bruce-
do you have a favorite ZCD schematic that you can share?
many thanks in advance!
-Brian, WA1ZMS
On Jul 25, 2009, at 9:36 AM, Bruce Griffiths
<[email protected]> wrote:
Bert
If you intend to use a USB interface then its probably essential to
use
an optical isolator or equivalent between the zero crossing detector
(ZCD) output and the counter input to minimise potential low frequency
ground loop problems. JPL found that low frequency ground loops limit
the performance if the zero crossing detector outputs aren't isolated
from each other.
A resolution of better than 1E-14/tau should be relatively easy to
achieve with a good ZCD design.
Maintaining such resolution for long tau will be dependent on the
stability of the mixer and ZCD temperatures.
Mixer drift can be as large as 10ps/C or more at 10MHz.
The tempco of the low pass filter components in the first few stages
of
the ZCD will dominate the ZCD phase shift tempco.
Using a linear amplifier and filter in front of a zero crossing
detector
built into the counter is not the best way to implement such a system.
The amplifier filter limiter chain should be designed to suit the
offset
frequency.
The goal is to increase the zero crossing slope to a point where the
noise of the counter input circuitry doesnt contribute significant
jitter.
To do this linear amplification is counterproductive, every stage of
slope amplification needs to incorporate a low pass filter and
clamping
to reduce the output noise whilst increasing the output slew rate. As
long as the signal level is sufficient to ensure that each stage is
driven well into limiting, linear amplification serves little purpose
other than increasing the noise by more than necessary.
Bruce
[email protected] wrote:
Bruce, Thanks for your comments. It is my opinion that a dual mixer
system
can be built including five counters for a material cost of less
than $
200. The computer interface would be USB. My challenge is the
programming of
two u processors and the software that need to be written for the
computer.
How ever there is so much expertise in this group to make it
happen as a
joint effort, if the interest is there. By integrating a dual
channel counter
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