of pogo pins, and with the help of a couple of index pins in the tray will line up nicely. 

In stead of connecting to a multimeter, i'll make a small test circuit, that will simplfy it down, and will just beep two tones,  indicating if its in the most common central bins, or if its outside. If its outside, we will just use a small vacum pick up tool to discard them.    Probably will end up with keeping about 60% of the parts, and chucking the rest.    Given the very low cost of the the parts, compared to the labour.

The tray can then go back on teh pnp machine.. I wont' need to tell it when there is missing parts in teh tray, as it will figure that out when it can't pick them up.    and will retry in the next spot.





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