TF1245A injection transformer update
The wet weather of the last few weeks persuaded me to bring forward my transformer injection experiments mentioned in post #1703.
The FT50-43 and 15mm copper pipe end cap is not a perfect combination as the capacitance betwen the toroid turns on the outside and the cap walls produces a resonant effect at about 110MHz, or slightly lower when the toroid is mechanically stabilised, resulting in the coupling increasing with frequency by about 0.5dB at 50MHz.? At some time I will try fabricating a slightly larger cup, perhaps 18mm internal diameter, in the knowledge that the stray capacitance will be lower and the hope that the HF coupling flatness can be improved.
For now, the existing transformer is adequate in conjunction with the HF compensation trimmer (C14*) in the "Q multiplier" meter circuit.
A 2:1 voltage divider comprising two 27 Ohm 0805 resistors across the transformer input winding provides the correct tapping point to preserve the non-linear "Q multiplier" meter calibration (0.5V RMS across the original injection resistor assembly, 1V RMS across the 50:1 transformer input).? The input impedance at the front panel is close to 50 Ohms (VSWR <1.1 at 50MHz) which will avoid any standing wave peculiarities in the generator lead regardless of length.
The TF1245A Input I (40kHz - 50MHz) can now be driven by any signal generator that can deliver ~14dBm into 50 Ohms, with acceptably accurate direct indication of Q and more precise measurement by delta-C or half power bandwidth methods (easy with a synthesised signal generator).? I am not looking for absolute values, but repeatability is important to me for comparisons.? I will probably build a dedicated exciter with ALC and vernier amplitude adjustment as I already have suitable prototypes from other projects.
The picture shows the transformer RF injection assembly installed in the TF1245A.
PeterS??? G8EZE
