[time-nuts] NIST isolation amplifiers

[Bob Camp]

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 <bruce.griffiths at xtra.co.nz> 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 <csteinmetz at yandex.com> 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 -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
> 
> 
> 
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.