[time-nuts] distribution amp question + hp 59309A
kb8tq at n1k.org
Sat Jan 28 16:28:12 EST 2017
The gotcha with transformer coupled coax is keeping it terminated over a wide range of frequencies. If the coax is miss terminated
and the end of the cable is floating, you have a pretty good opportunity for noise to get into the system. Floating shields are also a
pretty good way to get crosstalk. In many situations, isolation between outputs is a pretty big deal.
> On Jan 28, 2017, at 12:29 PM, walter shawlee 2 <walter2 at sphere.bc.ca> wrote:
> I notice that in the distribution amp being discussed at the moment,
> the BNC output connectors are grounded, and tied to the chassis,
> which in turn has a grounded emi line filter. this seems like an unavoidable noise pathway to me.
> I notice that some commercial amps are grounded, but more advanced and transformer coupled units have floating connectors. it makes the most sense to me to be floating, since this frees the return from line noise and spurious, and avoids the significant problem of shifted AC voltages on the return from distant units connected to the amp which are on other ac line circuits.
> What is the general feeling here about this issue? I confess that if the amp output is transformer coupled, I see exactly zero benefit in a grounded connector as the feed from the amplifier.
> Also on an unrelated topic, I found an HP 59309A HPIB clock on a forgotten shelf and looked at it, and was surprised to see such a poor primary time standard oscillator inside, just a 1Mhz crystal using a cmos buffer oscillator. It can accept an external standard, but it did feel odd for a device that is meant to provide coordinated system time to be so modestly executed. it's like an uncorrected PC desktop clock.
> This same issue pops up in many hp/agilent counters, signal generators and related objects. I have always been puzzled by the decision to make such marginal instruments that have time/frequency as their primary parameter, when so little additional effort would have dramatically improved them. I do get the concept of an external standard reference, but it's a pretty weak argument for making a $5K generator or counter with poor performance. Just curious to know everybody's thoughts on this.
> all the best,
> Walter Shawlee 2, President
> Sphere Research Corporation
> 3394 Sunnyside Rd., West Kelowna, BC
> V1Z 2V4 CANADA Phone: (250) 769-1834
> walter2 at sphere.bc.ca
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