Hi,

Is it possible to convert an 8753ES from 50 to 75 ohm operation simply by changing out the two test port couplers?

Alternately, I am guessing it is possible to do it with matching pads, but with some loss of sensitivity?

George G

Is it possible to convert an 8753ES from 50 to 75 ohm operation simply by changing out the two test port couplers?

Alternately, I am guessing it is possible to do it with matching pads, but with some loss of sensitivity?

George G

The easiest alternative is to add two min-loss pads. This cuts the source power by 5.9 dB, as well as the receiver sensitivity by the same amount, for about 12 dB loss in dynamic range. The main disadvantage is that the directivity is also reduced by 12 dB. The main advantage is that the source and load match are improved by 12 dB.

An alternative is to just use a 50-75 ohm adapter (no min-loss pad), and calibrate in 75 ohms. This will also give a very good result if the cal is good, but as the source and load match are not too good (14 dB nominal), the transmission tracking will suffer when compared to the min-loss pad case. But, in this case there is essentially no degradation of dynamic range, and while the directivity is degraded (to 14 dB), the directivity drift is not affected.

Replacing the bridges is the best alternative. In the 75 ohm bridge, the source and coupled ports are still 50 ohms (nominal) to allow connecting to the 50 ohm internal source and 50 ohm receiver. The bridge resistors are designed in such a way that the function similarly to the min-loss pad, but we absorb much of the loss in the bridge factors, so the added loss of the 75 ohm bridge is only about 1.5 dB.