[time-nuts] TADD-1 Design (was Re: Stepping up the output of an OCXO

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Feb 4 20:31:12 EST 2007

Christopher Hoover wrote:
>> The TI regulators are, from my perspective extremely noisy, one can do 
>> much better at least for higher output voltages.
>> They also don't have a high enough output for the TADD-1.
> Agreed, that that regulator doesn't have sufficient margin for the entire
> TADD-1.
> I was suggesting using the low-noise high PSRR linear reg only to establish
> the bias level.  
> There are indeed better parts.  This one looks pretty good for an
> integration solution (not include three C's):
> http://datasheets.maxim-ic.com/en/ds/MAX6126.pdf
> This app note, which filters the MAX6126 with an additional pass transistor
> and opamp, shows how to get down to 6 nv/sqrt(hz):
> http://www.maxim-ic.com/appnotes.cfm/appnote_number/3657
That circuit is only for about 3.2V output, you can do better for higher 
output voltages using the same technique with a buried zener reference. 
The TADD-1 has a 9V supply and requires a 4.5V reference. A 10V supply 
with a low dropout low noise regulator would be better as one could then 
obtain an output of slightly more than 1V rms instead of slightly less.
>> < other circuit suggestions >
>> This avoids blowing fuses if the supply polarity is reversed. 
> Or a PZT.   We use a lot of them in the power circuit for hot plug sockets,
> like on disk shelves.  (A fuse is cheaper wrt to the BOM, but the warranty
> costs kill you.)
>> The problems of instability and coupling of power supply noise into the 
>> signal path were also present when single supply discrete transistor 
>> amplifier were the norm.
>> Quite elaborate bias divider decoupling circuitry was used to minimise 
>> the effect.
> Yeah, at some point a bipolar design becomes easier, right?
> -ch

Yes, an isolation amplifier consisting of a cascade of 3 stacked common 
base stages with transformer coupled inputs and outputs should achieve 
an input output isolation of 120dB or so at 5-10 MHz with a total power 
supply requirement of around 20mA @ 24V per amplifier independent of the 
load (open circuit, 50 ohms or short circuit).
6 such amplifiers would required about 120mA @ 24V, considerably less 
power than a fully loaded TAD-1. Having the power supply current 
independent of the output loading is also advantageous. The phase noise 
floor should be -174dBc/Hz or lower with  13dBm input and output. The 
amplifier delay tempco should also be very low (~1-3 ps/K).

This configuration could be redesigned for 12V operation but would 
required transformer coupling the output of the input common base 
cascade pair to a pushpull common base output stage. Power requirements 
would then be about 60mA @12V per amplifier or 360mA @12V for 6 
amplifiers independent of the the individual amplifier load impedances. 
Performance would be degraded slightly with a phase noise floor a few dB 
(1-2) above that of the equivalent 24V amplifier.


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