life speed wrote:
Message: 1
Date: Thu, 11 Feb 2010 12:42:27 -0500
From: "Bob Camp"<[email protected]>
Subject: Re: [time-nuts] Advice on 10 MHz isolation/distribution
amplifier(Clay)
Hi
I really should learn how to read the whole message ....
====
Cancel the second request on vibe info.
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The gotcha with vibration isolation is that it will stop working at some
lower frequency. Aircraft have plenty of vibration running around at low
frequencies.
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.
Bob
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.
Clay
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.
Bruce
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