[SI-LIST] Re: testing LVPECL signal with 50ohm system
- From: "Michael Poimboeuf" <Michael_Poimboeuf@xxxxxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Thu, 4 Sep 2003 09:34:52 -0700
FWIW I've found that using differential probes is the best way to check
LVPECL signals since they allow you to probe differential single-ended =
and=20
common-mode. I just bring the signals to .1" testpads (which can be
probed directly with a diff probe or can take a header).=20
Although you can cook up a quasi single ended method of testing, it
often won't allow you to see characteristics of the signal which affect
performance in the product you're testing.=20
For instance, on one board I worked on the "LVPECL" levels out of an
fpga didn't meet common mode or single ended requirements while they
did meet differential. I had to use a wider CM range follower to restore
them in one case and external TTL->LVPECL converters in another case.
This is one reason that I've found little use for vendor's evalation =
boards
that screw with the Vcc/Vee rails to kludge the boards for single ended
testing.
---
mkp=20
-----Original Message-----
From: Bob McNamara [mailto:electronbob@xxxxxxxxxxxxx]
Sent: Wednesday, September 03, 2003 6:50 AM
To: si-list@xxxxxxxxxxxxx
Cc: dan.zhu@xxxxxxxxxx
Subject: [SI-LIST] Re: testing LVPECL signal with 50ohm system
[Sorry, forgot to copy the reflector the first time]
What I've done in the past is to terminate the signal in
a Thevenin equivalent to Vcc-2V and use the 50ohm termination
in the scope as part of the Thevenin circuit.
For example in your case of LVPECL at 3.3V, you could use
127 Ohms to +3.3V and 83 Ohms to ground. That gives you a=20
Thevenin equivalent resistance of 50.2 Ohms and a Thevenin
voltage of 1.304 Volts. The 83 Ohm resistance consists
of 33 Ohms on your module and 50 Ohms in the scope.
Then you set up your scope with an external attenuation
of 1.66:1 as a result of the voltage divider formed by
the 33 Ohm resistor and 50 Ohm termination.
Bob
> ------------Original Message-------------
> From: "Dan.Zhu" <dan.zhu@xxxxxxxxxx>
> To: si-list@xxxxxxxxxxxxx
> Cc: dan.zhu@xxxxxxxxxx
> Date: Tue, Sep-2-2003 2:27 PM
> Subject: [SI-LIST] testing LVPECL signal with 50ohm system
>=20
> Hi,
>=20
> I have a question on how to test LVPECL signal with 50ohm system like =
digital oscilloscope or Spectrum Analyzer (not=20
> probeing, just SMA connector on board ->cable-> scope).=20
>=20
> Background -
> For the LVPECL(VCC=3D3.3v & VEE=3D0v) output, normally a 50ohm =
termination resistor is used between LVPECL output pin and=20
> a voltage source of Vtt(=3DVCC-2v) to get a common-mode PECL output =
level of VCC-1.3v. When testing a standalone PECL=20
> device, people can lower the supply/gnd of pecl device by -2v so =
VCC'=3D1.3v, VEE'=3D-2v -> Vtt'=3D0v, then a digital=20
> oscilloscope with 50ohm input resistor(to GND) can be used to replace =
the 50ohm termination resistor and test the=20
> signal directly.=20
>=20
> Question -
> My question is that if due to some reason, the supply of PECL device =
can't be changed(for example, the pecl device is=20
> integrated on a mixed-signal IC and change of supply of PECL device =
is not allowed), there is any other way we can=20
> still test the PECL output signal directly(not probeing, just SMA =
connector on board ->cable-> scope)?
>=20
>=20
> Thanks for your suggestion
>=20
> Best Regards,
>=20
> Dan Zhu
>=20
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>=20
>=20
>=20
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