Li Ang wrote:
Hi Charles,
I'm making a 1-to-4 distribution amplifier for 10MHz. Can you give
any suggestion
The opamps I'm considering are LMH6609 LMH6624 LMH6702.
Does the piezoelectric effect of capacitors need to be considered here?
I'm working on a response to your questions. In the meantime, since
I do not know what you plan to do with your distribution amplifier
(feed a reference oscillator to several instruments and/or radios to
keep them on frequency? feed an oscillator as an input signal to a
phase noise analyzer or DMTD system to characterize its frequency
stability?), I'm repeating something I posted in November 2014 for
you to think about:
This brings up the distinction between *isolation* amplifiers and
*distribution* amplifiers. Most of us need a [number of] feeds for
various test equipment, radios, etc. These feeds should have 50 ohm
output impedance, moderate isolation (35dB or more), and should not
noticeably degrade the noise, PN, distortion, or xDEV of the
source. That is the job of a distribution amp.
I would generally [use] some version of a two- or three-transistor
Class A buffer amplifier [or an IC-based circuit for this]. There
are lots of circuits to choose from. Many are transformer (or
autoformer) coupled, some are not (the JPL circuits come to mind)
and can also be used to distribute lower frequencies. You can get
[fanout] the NIST way ([high] buffer amp input impedance so you
parallel a bunch of them at the input connector), or by using one
stage with low output impedance to drive a number of output
amplifiers in parallel, or by using an amplifier with very low
output impedance (perhaps a high-current monolithic amplifier) to
drive a number of 50 ohm build-out resistors, or by fanning out with
CMOS logic and following each CMOS final buffer with a Tee network
to [re]generate sine waves.
Then there are the times when you are making measurements of
oscillators and must absolutely ensure that there is no interaction
between them. That is the job of an isolation amp[, with isolation
of 80dB or more]. Rarely will you need more than two or three feeds
per oscillator, so what you need are several, one-to-three iso amps
(one for each oscillator). Here, something like the NIST amplifiers
makes sense.
I uploaded a basic tutorial on various distribution amplifier
topologies to Didier's site, but it is still in the "recent uploads"
section and not yet available for download. I'll post again when it
can be searched and downloaded.
Best regards,
Charles
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