AGC oscillation- Ground at Ant. Connector stopped it for me.

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AGC oscillation- Ground at Ant. Connector stopped it for me.

Postby kc7dax » Mon Aug 22, 2016 5:19 am

I have read several places about AGC oscillations while using SDRUno. Tonight I stumbled across an easy way to stop it, at least for my setup here.

I was reading about this happening while tuning VLF today and so I tuned from VLF and up to the AM BC Band tonight to see if it happened with my SDR Play. For quite awhile, I had no problems other than the tuning scale not moving with the waterfall's frequency bar as others have reported, and I also had problems tuning, while using the mouse wheel over the frequency meter digits. The mouse tuning was erratic and wouldn't tune but a limited amount, but I could tune by clicking on signals on the waterfall and the frequency display would reflect the new frequency. The erratic tuning also seemed to get worse over time.

I toyed around with the (RF) AGC On and Off on the Main settings and had no AGC oscillations for some time, but it finally happened around 250 to 300 Khz. I tried seeing if the "other" AGC made a difference, and it didn't. It was much as described by others for me. It would stop when the "Main" (RF) AGC was turned to manual control and adjusted properly.

Later, I tuned to AM BC and it would oscillate on that band also. At one point, I tried clipping a DC ground to the Antenna connector while the AGC was doing it's bouncing thing and it immediatly stopped the AGC oscillations when the ground was connected.

I am using a Sony AN1 active VLF to HF antenna powered through a wall outlet transformer. There is no way to ground it at the mount and the coax out to the antenna controller, as Sony calls it, comes from within the molded antenna case. I'd bet there are a good many others experimenting with SDR using makeshift antennas, such as random wires strung out a window or telescoping antennas that come with some dongles that would not provide a good ground at the antenna input connector on the SDR (or an HF upconverter if used). I thought I would pass this on so others with the AGC oscillation problems might also try grounding their equipment to see if it might help you too. I had previously tried the ground at higher frequencies, but I didn't notice any effect on reception there.

73 and good luck,
Brian, kc7dax
Last edited by kc7dax on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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