RSPduo problem in the 15 m band

General discussions
Post Reply
glovisol
Posts: 662
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

RSPduo problem in the 15 m band

Post by glovisol » 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?
Attachments
Front End oscillation @ 21 Mhz Oct. 20 08.51.jpg
Front end instability on the 15 m band
Front End oscillation @ 21 Mhz Oct. 20 08.51.jpg (197.69 KiB) Viewed 7867 times

Reason: No reason

glovisol
Posts: 662
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

Re: RSPduo problem in the 15 m band

Post by glovisol » Sat Oct 20, 2018 7:16 am

The same problem with Tuner 2 as well. There is no problem of conditional stability when using the HI Z input. Tuner 1 and Tuner 2 50 Ohm front ends appear to be conditionally stable: e.g. amplification stability depends on amplifier's input termination. Probably a small input attenuator will provide the required stability. Tests to follow.

Reason: No reason

glovisol
Posts: 662
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

Re: RSPduo problem in the 15 m band

Post by glovisol » Sun Oct 21, 2018 7:38 am

There is no problem with the RSPduo at 21 MHz. This problem has been solved once it was discovered that anything connected to the 50 Ohm antenna input (save for the 50 Ohm termination) would start the apparition of birdies spaced at 8 MHz, so this pointed to radiation from the PC laptop. Placing the RSPduo 12 m away from the PC (with a buffered USB cable) reduced the level of the birdies to noise floor level.

To conclude the problem was caused by PC interference getting into the 50 Ohm ports, interference that, on the contrary, appears to be totally rejected by the HI Z balanced input. This is another demonstration of the advantages of the balanced input in HF & lower!

Reason: No reason

Post Reply