[time-nuts] Advice on 10 MHz isolation/distribution (Clay)
bruce.griffiths at xtra.co.nz
Thu Feb 11 23:28:23 UTC 2010
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.
The input npn transistors for each stage have a collector current of
around 2mA the ft of a 2N3904 is relatively low at such currents.
Use a transistor with a higher ft (at 2mA) like a 2N5179 or its SMT
equivalent for the input transistor.
Increasing the collector current of the input transistor will also help.
There will be a nonzero phase shift at 10MHz due to the finite bandwidth
of the transistors used.
To a first approximation the phase shift is equivalent to a fixed delay.
This phase shift is relatively unimportant and should have a low tempco.
The gain of the amplifier is more important.
LTSpice predicts an output distortion below -40dBc with a 10MHz input
and +10dBm output.
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