EMRFD Message Archive 8229

Message Date From Subject
8229 2013-02-05 21:17:14 kb9fko ills of DC receivers
I never know if I should post here or in the r2pro list...

I've been experimenting with a softrock transceiver recently. The RX is straightforward. RX HW: Si570 LO, commutating passive fet mixer, audio baseband preamp, and PC soundcard.

I had it hooked up to an antenna tuner with two BNC patch cords with a m-m adapter. I bumped the patch cord and heard a nice thump. Hmm... Having seen bad cables and connectors more than once or twice, that was my first suspect. I just wasn't sure and tried another, superhetrodyne, RX. It wasn't experiencing problems the day before with the same cables. The superhet would NOT thump no matter how much I wiggled the connectors and cables.

Yes, it was exactly the problem outlined in EMRFD. I understood the math, but I still chased the wrong hypothesis first. And, yes, it can be loud! Note the absence of a buffer that is recommended before the mixer in the HW description above.

Fun stuff!

Best regards,

8230 2013-02-05 21:49:44 victor Re: ills of DC receivers
The original softrock has the antenna connection isolated from the Rx ground. Didn't you by any chance connect the antenna cable or the antenna tuner ground to the Rx ground because if so low frequency noise on the antenna ground or cable gets into the Rx.
On other direct conversion receivers that I use I usually isolate the antenna from the receiver by using two series 10nF capacitors