I have a HP8714C with the attenuator option installed. CW signal is level accross range coming out of the stimulus module but after passing through the attenuator module and directional coupler drops off rapidly below 1 GHz to around -40 dB down by 30 MHz. As a result, the trace is very noisy on the lower frequencies. Seems like something in the signal path has opened up and is rolling off the low end. Any idaes?