| Author |
Message |
Jim Thompson
Guest
|
 Posted:
Fri Dec 09, 2005 5:35 pm Post subject:
Re: electrical interface problem |
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On Fri, 9 Dec 2005 16:27:44 +0100, "Melanie Nasic"
<quinn_the_esquimo@freenet.de> wrote:
| Quote: | Hi,
I wonder what is the characteristic impedance of the circuit proposal
CMLmystery.pdf? I think this is the easiest way to do it but I want to be
sure that the CML transmitter still "sees" 50 Ohm line impedance. Any
comments on that?
Bye, Mel.
[snip] |
qrk/Mark implies that the transmitter can be operated with less output
common mode (as I suspected), so just use my version without caps, use
no divider to set output CM, just twiddle the pull-down R, as in
Mark's version until you get the +1.15V at the receiver. (My values
were line-match based.)
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
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Melanie Nasic
Guest
|
| Posted:
Fri Dec 09, 2005 5:35 pm Post subject:
Re: electrical interface problem |
|
|
Hi Jim,
don't the changes you suggest effect the line match (twiddling the pull-down
Rs)? Nonetheless I would like to calculate the impedance of Mark's circuit.
Any suggestions how I can find out about the impedance?
Bye Mel
"Jim Thompson" <To-Email-Use-The-Envelope-Icon@My-Web-Site.com> schrieb im
Newsbeitrag news:dq8jp1hkpvvr9l2og58amk4m08jdrht3nf@4ax.com...
| Quote: | On Fri, 9 Dec 2005 16:27:44 +0100, "Melanie Nasic"
quinn_the_esquimo@freenet.de> wrote:
Hi,
I wonder what is the characteristic impedance of the circuit proposal
CMLmystery.pdf? I think this is the easiest way to do it but I want to be
sure that the CML transmitter still "sees" 50 Ohm line impedance. Any
comments on that?
Bye, Mel.
[snip]
qrk/Mark implies that the transmitter can be operated with less output
common mode (as I suspected), so just use my version without caps, use
no divider to set output CM, just twiddle the pull-down R, as in
Mark's version until you get the +1.15V at the receiver. (My values
were line-match based.)
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food. |
|
|
| Back to top |
|
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Jim Thompson
Guest
|
| Posted:
Fri Dec 09, 2005 5:35 pm Post subject:
Re: electrical interface problem |
|
|
On Fri, 9 Dec 2005 16:42:15 +0100, "Melanie Nasic"
<quinn_the_esquimo@freenet.de> wrote:
| Quote: | Hi Jim,
don't the changes you suggest effect the line match (twiddling the pull-down
Rs)? Nonetheless I would like to calculate the impedance of Mark's circuit.
Any suggestions how I can find out about the impedance?
Bye Mel
[snip] |
Oooops! In Mark's version they do affect impedance. Use a single
midpoint pull-down... my divider without the upper resistor.
Insert transmission line into your drawing, then you can visualize
source and termination easier.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
|
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qrk
Guest
|
| Posted:
Sat Dec 10, 2005 1:35 am Post subject:
Re: electrical interface problem |
|
|
On Fri, 9 Dec 2005 16:42:15 +0100, "Melanie Nasic"
<quinn_the_esquimo@freenet.de> wrote:
| Quote: | "Jim Thompson" <To-Email-Use-The-Envelope-Icon@My-Web-Site.com> schrieb im
Newsbeitrag news:dq8jp1hkpvvr9l2og58amk4m08jdrht3nf@4ax.com...
On Fri, 9 Dec 2005 16:27:44 +0100, "Melanie Nasic"
quinn_the_esquimo@freenet.de> wrote:
Hi,
I wonder what is the characteristic impedance of the circuit proposal
CMLmystery.pdf? I think this is the easiest way to do it but I want to be
sure that the CML transmitter still "sees" 50 Ohm line impedance. Any
comments on that?
Bye, Mel.
[snip]
qrk/Mark implies that the transmitter can be operated with less output
common mode (as I suspected), so just use my version without caps, use
no divider to set output CM, just twiddle the pull-down R, as in
Mark's version until you get the +1.15V at the receiver. (My values
were line-match based.)
...Jim Thompson
Hi Jim,
don't the changes you suggest effect the line match (twiddling the pull-down
Rs)? Nonetheless I would like to calculate the impedance of Mark's circuit.
Any suggestions how I can find out about the impedance?
Bye Mel
|
See posting in abse, "electrical interface problem - Melanie_attn.gif"
I'm not quite sure of your electronics knowledge, so I'll show the
steps to finding the equivalent input impedance of the network. This
will work for both mine and Jim's circuits since they are essentially
the same.
Item 1, in Melanie_attn.gif, is the full network. R7 has no effect on
the differential impedance. R7 is only to set the DC bias point for
your receiver. In Jim's schematic (MelanieCML.pdf), the bias parts R8,
R9, C3 have no effect on the differential impedance.
Item 2 shows the network without R7 since the node R7,R4,R5 is at zero
AC potential.
Items 3, 4, and 5 show the step by step simplification of the network
to get the equivalent input impedance of the network. The double bars
(||) mean parallel, i.e. R1||Rb means R1 in parallel with Rb.
Your Rocket IO wants to drive an impedance of 100 Ohms differential,
not 50 Ohms.
The difference between Jim Thompson's circuit and mine:
Jim thinks you need to add termination resistor at the input to the
receiver (the pair of 49.9 Ohm resistors). Jim's circuit gives 17.6 dB
attenuation which might be too much.
I think that the termination resistor is included on the receiver
silicon die. That would be R6 in the schematic. My circuit gives 10.1
dB attenuation. I also use non-standard resistor values only because
I'm too lazy to look up standard values. Boy, it's nice being
unemployed and lazy!
Before this discussion goes on, we really need to know what your
receiver device is. John Larkin brings up some valid points about
simplification of the interface which could null this discussion of
level shifting and attenuator pads.
---
Mark |
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