Termination Options—1kΩ or 50Ω?

Most unamplified (D-series) products normally come internally terminated at the output with a 1kΩ resistor. This provides the highest conversion gain when attached to a 50Ω measuring instrument, such as an oscilloscope or a spectrum analyzer, because virtually all of the detector’s output current is delivered to the 50Ω load. In many cases a 1kΩ termination also enables the cleanest pulse response.

However, there are measurement situations where reflections between the detector and its load need to be minimized. This is especially true in analog, microwave applications, or where very long time-domain signals need to be accurately measured. In these cases, a 50Ω termination is preferred, in spite of the fact that only half of the output current is made available to the load, thus reducing the conversion gain by 50%.

Several of the unamplified communications detectors are available with a 50Ω termination option. This option must be specified at time of order with a “/50” suffix added to the model number (e.g. D-70xr-FC is 1kΩ terminated, D-70xr-FC/50 is the 50Ω version). Other models are only available with a 50Ω termination for the same reasons.

Note that the 50Ω termination does affect the pulse and bandwidth performance of the detector by shortening the pulse duration and increasing the bandwidth to some degree. The specifications cited for those products with the termination option are for the 1kΩ version.