[time-nuts] Advice on 10 MHz isolation/distribution (Clay)
lists at cq.nu
Thu Feb 11 23:27:08 UTC 2010
If you dig into the data on some of the conventional isolation amp designs, their noise floors are down around -150 dbc / Hz in the 1 to 10 Hz range. Taking out the transformers and going to a push pull output might bump things up into the -140's in the 1 to 10 Hz range. Essentially you are building the "old NIST amp" in the paper:
That's not to say that the old NIST amp is the best way to go, only that the close in noise performance should be similar to that amplifier.
Simple answer = the 2N3904 / 2N3906 should be fine for what you are trying to do. You can find lower noise parts that don't have enough Ft to be useful. You can also find RF parts that have great Ft, great broadband noise, and lousy flicker noise.
On Feb 11, 2010, at 5:57 PM, life speed wrote:
> Message: 1
> Date: Thu, 11 Feb 2010 12:42:27 -0500
> From: "Bob Camp" <lists at cq.nu>
> Subject: Re: [time-nuts] Advice on 10 MHz isolation/distribution
> I really should learn how to read the whole message ....
> Cancel the second request on vibe info.
> The gotcha with vibration isolation is that it will stop working at some
> lower frequency. Aircraft have plenty of vibration running around at low
> That all sounds like bad news. Actually it's not. Since the phase noise
> isn't going to be all that good below the cutoff of the isolation, the amp
> doesn't need to sweat super low phase noise very close in. That can make the
> choice of transistors easier.
> Thanks Bob. I am aware of all the vibe issues, low freq corner, resonance peaking, etc. And yes, I have seen the Wenzel spreadsheet. Wenzel is a good resource for info. These issues have all been looked into. The phase noise numbers are what is predicted under vibration (10 Hz number might degrade a few dB). The amplifier will need to be better.
> 1 Hz < -100 dBc/Hz
> 10 Hz < -125 dBc/Hz
> 100 Hz < -140 dBc/Hz
> 1 KHz < -150 dBc/Hz
> 10 KHz < -155 dBc/Hz
> Are you aware of any bipolars that are better than others in 1/F noise performance? I noticed Gerhard Hoffman's design used BFG198 and BFG31, although those are SOT223 parts, which are somewhat large for my design. If I'm not mistake 'low saturation' correlates to low 1/F noise . . .
> I simulated the circuit with two outputs you sent in .GIF format. It appears to be tuned to a somewhat lower frequency than 10 MHz, perhaps 10 KHz to 1 MHz where the overall gain is near 0 dB, and the phase shift is near 0. I am using MMBT3904 transistors with Ft near 250 MHz. Perhaps that is the issue.
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