I use SIGNALINK USB modem interface for digital modes on HF (mainly FT-8)
and Meteor Scatter on VHF. It is very handy because the volume of TX
and RX can be adjusted by pots on the front panel, and it has a built-in VOX
circuit - eliminating PTT keying through a serial port. And as I always
connect up the transceiver for DATA through the DATA ports on the back
panel, the built-in VOX of the transceiver is not active (at least : with
ICOM rigs).
What can be improved on this modem ? There are several things to do ...
Very noisy on the low end of AF band, from 0-100 Hz about : this can be checked for instance while in FT-8 mode, and setting the waterfall to start at 0 Hz ! Off course, this is outside of the normal passband for FT-8 operation, but as a matter of facts, the less noise you feed to the decoding software, the better ...
Audio passband not 'flat' : it is indeed desirable to have a flat response from 300 - 2.700 Hz. This is not the case, we see (for example in WSJT FSK441 mode) a peak around 2 - 2.3 kHz, and a significant drop-off on the low frequency end - about -10dB @ 1kHz and -20dB @ 500 Hz ! This can as well be observed in the FSK441 waterfall, where the lower part is 'dark'. This can be somewhat corrected by changing the audio response of the transceiver, but I don't think this is the right approach.
Original audio passband
Several HAMS have done a wonderful and valuable job to improve the modem, e.g. Peter OZ1PIF - see http://www.frenning.dk/ and Matt AB8VJ.
I can only recommend the following modifications (download pdf summary, should above hyperlink not work), which proved to be very effective :
in my unit the 4,7�F was factory replaced by a 33�F - if not, replace !
solder a 0,1�F capacitor on the PIC power supply pads
insert a 100�H choke in 5v supply line, and add 2x 0,1�F capacitors (can be soldered on the ground 'via's' in the neighbourhood) to decouple supply
Use the 3.7V reference from the PIC to BIAS the op-amps - the mod of AB8VJ - it is really very effective ;o)
Exchange the original 600:600 Ohm transformers (type TL016-R by XICON - see datasheet here) by quality transformers, typically used in high speed data modems. I used ETAL type P3356 (see datasheet here) which have a flat response (100 Hz to 4kHz +/_ 0.1 dB !) with low distortion. I removed as well the two 10nF SMD capacitors on transformer input. See result hereunder for FSK441, where the waterfall is now uniform from top to bottom. On HF also for FT-8 the sensitivity in the low end of AF has significantly improved !
Passband with ETAL transformers
Good luck with the mods and CU in digital mode !