Luciano wrote:
The Extron is a very nice solution, certainly of modest price, but we must
consider * * * the low separation between the output channels.
Well, there is a distinct difference between DAs that use a single
amplifier (discrete or integrated) to drive each output channel (as your
design does), and DAs that drive multiple outputs from each amplifier
using build-out resistors. In the case of the Extron integrated circuit
DAs using CLS409 opamps, the isolation is 30dB between outputs fed from
one amplifier, and >90dB between outputs fed from different amplifiers
(see my document detailing recommended modifications to the Extron DAs,
downloadable from):
<http://www.ko4bb.com/getsimple/index.php?id=download&file=02_GPS_Timing/Extron_ADA_6_modifications_for_use_as_10MHz_distribution_amp_STEINMETZ.pdf>
So, simply by using only some of the outputs of an Extron IC-based DA,
one gets isolation roughly equal to your design. Of course, one could
easily build a one-output-per-amplifier DA using the Extron circuit
(with LMH6702 or other suitable opamps) to have a DA with ~100 dB of
isolation between all outputs (the exact isolation will be dependent
upon the layout, just as it is with your design).
One advantage of the IC design is considerably lower distortion -- the
harmonics are all below -65dBc at 10MHz, +16dBm output, compared to
-45dBc at +13dBm for your design (per your table). Further, the
distortion products of the IC design are essentially all odd-order
harmonics, while your design shows predominant even-order harmonics.
Odd-order harmonics are symmetrical, so they do not generate additional
amplitude-to-phase modulation when the output feeds a squarer/zero-cross
detector. Even-order harmonics, conversely, *DO* generate additional
amplitude-to-phase modulation when the output feeds a squarer/zero-cross
detector. This is very important because the vast majority of
measurement equipment likely to be fed by a DA/iso amp feeds its inputs
directly to a ZCD. See reference in next paragraph.
We have, in the past, disagreed about the importance of low distortion
in DA/iso amplifiers, but I have satisfied myself that a timing DA/iso
amp should have, at a bare minimum, -65dBc harmonic suppression. For
just one reason, see the NIST publication, "The Effect of Harmonic
Distortion on Phase errors in Frequency Distribution and Synthesis"
(Walls and Ascarrunz), available at:
<http://www.ko4bb.com/manuals/73.134.121.2/Harmonic_distortion_and_phase_errors_in_frequency_distribution_and_synthesis_NIST_1995.pdf>
Best regards,
Charles
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