RSPduo problem in the 15 m band
Posted: Sat Oct 20, 2018 7:13 am
While working in HF I have always been using the HI Z balanced input. In the last few days, to investigate antenna noise and the NR function, I have started using the Tuner 50 Ohm input as well, using a transformer type balun made with a type 43 Ferrite toroid, 10 mm dia., 10t primary & 4t secondary to match the 560 Ohm antenna to the 50 Ohm RSPduo input.
With this input termination, if the gain is set above 37.2 dB, the front end appears to oscillate when the RSPduo is set to the 15 m band,. If I terminate with a 50 Ohm resistor, or if I leave the input not terminated, the oscillation disappears. The oscillation appears to be at 8 MHz, as per the uploaded PC screen. Even reversing the balun, e.g. 10 t to the 50 Ohm input, the oscillation still persists. It would appear that the front end amplification is conditionally stable.
Any suggestion?
With this input termination, if the gain is set above 37.2 dB, the front end appears to oscillate when the RSPduo is set to the 15 m band,. If I terminate with a 50 Ohm resistor, or if I leave the input not terminated, the oscillation disappears. The oscillation appears to be at 8 MHz, as per the uploaded PC screen. Even reversing the balun, e.g. 10 t to the 50 Ohm input, the oscillation still persists. It would appear that the front end amplification is conditionally stable.
Any suggestion?