My attempt to make an adapter to excite the TF1245 from a signal generator wasn't going too well so i decided to repeat and extend my measurements.I borrowed the TF1246 HF exciter again and also a HP 411A RF millivoltmeter. Well that turned out to be quite a revelation! The TF1245 needs a RF source that feeds 0.5 volt of RF into a 0.5 ohm resistive voltage divider tapped at 0.02 ohm to get 20 mV for the measuring circuit. This implies a current of 1 amp over the frequency range of 50 kHz to 50 MHz. When the TF1246 is used the voltage at the TF1245 input BNC is indeed around 0.5 volts. However when you measure it at the TF1246 output jack it's much higher, more than the HP411A upper limit of 10 volts. This implies a significant voltage drop in the special BNC link cable that is specified by Marconi. The DC resistance of that cable is pretty much zero. Substituting the BNC link cable I made for my project, which is shorter than the Marconi cable, produced the same result. It seems that there is a significant voltage drop in the connection consisting of a short (about 100 -150 mm) cable and two BNC connections.? I'll have to re-think the coupling from my amplifier into the TF1245!
Morris
