Date: Thu, 18 Feb 2010 15:22:40 +1300
From: Bruce Griffiths <bruce.griffi...@xtra.co.nz>
Clay
What's the effect of assigning the same label (Vout_2) to the outputs of
both output amplifiers as shown in your circuit schematic?
Bruce
Hi Bruce,
Duplication of the Vout_2 net labels (copy and paste is great) did not help the
isolation. Neither did the 10 ohm base resistors. Now that the schematic is
correct, the base resistors do not ruin the noise performance. Sigh . . .
So this circuit looks like it will work well (I also get 100 dB simulated
isolation between outputs, as well as noise of less than 2 nV/rtHz). I do
believe isolation is important, as there are several subsystems using (and
possibly trying to contaminate) the reference. I don't know all the details of
each subsystem, and may not be privy to such info. So I can't put a number on
the required isolation, but 20 log N applied to spurious within the PLL
bandwidth is very unforgiving.
The opamp solution is appealing, but I am unsure if it will work. My favorite
low-noise parts right now are the TI OPA211, and the Analog AD797. I know the
AD797 can be a bit tricky to tame oscillations. They both are probably
difficult when configured for more than 10 MHz BW. I will run some
simulations, but at 2V p-p into 50 ohms (10 dBm) the required rails would need
to be more than 5V, although I have 15V available. I am unsure of the
distortion performance at 20 mA peak output drive. Although the AD797 can
source more current, neither amp is specified with good distortion numbers at
50 ohm loads. They always show audio (600 ohm) loads for their good THD
numbers. I need to provide harmonics lower than 30 dBc. At first glance it
does not look a likely solution.
Lastly, my customer, the system designer, would now like to be able to add a
switch function to the 10 MHz distribution. I will have to check and see if
switching of bias current to these two-stage transistor amp circuits can
accomplish this function. Jelly-bean analog switches are unlikely, I think, to
have the requisite noise and isolation performance. Nothing like hitting a
moving target.
Clay
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