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Saa2 output port attenuator


Stephen Laurence 2020/08/12 03:00

Perhaps a silly question from a retired doctor.......

On the output port, according to Gabriel’s circuit diagram on the bottom left hand corner, is a pi attenuator with 75ohm parralell resistors and 120ohm series.

This gives 14db attenuation of the output signal. I appreciate that its main function is to stabilise the output impedance seen by the DUT.

However I am more intersted in dynamic range rather than small imbalances to the connected devices. Would a reduction to 4db attenuation give a 10db increase of dynamic range? If I needed greater stablity of output impedance, an addition of an external 10 db attenuator would restore the original specification.

Steve L. G7PSZ

Stephen Laurence 2020/08/12 03:00

Perhaps a silly question from a retired doctor.......

On the output port, according to Gabriel’s circuit diagram on the bottom left hand corner, is a pi attenuator with 75ohm parralell resistors and 120ohm series.

This gives 14db attenuation of the output signal. I appreciate that its main function is to stabilise the output impedance seen by the DUT.

However I am more intersted in dynamic range rather than small imbalances to the connected devices. Would a reduction to 4db attenuation give a 10db increase of dynamic range? If I needed greater stablity of output impedance, an addition of an external 10 db attenuator would restore the original specification.

Steve L. G7PSZ

OwO 2020/08/12 18:04

Yes you can reduce the attenuation to improve the dynamic range, but the
receiver might saturate with a thru cable connected so you may need to
calibrate THRU with an external attenuator connected.

On 12/08/2020 18:00, Stephen Laurence wrote:

Stephen Laurence 2020/08/12 06:08

Thankyou.

So if I did change the input resistors to give 4db rather than 14db attenuation, I woud need a 10 db external attenuator to attach when required.

I was just about to add to my previous question by asking if a broadband low noise preamp of 10 or 20db in front of the receiver port would also improve dynamic range, but you have effectively answered my question. A preamp would also add a significant physical path/ time delay and presumably a 180 degree phase shift, both of which I suppose could be calibrated out.

The 4 inch screen version would however possibly have enough empty space to put it somewhere.

On a related subject, what is the input 1db (or .5db or .1db, depending on ones personal threshold) compression point of the v2 input? ( sorry if I have overlooked it in the wiki specs)?

Steve L

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