Re: Carrier suppression on uBITX v5 - -35 to -40 dB ok?
Gordon Gibby <ggibby@...>
A significant number of sound interface systems will have a TRANSFORMER here and there. It is important in most cases to provide DC isolation to prevent the transformer from dramatically changing the biasing conditions of the mic input amplifier of the
radio to which it is connecgted. This blindsided me once when I connected a homebrew isolator (with transformers) to an icom 2 meter rig to do packet -- and the signals sounded horrible. You won't damage the transformer, but you can throw the linearity
of the mic-amp stage out the window. A Signalink in the same situation did FINE -- because it had capacitor dc-isolation. I learned the hard way to DC isolate.
From: BITX20@groups.io <BITX20@groups.io> on behalf of Tom, wb6b <wb6b@...>
Sent: Saturday, August 17, 2019 4:04 AM
Subject: Re: [BITX20] Carrier suppression on uBITX v5 - -35 to -40 dB ok?
I just double checked the schematic. It looks like there is adequate DC blocking capacitors in the mike input circuit. Also, the mike input already has a DC voltage applied to power the microphone. So, unless the sound card doesn't like the DC voltages and causes distortion, it is likely not the source of the carrier leakage.
I'm not sure how much is good or bad, but the topic of carrier suppression and fixes has come up on this group. Hopefully, other may be able to shed more light on what level is acceptable.
I have had good success with connecting the high side of the volume control directly to the microphone input of a USB sound dongle. And the headphone output of the USB sound dongle directly to the microphone input of the uBitx. Some sound adaptors may work better or worse in this configuration, but the generic adaptor used seems fine in this arrangement. It is possible some sound adaptors might not like the DC voltage (but low current) the uBitx puts on the microphone input applied to the headphone output.