J105

Mon Jan 25 02:11:25 CET 1999

>>>>> "IF" == Ian Fritz <ijfritz at earthlink.net> writes:

Hi!

 IF> Magnus Danielson wrote:
 >> I know for a fact that the LM311 is a real noise-makes when I turn the
 >> FET of in the ASM-1 VCO. I get all kinds of frequencies. I haven't
 >> cured it but it is time to do so, I am considering a 100 nF or so
 >> sitting across the LM311 body.

 IF> Comparitors are notorious for oscillating as they switch. Good layout
 IF> and bypassing are absolutely necessary. I had a lot of trouble with the
 IF> LM319 in my version of this circuit. The main problem turned out to be
 IF> too large a resistor on the + input. Anyway, you have to have the power
 IF> supplies bypassed with about .1muF right at the IC pins.

This is certainly true.

 >> So, in true EMC spirit there is several ways to attack the problem,
 >> one is to ensure that it doesn't pull that much current (keep the
 >> capacitance of the JFET down) or we could slow down the transient.

 IF> Isn't most of the comparitor current going through its output resistor?
 IF> I don't see how the small FET capacitance is an issue here.

It has to pull both the cap and the resistor, the faster the transient
is, the higher current value for the cap. If it is an issue, this is
how it interact with the cap. The quick pulling will upset the power
supply lines which will act pretty inductive, a power decoupling is a
good cure for that.

 >> Actually, if you consider the time margins we have we could reduce the
 >> slope of the LM311 quite a lot before it becomes a real burden on us,
 >> a Miller capacitor (a capacitor in the negative feedback over the
 >> LM311) is one possible way to acheive this. The benefit of reducing
 >> the slopespeed is that the current peak will be less and thus the
 >> source of the spike becomes less.

 IF> This might be interesting to try, but the comparitor ICs are optimized
 IF> for fast switching, so wouldn't that just be defeating the LM311's
 IF> purpose? If you want slower switching why not just use an op amp (like
 IF> some of the simple circuits you see around)?

Well, maybe. But then, part of the point with using the LM311 is the
output arrangement (it is hooked up as a open collector). Deliberately
slowing down the slope kills the problem much more at the source than
the power decoupling cap does.

 >> Just look at the slow slope (the red signal in the picture) as we
 >> start turning the JFET on, why not just let the turn off slope become
 >> something similar to cure away many problems?

 IF> I'm afraid I wasn't able to see your figure. It came through as garbage
 IF> text. 

I realized that after I sent it. Let's see if you can do anything with
it this time... (somebody got to fix that mail-tool!).

 >> So, to end this little adventure, I think there are several ways to
 >> cure the problem and changing the slope speed rather than keeping to a
 >> low capacitance is one things one can do. The falltime of the raw
 >> sawtooth is 737 ns so a slower turnoff time is reasonble.

 IF> I wonder if you aren't confusing a couple of different effects here. The
 IF> problem with the JFET capacitance is that it couples the switching
 IF> signal (at the gate) onto the timing cap (source and drain), so it puts
 IF> glitches on the sawtooth waveform. (See Terry's earlier post on this
 IF> effect.) The JFET capacitance has a *negligible* effect on the switching
 IF> speed. The switching speed is mainly limited -- as I see it -- by (1)
 IF> the the integrator opamp's slew rate, (2) the series resistance of the
 IF> JFET and (3) the JFET's saturated current limit. I like the idea of
 IF> trying the J105 because it improves (2) and (3).  But don't overlook (1)
 IF> either. The BB OPA132 I use makes a big improvement and an OPA602 would
 IF> probably be even better.

I may confuse things up, but then you really should see the scope
picture that I have in mind to understand some of what I am talking
about.

In this scope picture I have hooked up a probe onto the integrator
output, a probe at the LM311 collector output and a probe at the CV
summer output. When one moves in on the signal (to a 250 ns/div scale)
you can clearly see that when the LM311 falls you get a very noticable
click on the integrator output. Putting a J105 does not have anything
to do with this click, or at least not as a cure. I noted the strong
time correlance between the LM311 fall and the strong click on the
waveform and started to consider a number of cures:

1) Power decoupling

2) Slew-rate limiting

The benefit of the second is that you avoid getting a strong peaking.
While one might say that the capacitor effect of the FET is small onto
the LM311, the LM311 itself cause enought trouble on the powerline and
reducing the slewrate could be a path to try, this is what one does on
digital systems to keep them quiet (see "High-Speed Digital Design - A
Handbook of Black Magic" by Howard Johnson and Martin Graham").

 >> As this impulse moves along the -15V line it sneaks in to place we
 >> would not like it and there it can cause a form of syncing of the
 >> other oscillator.

 IF> You might try separate power lines for the LM311 (from the point where
 IF> power enters the board directly to the IC).

 IF> Good luck, and please continue to keep us informed on your experiments.

I will.

BTW, did the scope-picture (an PNG format thing) get there now?

Cheers,
Magnus

 IF>   Ian
-------------- next part --------------
?PNG

IHDR  <  ?   ??	   gAMA  ???a   8tEXtSoftware XV Version 3.10a  Rev: 12/29/94 (PNG patch 1.2)?.I    IDATx??????*?(P??y?W????,&???9F???+U??O???~' ???????\????N????Y?=????ej??????S?v4???m?T?*?????&?8tJ?? p??J?f3?????z ??????,+-e?O??>?b)]+??-???%?w???????T?ID?????????O??a??4????"kv?f^?1??????4??x	?o?6ve??
cG?uS!??????9;???g?e??%\?[????G?][Mk?HI???????K??? ?6?:S8)?1???*?SQ
?mK?`][)??v?O??b???2*??ZH8?T???????Z+?\?-h?6}??jj/?-q????jW???A'??vg?@?}Nj?6???f???j??G=??????Y?8@??_?\i	?-??
u????Zd7?????LZ??U8J?PCMk.`????V?dh??me??`=,e?6UZ?????+}J?8WeEi??q ?V:?t;????~??	???y : h0M: CM?a??FNu?@c????S??}???o n?)?ux<c<?????L??f	N??#P??L?aZ C?`?iu???]e?M?V?????#)?1?O?  ?? ???0-?x??@G??}?????%/p?)?D ??y?a??<?? ?8?_ug>?*?????i?I&?dR8?4???A ?!h0??z?????
??
?4SO9??? ?? ???0- ?q?8?e????#???,??g??cp1<h9-???[?Y?lN??y?aZ C?`? ?? ???0- ?ap1/gp1?3<????????????v????0- ?!h0A?aZ C?`?r?????\??\L?.n?y?aZ C?`? ?? ???0- ?ap1/gp1?3<????????????v????0- ?!h0?3;b????3e?~' ??0h?tg?O?h??U ???yP?M.????wZeH|*?N
X???6P??pp?#/???M2????;?#?x??l,??R???????u??!??????3???LQ??0?QK4????l?M?Z??wTa???????????sJ~??????????ju[????n?t????x??a?|???{?qu?9f/???gf?_iR%??npe???3M?w?_?kn?I[~?9??????Ev'n??[?U?s????n?K{rj(?????hjt???????~d<@??VX?j??7*]Q????-??h?e?????[?&?=???e????m?4?????$~??Kt?Ei??!-?7{?v?Y??R6???<~q??I??c?D7???7?)[F,'Vv?T>?????^????JS?????N?
Wc??J?IS??>??n???p?h??|?Y?F???RBZ????4?m????QH?X?TY???s???tj?xvC?G!?czJd???B?????F{2?Q?}F?Ea!????g#S?6??Qin?)?????=?-'C:?????y1?????F?N?)9?Jeh???D/????l???^???P*?J?0??G)???Qt??
?\=^??TB{zK?R?U?7?L??]?3?`JYJ??????n`?`8C??L3\??l??2??%???5???????9??????^????y^]n?+??4???6???+?71?k??XU??#?D?+???#R/??DJSW??<X?YjW??hY6=}?2[ V???|A??????('??%?h????,X/?W?WrO???R?V???J???\)?W??r?????h????g???Oj\?q?????????W??D???WrX?e)???????\?_.?/????3?a?r?E??;}?n??N??6]u?I]1i?G.?4H?\c?}\????
??,??(9????qsz???]u???j-WS?>oI???)?L=???#??V???b????Q???X????_.?{7w?nihM?T#'????4?B.:N???_|?j#g?????d$C???~????FU????????F?9J?w???;????ov??????????P?jVK?????.????????y?0&?????q:?
v???+:?T:
~???u?fi;???K???T? ?d?#??9[z?n??????????0??1fi?;qu?????5%q???zU?tx????9^?x??S??gN-/???9???+?F=/?B?L?!T??<??=Yo???:Oi????????????????iG??z?V????u???#|K?X??HKN?=??3?i?S?!??_?u?qc????,he?? ~??c?W_[s?(????/??+???8?-??n?X??????q?O?wy????zg5???0?????oY???)?????v???)??\?3?????H?0Z??????o?O=.?S\L?n?r??4f???'A?aZy*[ ?j?-??ZE???A?aZ ????9:??????.??.???<????Ak??s?NZ C?`? ???x5@- ?q????tW?{???K??'??8V??
?(????????? ?? ???0- ?!hm???S?m???? O??b^???????vo\_.??\?N?  ?? ???0??4=?O??C?w?GZ C?`?r????\??\L?.n?yp?}1 n&h0A?aZ ??? th??l??R????M??k?!'3a|j?J?U?[A??????`W?%?ea??h?????,??g??cp1??h??=c?????tl{
.??.??.?D??<?i<*??I????rO???=2?PS? 8?
/??????GU?:?|rx??????i??Wtn?? ^?1>Uf?4?4s}??&??????????}O??cp?i<??????o?????5?.P???? ??{????:A?E???wS:?=?u???*}F?4?D?3?@?XZ??a??O
Z ?(l?????
w?????j#???m?????@??m??o??3?x6??<???
?h????????[  -nxS????l
????? C?`???:y???N^e{JV?? 8?? ????o>??S?9??p???i??????3????r1?;???:<t?(?4teI?)???n?(??]??????b?w???+q+????I???D???B??}?U???U?2nvJ????$??z?@?n?-??????=4?k??B?~??<=hjZWY??k???????z???????-eZe?;???u?? ?U?uV_e[??=?M?J????  ?ep??<?z???t?4??
??y9???S?}H???????????gZ w???+X/?u?L?V/?R????k??E???.??????7^W?? sg???e???*?Sy
??1L????Z?,?C?d?3?????c`F?!h?M????b^??b????X?94??&?~/?
???V?d????K?>n?^?m???;M:b ??????c?H'?pY??l???*??O??L=??CG??x?
p???0- ?!h0A?a\??\L?t?jgp??1?????????#?<?0- ?Ql?L?g9iW9?B???A?3}?(0 xD?y??^??*?O?$?!???l?8?LK%????0,}?O??4M??+n??????2WzD??????Du^??.?????????r~%??M??>h?????5g(???????????d?I7O?.?????1??{>???????`y??w? ???{????m?0??Bb'oy(s???????
??
????Ic???e?S??#???c?s? ??? ?3??????t. ??S???[???5???\?????s??=?$?[?\i??>???b^?????m_.^??3??6?~??y?aZ C?`??????t??c??g??j?????(???~?@W????????????/?O{???+h??????@????????? ^n????o?,;???{???V\??\L???c???r??/??.??.??
..????Q? {i?(?tyx??
???a??8?????@\???W? ~Z?MsZ!?tuYzv??9???^???s??gZ??;UB?)?6%???G?  ??:?;@?????????
k;??{??????J.??.?????[ u? ?? ???0- ?!h?E_?
A?G?@????Q?j4??Z?W???|;??I:?x???????i`? ?? ???#}1 ?- ?!h?Ke+K5~???)??????N??? 8?
k??w2???l????F?T??i????a?mdC6*????<?0??ur?7.???2?V?D?#?N?v#??????+???????u?
?V??????7	Z?G????????|Z??;s???)??6?"?????\?N?? <???????M? ?????0	Z D????Q?~??5?????????????_?F`????Y/j
??????]Z;??^????????i`??x~??p??_???_??m|???O??	H-?? ?l?O???4??[%??jZ?G?;?X??????+uw:???jZt?W?aD??v??O?e?????_?F?5?oI
.??G-.?????FO?Snbp??????|?E??:?8}?u??<?/zY? s-??w????4o?/?{???????l???v?Y??n???Nl,??Y????Z;??_?h??????]?G??Gu??*p????0?s?U???L?
??.??F?2?{?U??~????2??=??5t?wz????;?E;??+=u????U?l??.&f???v?}??"?5,J?????c?gZ????s??5?????*|?w1b????6??.8tA3???? ?????<X-?J?`??6???O??|
?^c????\???%?F??w?!?????????Q?????B?|?!?a??r.?d????????%?z:-???????[p???S????Ak?B?s???E???]G???~?6?????p?4?_z???Mw??}wc??`?k?p??d#V?f\	ri:???????????????\????????6??JTe???<?4S?????~8/?+??_-7?zM.,??)G=J???????U?gri??i?z|??n?`eGe?e?????????9??????G
Ak?|???S ??????J=???W8??4}?4?s???nSy?>?Z?N?9?v?>????l?????O?j7?h?t?eESW????mb]6???I	???+Q-L-?"???4?]?????:???`??j?"f?y???5Q???$-???"{"}??^1???a?z?e??/iTH???K>?????*??K?~6??????h?{&=Q??U?g)???lJwW?T4??'?A???????????~?}??\?v^??????}?G\ 
?K{D??5??L??? ??????u'7??qE??Uq?????????;(K??K?\?Z?V?E?w???|???/??/Qj?FD????Kj??VQJ??S4????c?Z??ST
??j*??gG
?w????{py?(?C????~?]?[N?p???o\Kvjn????D??`?%rn??[?????"!n?_"?s?+W???,n?u?????????*u??k????g??k?????G?(B?U??"?R??C???i??_RX*4?y?=??F??T?????~?SRs
qKy?l?>??????[?????<m?
?iO93???Uhd????wv-a??u??+mB??r??GM@??qC??Y????????Z?"???????????H?7-??<?D???p?4?D??(mH}????(Bd??
5??.?'?Z?BH?Ki??n1?kW?}X??????\?ceS???Ww??D??
?n???^?#??r??O]????t???\k?9?ZL?1?E???wNVF>?????s???????6?-v4??????)5??P ???????{-?/?Wz=G??
??????????,??,j??_.??????6?v?s?5/?????L??A?}???}?Kmu?m?=S?
r?
=5??]????????????7 {u?Z???P~?y0G{i??????`p/?i???R|? F???????wy??b.(?1?nfxV??.?`??^H?.n7rG???U0??????O?
T ?`??}8X??
????? ??q?V??yx??;b?J????~?hAK5??
2??xa?2???iNh????P`p1?3??]????aS? ?i????-? L??o?Z??y??????_?? ????*?@Oo???
??njZ???????#0??????????s,?g3???\????A X?A????6- h?t?R????Q????)?H???????T`?GkZ????G? ??m?C??/?cu??^6?f??r???7d?7?1??o?D??VA v??-?n"????`?q7-, ?1N?aZ ????m?????? '????^%?s|?????????
???????????0.Z?_ p??:b??p?k????? 8???????_?C?8?jN?1]?LK??
r

H?A`pb?/?`p??0?`)??z??0?????Os=ntQG??????????w??????????a???????O?????1?;?#?RKj???'x??Sn?VR?N
wNKN6????.????CJ)??gC6???L? ??????}?H??8???N]7K:???????Z/?6?h??g?*??%??e?q%b-IE??r??c?E?c?.????????1Oe???_.?Z?h??1}??????ay??????e?D?~}?l?QN?MS?6?T:????M??V?N?F?e?p???D{)?X?lv??&d?
?gJ?Z?c?6%=??X?f/{4??U??Z?,* ???"*???K?K?{5+=??5[s?????r?9??e??Qn?D??r)?Fk/??????/???Ok~?.???U?c???>?r?
?????N?S??;N?N?6?x??M??H?V"?j}+?B?|l?^????S9?V?l???U?M??U?J?J??r?Z?^??=G??	5-F??@i)??5??JCKqs?<?z??Co?;j??3?u?JU??X?a??N)z5.NB??Wi??p??????[???^rd???E?y??? ?qDx?-_.?}?8?|???\???/?C|????rq;?????0- ?!h0A?aZ C?`?r?????\??\L?.n?y?aZ C?`? ?? ?A????I?!?!??5- ?!h0???[???;M?????k?E?y]?/&??-iV?G????l?7????74}?????w?Rgec5?e?t???e????&?V9???u??R?-`v?m'U?B??7?>[)??[u?9-???dx?3_??????qE??=??4?N??????}?D?]LGK?R????.ZueR??t?E??????5??}??i???x????????~G[Z????_pU???"V?????p?K??NY???j??Y??:?[?????N??????]?i?K?)??}z??x?????UU?: ?????????p,Q<?z?:?k)_Ve?????:ki???#?B??cSs??6??V????I?M~&hu{^?^c9?}?C??k??"[?5??1I6K;?~?k.???U?-
??E{?????1??Ct?n},R?{??'UZn?	Z?9?W??????O??_???N?????n)??;?????S?"??????=?~??r_U?.W?|?)???M?_<?<?n=?r?]??67?]?r??;?{????'7T???Ecw?#k?b?Y????ww09.??J?XA??y?#?;?_ws?????7|-?:?????W?v?&9QK?????_???_??_??z????6??>7G?tX??K????????(DV??????`]?An???????X??sJ?????$?N??m?4i
?N:???V?/?@??V???G?7?[?K?[??????U&e????c???l:???p???o????{_???+????????u???&2??????t???hR?P?2?s????;M?[?z#ge?M1?>sce??[/??ZX;???????????-???  ?IDAT?'>????e??Tu? ?????????fV???K??o?k?????|??J?K???4?e???j?`
?????????K??M??NZ?i{?*?J'??	 ????????????K??)=^J?3'??Z??<?P?o?j??pG??????d,?GNl8?sA??(???'??C???b? ??C5??P?[??VK?G?)???????- ?fp?????R58O??F??W_?T??qdq%?z?'?2he@???{D#?????j_}[??3????F????4??!??S??E.Z??g'???.U_???????V??? ?? o?^%?????|PF???/??~?`?1X}????=@?????n??/?????-?b????hx??W????0??~?N?_@???,5-???????Dn??m????o??wh`?*??????F   tIME?gC?   ?tEXtComment Image generated by Aladdin Ghostscript (device=ppmraw)
dC    IEND?B`?