[time-nuts] Build my own dist. amp ??

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Dec 13 09:52:56 UTC 2008


Randy Leifer wrote:
> Can I build my own 10 MHz distribution amp, using fast opamps ?
> I have searched the archives, but all of the designs are fairly complex for me to build using "point-to-point" on a proto/perf board.
>
> I want to feed an oven-ized reference 10 MHz sine signal (50 ohm), as a master clock for my:
> HP 3325A
> Keithley Counter
> PTS 160
> and future gear.
> (let's say, 1 in / 6 out)
>
> Parts I have in stock are 
> AD811
> EL2030, 2070, 2090
> CLC430
> CLC200
> LM7121
> LM6165
> Plus some LH0033 "damn fast" current boosting buffers, and some MH0007 clock drivers.
> I don't build much high-freq stuff....but a lot of audio freq stuff.
> My first thought is an input opamp buffer, branched off to separate line drivers.
> Am I anywhere near to achieving my goal ?
>
> Thanks.
> =Randy=
>
>   
Randy

One can easily do this if the resultant performance is acceptable.
A discrete implementation almost invariably has a lower phase noise
floor and higher reverse isolation.

Current feedback amplifiers tend to be relatively noisy due to input
current noise flowing in the feedback resistor.
To keep the close in phase noise down the dc noise gain should be as low
as possible.

LH0033's were used in an early JPL design, however they are relatively
noisy especially in the flicker noise region.
If you really want it I have the circuit schematic.

The TADD1 designed by John Ackermann and available from TAPR is one
example of an opamp based distribution amplifier.
However a good discrete design can have a phase noise floor up to 20 dBc
lower.

If you do decide to use opamps then a low noise wide bandwidth voltage
feedback amplifier is usually a better choice than a current feedback
amplifier as the total ouput noise will be lower.

For a 10MHz isolation amplifier you dont need any transistors much more
exotic than 2N3904's in an appropriate package.
3 or 4  2N3904's per isolation amplifier stage will with careful design
and layout achieve a reverse isolation of more than 130dB.


Bruce



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