Hi Run in common base ( with things done properly) you can get well over 60 db isolation on a single stage.
Bob > On Nov 26, 2014, at 2:59 AM, Bruce Griffiths <[email protected]> > wrote: > > A single 2N2222 or equivalent transistor in a suitable circuit dissipating > about 200mW or so can achieve a reverse isolation of 35dB with distortion of > around -40dBc (output +13dBm) with a gain of unity, and an output impedance > of 50 ohms with a PN floor of around -180dBc/Hz or so. > Bruce > > On Wednesday, 26 November 2014 9:13 PM, Charles Steinmetz > <[email protected]> wrote: > > > 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 -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
