[time-nuts] HP 5087A Input Doubler and 10 MHz Ampifiers

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Jul 8 01:49:20 EDT 2008

John Miles wrote:
>> Just watch for the odd error in those references.
>> eg in the first reference (.../498.pdf) the captions for figure 2 and
>> figure 3 should be swapped.
>> If anyone wants to try it, I have an even quieter, lower distortion 3
>> transistor isolation amplifier design that runs from a 12V supply.
>> I'll add it to that page in the next few days. It has a 50 ohm input
>> impedance and can be driven from a splitter which is in turn driven by
>> another such amplifier to form a high reverse isolation and channel to
>> channel isolation, low noise distribution amplifier.
> Certainly, put it up there -- that's a handy collection of ideas.  I'd like
> to add a couple of low-noise outputs to my 5087A at some point, actually.
> There's a lot of empty space on the front panel that bugs me.
> -- john, KE5FX

Will add the general configuration shortly and follow up with a 5087 
specific version that is tailored for the 5087 +18V supply.

Have been looking at the 5087 amplifier schematics and it appears that 
most of the noise is due to the fact that the various output amplifiers 
have a gain of somewhere around 10x or so judging by the waveform 
amplitudes given on the schematic. The amplifiers have a fixed gain (for 
a given load) and large amplitude inputs are accommodated by attenuating 
them. Lower noise for high amplitude inputs is achieved if the amplifier 
gain is adjusted to suit the input signal level rather than attenuating 
the input to suit the amplifier.

You will also need to change the 5087A input amplifier - although its a 
relatively low noise emitter follower it wont drive a 50 ohm load or 
several in parallel especially with a +13dB input signal.

For those who like to use wide bandwidth opamps I'll indicate how to 
achieve a phase noise floor within 6-12 dB of a low noise discrete design.

As NIST have shown there is no need to use particularly exotic 
transistors for 10MHz distribution amplifiers.


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