Re: [PEDA] 50 ohm differential impedance connections and traces
Variations in the length of the twisted pair can easily be kept less than 1/4", about 30 ps. If this range causes concern, you probably want to use a rigid connection. Richard At 07:18 PM 12/6/2001 Thursday -0500, you wrote: >I agree. My only concern may be how you attach the twisted pair from PCB >to PCB. >Will there be a consistency issue from each unit manufactured to the next? > > >Brian Guralnick > > >- Original Message - >From: "Richard Sumner" <[EMAIL PROTECTED]> >To: "Protel EDA Forum" <[EMAIL PROTECTED]> >Sent: Thursday, December 06, 2001 3:58 PM >Subject: Re: [PEDA] 50 ohm differential impedance connections and traces > > >| Scott, >| >| Maintaining signal integrity is a matter of degree. Any discontinuity in >| impedance will produce a reflections and changes in the received >| signal. But, if you are using ordinary pecl, 1-2 ns rise times, what you >| want to do is easy. With high speed pecl, 200-300 ps rise times, you >| will have to be more careful, but extreme measures are usually not >| required. Discontinuities in the controlled impedance signal path that are >| short compared to the signal rise time will have very small effects on the >| signal. >| >| First, keep the pull down resistors on the pecl output close to the driving >| chip. Do not place a Thevenin equivalent termination on the other board >| (you don't want dc power supply currents between the two boards). The best >| is a simple parallel termination (no connection to ground) at the receiving >| end. >| >| If you can't start the twisted pair at the signal source (close to the >| chip) then run two 50 ohm controlled impedance lines to where the twisted >| pair will start. For ordinary fr4, a line (on an outside layer) about twice >| as wide as the layer spacing (outside layer to next layer down, which >| should be either a ground (best) or power plane, not a wiring layer) will >| give you about 50 ohms. The two lines spaced 3-4 line widths apart >| (uniform spacing) will give you an approximately 100 ohm differential line. >| Connect to the next board with a short piece of twisted pair. Typical >| twisted pair has an impedance about 100-120 ohms. One inch is only about >| 130 ps delay. >| >| There must be a ground connection between the boards, but this does not >| need to be extremely close to the twisted pair signal path. You can also >| use two pieces of 50 ohm coax, which will connect the grounds. Unless you >| are very sensitive to emi, twisted pair will be fine. >| >| Continue on the next board with controlled impedance if necessary. >| Terminate with a 100 ohm resistor between the 2 lines of the differential >| pair. This should be very close (or a little beyond) the receiver. Do not >| terminate with resistors to ground or to a power supply. >| >| If you can launch the twisted pair close to the driver, and land close to >| the receiver, you can dispense with the controlled impedance lines on the >| pcb, and use a longer piece of twisted pair. This will improve the received >| signal, by eliminating the discontinuities in the total path (where the >| path changes between pcb and twisted pair). You can transmit differential >| pecl (or ecl) over several feet of twisted pair with very little signal >| degradation. >| >| I hope this is clear! (and helpful) >| >| Richard >| >| >| >A couple of questions. >| > >| >Does anyone have a good way of doing board to board connection (only >| >about an inch) for a differential 50 ohm connection? >| > >| >Can I make two parallel connections with 100 ohm twisted pair? >| > >| >We have done some calcs for a FR-4, 1 oz copper, double sided, 1.6mm >| >thick, and we get traces 140thou, 8thou spacing over a gound plane for a >| >50 ohm differential impedance. Can anyone confirm our result? 140 thou is >| >getting up there! >| > >| >The reason for the questions is that we need to take a differential PECL >| >signal from one board to another and maintain the signal integrity. >| > >| >Scott Ellis >| >Manager >| >Novatex Research - Excellence in Electronic Research & Development >| >[EMAIL PROTECTED] >| >41 Yule Road, Merewether, Newcastle, NSW 2291, Australia >| >Ph 0412 988408 Fax 02 49636058 >| > >| >| Cheesecote Mountain CAMAC >| 24 Halley Drive; Pomona, NY 10970 >| 845 364 0211, www.cmcamac.com >| >| Cheesecote Mountain CAMAC 24 Halley Drive; Pomona, NY 10970 845 364 0211, www.cmcamac.com * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * To post a message: mailto:[EMAIL PROTECTED] * * To leave this list visit: * http://www.techservinc.com/protelusers/leave.html * * Contact the list manager: * mailto:[EMAIL PROTECTED] * * Forum Guidelines Rules: * http://www.techservinc.com/protelusers/forumrules.html * * Browse or Search previous postings: * http://www.mail-archive.com/proteledaforum@techservinc.com * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
Re: [PEDA] 50 ohm differential impedance connections and traces
I agree. My only concern may be how you attach the twisted pair from PCB to PCB. Will there be a consistency issue from each unit manufactured to the next? Brian Guralnick - Original Message - From: "Richard Sumner" <[EMAIL PROTECTED]> To: "Protel EDA Forum" <[EMAIL PROTECTED]> Sent: Thursday, December 06, 2001 3:58 PM Subject: Re: [PEDA] 50 ohm differential impedance connections and traces | Scott, | | Maintaining signal integrity is a matter of degree. Any discontinuity in | impedance will produce a reflections and changes in the received | signal. But, if you are using ordinary pecl, 1-2 ns rise times, what you | want to do is easy. With high speed pecl, 200-300 ps rise times, you | will have to be more careful, but extreme measures are usually not | required. Discontinuities in the controlled impedance signal path that are | short compared to the signal rise time will have very small effects on the | signal. | | First, keep the pull down resistors on the pecl output close to the driving | chip. Do not place a Thevenin equivalent termination on the other board | (you don't want dc power supply currents between the two boards). The best | is a simple parallel termination (no connection to ground) at the receiving | end. | | If you can't start the twisted pair at the signal source (close to the | chip) then run two 50 ohm controlled impedance lines to where the twisted | pair will start. For ordinary fr4, a line (on an outside layer) about twice | as wide as the layer spacing (outside layer to next layer down, which | should be either a ground (best) or power plane, not a wiring layer) will | give you about 50 ohms. The two lines spaced 3-4 line widths apart | (uniform spacing) will give you an approximately 100 ohm differential line. | Connect to the next board with a short piece of twisted pair. Typical | twisted pair has an impedance about 100-120 ohms. One inch is only about | 130 ps delay. | | There must be a ground connection between the boards, but this does not | need to be extremely close to the twisted pair signal path. You can also | use two pieces of 50 ohm coax, which will connect the grounds. Unless you | are very sensitive to emi, twisted pair will be fine. | | Continue on the next board with controlled impedance if necessary. | Terminate with a 100 ohm resistor between the 2 lines of the differential | pair. This should be very close (or a little beyond) the receiver. Do not | terminate with resistors to ground or to a power supply. | | If you can launch the twisted pair close to the driver, and land close to | the receiver, you can dispense with the controlled impedance lines on the | pcb, and use a longer piece of twisted pair. This will improve the received | signal, by eliminating the discontinuities in the total path (where the | path changes between pcb and twisted pair). You can transmit differential | pecl (or ecl) over several feet of twisted pair with very little signal | degradation. | | I hope this is clear! (and helpful) | | Richard | | | >A couple of questions. | > | >Does anyone have a good way of doing board to board connection (only | >about an inch) for a differential 50 ohm connection? | > | >Can I make two parallel connections with 100 ohm twisted pair? | > | >We have done some calcs for a FR-4, 1 oz copper, double sided, 1.6mm | >thick, and we get traces 140thou, 8thou spacing over a gound plane for a | >50 ohm differential impedance. Can anyone confirm our result? 140 thou is | >getting up there! | > | >The reason for the questions is that we need to take a differential PECL | >signal from one board to another and maintain the signal integrity. | > | >Scott Ellis | >Manager | >Novatex Research - Excellence in Electronic Research & Development | >[EMAIL PROTECTED] | >41 Yule Road, Merewether, Newcastle, NSW 2291, Australia | >Ph 0412 988408 Fax 02 49636058 | > | | Cheesecote Mountain CAMAC | 24 Halley Drive; Pomona, NY 10970 | 845 364 0211, www.cmcamac.com | | * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * To post a message: mailto:[EMAIL PROTECTED] * * To leave this list visit: * http://www.techservinc.com/protelusers/leave.html * * Contact the list manager: * mailto:[EMAIL PROTECTED] * * Forum Guidelines Rules: * http://www.techservinc.com/protelusers/forumrules.html * * Browse or Search previous postings: * http://www.mail-archive.com/proteledaforum@techservinc.com * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
Re: [PEDA] 50 ohm differential impedance connections and traces
Scott, Maintaining signal integrity is a matter of degree. Any discontinuity in impedance will produce a reflections and changes in the received signal. But, if you are using ordinary pecl, 1-2 ns rise times, what you want to do is easy. With high speed pecl, 200-300 ps rise times, you will have to be more careful, but extreme measures are usually not required. Discontinuities in the controlled impedance signal path that are short compared to the signal rise time will have very small effects on the signal. First, keep the pull down resistors on the pecl output close to the driving chip. Do not place a Thevenin equivalent termination on the other board (you don't want dc power supply currents between the two boards). The best is a simple parallel termination (no connection to ground) at the receiving end. If you can't start the twisted pair at the signal source (close to the chip) then run two 50 ohm controlled impedance lines to where the twisted pair will start. For ordinary fr4, a line (on an outside layer) about twice as wide as the layer spacing (outside layer to next layer down, which should be either a ground (best) or power plane, not a wiring layer) will give you about 50 ohms. The two lines spaced 3-4 line widths apart (uniform spacing) will give you an approximately 100 ohm differential line. Connect to the next board with a short piece of twisted pair. Typical twisted pair has an impedance about 100-120 ohms. One inch is only about 130 ps delay. There must be a ground connection between the boards, but this does not need to be extremely close to the twisted pair signal path. You can also use two pieces of 50 ohm coax, which will connect the grounds. Unless you are very sensitive to emi, twisted pair will be fine. Continue on the next board with controlled impedance if necessary. Terminate with a 100 ohm resistor between the 2 lines of the differential pair. This should be very close (or a little beyond) the receiver. Do not terminate with resistors to ground or to a power supply. If you can launch the twisted pair close to the driver, and land close to the receiver, you can dispense with the controlled impedance lines on the pcb, and use a longer piece of twisted pair. This will improve the received signal, by eliminating the discontinuities in the total path (where the path changes between pcb and twisted pair). You can transmit differential pecl (or ecl) over several feet of twisted pair with very little signal degradation. I hope this is clear! (and helpful) Richard >A couple of questions. > >Does anyone have a good way of doing board to board connection (only >about an inch) for a differential 50 ohm connection? > >Can I make two parallel connections with 100 ohm twisted pair? > >We have done some calcs for a FR-4, 1 oz copper, double sided, 1.6mm >thick, and we get traces 140thou, 8thou spacing over a gound plane for a >50 ohm differential impedance. Can anyone confirm our result? 140 thou is >getting up there! > >The reason for the questions is that we need to take a differential PECL >signal from one board to another and maintain the signal integrity. > >Scott Ellis >Manager >Novatex Research - Excellence in Electronic Research & Development >[EMAIL PROTECTED] >41 Yule Road, Merewether, Newcastle, NSW 2291, Australia >Ph 0412 988408 Fax 02 49636058 > Cheesecote Mountain CAMAC 24 Halley Drive; Pomona, NY 10970 845 364 0211, www.cmcamac.com * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * To post a message: mailto:[EMAIL PROTECTED] * * To leave this list visit: * http://www.techservinc.com/protelusers/leave.html * * Contact the list manager: * mailto:[EMAIL PROTECTED] * * Forum Guidelines Rules: * http://www.techservinc.com/protelusers/forumrules.html * * Browse or Search previous postings: * http://www.mail-archive.com/proteledaforum@techservinc.com * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
Re: [PEDA] 50 ohm differential impedance connections and traces
Scott and Brad, This URL has many interesting articles, including the one you mentioned. http://www.speedingedge.com/html/articles.html Regards Liane. Electrical Drawing with CAD Tutor Elecrical & Computer Engineering Department CPIT Ph 9408330 or 9408142 [EMAIL PROTECTED] * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * To post a message: mailto:[EMAIL PROTECTED] * * To leave this list visit: * http://www.techservinc.com/protelusers/leave.html * * Contact the list manager: * mailto:[EMAIL PROTECTED] * * Forum Guidelines Rules: * http://www.techservinc.com/protelusers/forumrules.html * * Browse or Search previous postings: * http://www.mail-archive.com/proteledaforum@techservinc.com * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
Re: [PEDA] 50 ohm differential impedance connections and traces
Scott, and Brad, This URL has many interesting articles by Lee Ritchey, including the one your mentioned. Physics Technician Faculty of Health & Sciences CPIT [EMAIL PROTECTED] ph 9408330 * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * To post a message: mailto:[EMAIL PROTECTED] * * To leave this list visit: * http://www.techservinc.com/protelusers/leave.html * * Contact the list manager: * mailto:[EMAIL PROTECTED] * * Forum Guidelines Rules: * http://www.techservinc.com/protelusers/forumrules.html * * Browse or Search previous postings: * http://www.mail-archive.com/proteledaforum@techservinc.com * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
Re: [PEDA] 50 ohm differential impedance connections and traces
There are probably many different expensive solutions out there. But, if you only need to transfer the signal 1 inch, a cheap solution which might work could be a high density flex connector. Get a 2 sided shielded flex cable with around 4 internal conductors. Use the middle 2 for the differential signal while grounding the outer 2 &, or course, ground the shield. I've successfully transferred a 4GHz signal with such a setup. When I say high density flex, I'm talking 0.5mm pin-pin. The really thin traces on the flex help keep capacitance & radio emitions down. Brian Guralnick - Original Message - From: "Brad Velander" <[EMAIL PROTECTED]> To: "'Protel EDA Forum'" <[EMAIL PROTECTED]> Sent: Wednesday, December 05, 2001 8:59 PM Subject: Re: [PEDA] 50 ohm differential impedance connections and traces | Scott, | I can give you an answer, however your engineers may choose to not | accept it. It is from one of the preeminent figures in signal integrity, Lee | Ritchey, it published in the March 1999 PCD magazine. | Contrary to popular belief you do not have to deal with differential | impedances in your PCB traces. This is the fallacy of this design dilemma. | The simple solution is two equal length (+/- 500 mils) singular controlled | impedance lines routed along roughly the same path. The two lines should | each have an impedance to Gnd of 1/2 Zo or 25 ohms. Each trace should be | terminated to Gnd in it's characteristic impedance. Voila, you have a 50 ohm | balanced impedance differential pair. | Your calculations are definitely out there, I don't know how you | have been calculating this figure but here is my solution using your basic | characteristics. Dielectric thickness: 0.008 inches, total copper weight: 1 | oz (0.0014in), FR4 (Er - 4.7). | | Your trace width should be 27.5 mils (0.0275 inches). Don't forget to | terminate each line with a 25 ohm terminator. | | | If you need a copy of the article, let me know, I will scan it and | forward it to you. | | Sincerely, | Brad Velander. | | Lead PCB Designer | Norsat International Inc. | #300 - 4401 Still Creek Drive, | Burnaby, B.C., Canada, V5C 6G9. | Tel (604) 292-9089 (direct line) | Fax (604) 292-9010 | Website: www.norsat.com | | | -Original Message- | From: Scott Ellis [mailto:[EMAIL PROTECTED]] | Sent: Wednesday, December 05, 2001 4:17 PM | To: Protel EDA Forum | Subject: [PEDA] 50 ohm differential impedance connections and traces | | | A couple of questions. | | Does anyone have a good way of doing board to board connection (only about | an inch) for a differential 50 ohm connection? | | Can I make two parallel connections with 100 ohm twisted pair? | | We have done some calcs for a FR-4, 1 oz copper, double sided, 1.6mm thick, | and we get traces 140thou, 8thou spacing over a gound plane for a 50 ohm | differential impedance. Can anyone confirm our result? 140 thou is getting | up there! | | The reason for the questions is that we need to take a differential PECL | signal from one board to another and maintain the signal integrity. | | Scott Ellis | Manager | Novatex Research - Excellence in Electronic Research & Development | [EMAIL PROTECTED] | 41 Yule Road, Merewether, Newcastle, NSW 2291, Australia | Ph 0412 988408 Fax 02 49636058 | | * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * To post a message: mailto:[EMAIL PROTECTED] * * To leave this list visit: * http://www.techservinc.com/protelusers/leave.html * * Contact the list manager: * mailto:[EMAIL PROTECTED] * * Forum Guidelines Rules: * http://www.techservinc.com/protelusers/forumrules.html * * Browse or Search previous postings: * http://www.mail-archive.com/proteledaforum@techservinc.com * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
Re: [PEDA] 50 ohm differential impedance connections and traces
Scott, I can give you an answer, however your engineers may choose to not accept it. It is from one of the preeminent figures in signal integrity, Lee Ritchey, it published in the March 1999 PCD magazine. Contrary to popular belief you do not have to deal with differential impedances in your PCB traces. This is the fallacy of this design dilemma. The simple solution is two equal length (+/- 500 mils) singular controlled impedance lines routed along roughly the same path. The two lines should each have an impedance to Gnd of 1/2 Zo or 25 ohms. Each trace should be terminated to Gnd in it's characteristic impedance. Voila, you have a 50 ohm balanced impedance differential pair. Your calculations are definitely out there, I don't know how you have been calculating this figure but here is my solution using your basic characteristics. Dielectric thickness: 0.008 inches, total copper weight: 1 oz (0.0014in), FR4 (Er - 4.7). Your trace width should be 27.5 mils (0.0275 inches). Don't forget to terminate each line with a 25 ohm terminator. If you need a copy of the article, let me know, I will scan it and forward it to you. Sincerely, Brad Velander. Lead PCB Designer Norsat International Inc. #300 - 4401 Still Creek Drive, Burnaby, B.C., Canada, V5C 6G9. Tel (604) 292-9089 (direct line) Fax (604) 292-9010 Website: www.norsat.com -Original Message- From: Scott Ellis [mailto:[EMAIL PROTECTED]] Sent: Wednesday, December 05, 2001 4:17 PM To: Protel EDA Forum Subject: [PEDA] 50 ohm differential impedance connections and traces A couple of questions. Does anyone have a good way of doing board to board connection (only about an inch) for a differential 50 ohm connection? Can I make two parallel connections with 100 ohm twisted pair? We have done some calcs for a FR-4, 1 oz copper, double sided, 1.6mm thick, and we get traces 140thou, 8thou spacing over a gound plane for a 50 ohm differential impedance. Can anyone confirm our result? 140 thou is getting up there! The reason for the questions is that we need to take a differential PECL signal from one board to another and maintain the signal integrity. Scott Ellis Manager Novatex Research - Excellence in Electronic Research & Development [EMAIL PROTECTED] 41 Yule Road, Merewether, Newcastle, NSW 2291, Australia Ph 0412 988408 Fax 02 49636058 * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * To post a message: mailto:[EMAIL PROTECTED] * * To leave this list visit: * http://www.techservinc.com/protelusers/leave.html * * Contact the list manager: * mailto:[EMAIL PROTECTED] * * Forum Guidelines Rules: * http://www.techservinc.com/protelusers/forumrules.html * * Browse or Search previous postings: * http://www.mail-archive.com/proteledaforum@techservinc.com * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
