Re: Function Generator Output Impedance

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Bart,

On 27 Feb 2002, at 7:19, Tesla list wrote:

> Original poster: "Barton B. Anderson by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <tesla123-at-pacbell-dot-net>
> 
> Hi All, 
> 
> Question: 
> While looking for low output impedance generators, I keep running across 50
> ohms which appears to be typical. Are there lower output impedance
> generators? Are there methods to reduce a typical 50ohm output impedance
> generator down to near zero ohms? 

Yes there are. I built one ;)  I used a couple of NS high frequency 
amps in parallel. Current sharing is performed by a pair of 
switchable resistors to give a range of Zo down to 0.5 Ohms. As a 
side benefit, the amps have an output for driving a LED to indicate 
when they are driven to current limiting (which means the source 
impedance rises steeply from then on of course). It is interesting to 
see how much a resonator looks like a black hole at resonance, 
especially with some loading.
     The best way to reduce Zo to ?? is to build a wideband stepdown 
toroidal transformer using twisted primary and secondary wires. 
Impedance transformation is 50 Ohms x (Ns/Np)^2  assuming the 
generator sources at 50 Ohms.
     Terry I think built a low impedance power amp using a module as 
well. My generator can source a couple of amps at 3VRMS before it 
runs out of steam (which is plenty to perform low power tests). The 
ICs are now obsolete I believe but could still probably be obtained. 
I don;t have the part numbers handy but could visit the file and dig 
them out if you like.

Regards,
Malcolm