[time-nuts] new op amp for distribution amplifiers

[John Miles]

> Stylish dead-bug! ;)
> On my board I increased Rf/Rg to 4k7, to get rid of the gain-peaking.
> With 4k7 resistors I see +20 dBc/Hz worse phase-noise with the THS3491
> compared to LMH6702 :(  (image attached)
> If noise power scales with the resistors then it explains about 4k7/200R =
> 14 dB, for one amp, and perhaps 17 dB for two amps. Still missing 3dB
> somewhere in that reasoning...

The 3 dB figure I mentioned on the page was just a fudge factor to make the numbers come out with the resistors I tried.  I didn't test with 4.7k.  Looks like the NF in your case is 45 dB while my ad-hoc approximation would predict +34, so it clearly doesn't hold up at that end of the scale.  Where does the 200R come from?  

> John: would you say if one can use the suggested 976R Rf/Rg resistors then
> below -160dBc/Hz for a 10MHz carrier is possible for a gain of +2?

The 27 dB noise figure with Rf=Rg=976 is awkward.  If your NF is 44 dB with 4.7k, you'd need +27 dBm to stay 17 dB above the kTB PN floor at -177, so that's out of the question, and with the 27 dB noise figure at 976 ohms you still need +10 dBm.  Disclaimer: no coffee yet, sanity-check all numbers. :)

> At least we can say THS3491 is much more tricky to work with than LMH6702
> so far... (?)

You do seem to be getting somewhat worse results than I am, and it would be interesting to drill down and  figure out why.  The peaking behavior is excessive -- certainly if you have to use Rg=4.7k to tame the amplifier, you can rule out the part for any low-noise applications -- and what's up with the flicker noise below 10 Hz, I wonder?  I haven't seen any excess 1/f noise at all, and -115 at 1 Hz is a lot.  It might be OK for a low-performance distribution amp, but good OCXOs will be degraded, and I'd assume it's impacting your overall phase stability as well.  Need to find out what's up there.

-- john, KE5FX
Miles Design LLC