I think you need to consider what happens at DUT resonance and either side of it.? Assume an ideal 50:1 turns transformer.? At DUT resonance, the impedance presented to the 1 turn will be approximately the series loss resistance of the tuned circuit, say 1 Ohm as suggested by Mikek, or 2500 Ohms at the input of the transformer.? Off resonance, the impedance presented to the 1 turn will be capacitive or inductive with a tiny series resistance, depending on whether the frequency is below or above resonance.? The impedance presented to the amplifier will be very high, shunted by the 75 Ohm resistor in the HP circuit to provide a manageable load for the power amplifier under all tuning conditions.? The behaviour of a real transformer will be somewhat different with stray capacitance and frequency dependent leakage inductance.
A while ago I modelled a variant of the HP impedance converter circuit in LTspice using 2N3866 rather than 2N5109 transistors for both Q1 and Q2.? I believe the model for the transistors was reasonably good at these frequencies, but the 2N3866 has multiple emitter connections for low parasitic resistance. The theoretical output impedance was about 0.75 Ohms 200kHz to 10MHz, rising to about 1 Ohm at 50MHz.? Unfortunately I never measured the impedance of the development breadboard as it worked more than well enough for my purposes.
PeterS??? ??? G8EZE
