Bruce wrote:
Another issue is that if even one output needs high reverse isolation and low crosstalk, then even those outputs that arent so critical will also need high reverse isolation and low crosstalk to avoid degrading the crosstalk to the critical output.
This brings up the distinction between *isolation* amplifiers and *distribution* amplifiers. Most of us need a dozen or three 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 not use anything like one of the NIST circuits for this, but rather some version of a two- or three-transistor Class A buffer amplifier. 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 build-out the NIST way (buffer amp input impedance high 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 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. 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.
Note that I'm advocating distributing sine waves exclusively, NOT square waves or pulse trains. You will find that it is hard enough keeping 1, 5, or 10 MHz from getting into everything in the shop (and radio room), without adding the much-increased difficulty of keeping all of the harmonics under control. Also, you would like the harmonic content to be rather lower than is often thought because (i) even harmonics cause asymmetry, which can cause phase modulation when the signal is AC coupled or feeds a comparator-type zero-cross detector, and (ii) variations in the phase of harmonics in relation to the fundamental cause phase modulation (this is "harmonic dispersion," which is caused by temperature changes and other circuit variations such as modulation of semiconductor capacitances by low frequecies). NIST published a paper on this (see Walls and Ascarrunz, "The Effect of Harmonic Distortion on Phase Errors in Frequency Distribution and Synthesis").
Best regards, Charles _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
