I had an interesting day back in the lab after 10 days on the road. I won't run through the grueling details but I now have a 20 dB audio attenuator cable with 3.5 mm ends and I also built a hybrid RF combiner that allows me to combine or split two 50 ohm sources with 3 dB loss in each one. Without any trimming i got about 25 dB isolation between the ports up to 30 MHz. It will be a handy tool.
My attenuator cable idea works for utilizing the unused noise floor in either a 16 bit or a 24 bit card. That is the good news. The bad news is there seems to be little point. On the high end, as Alan suggested, I found that by minimizing the PC input gain I was within a couple dB of the clip points on the op amps anyway. So, even though I was able to "raise the roof" on the sound card, I only gained 2-3 dB before the op amps clipped. This interested me so I ran through the 4 RXTX that I have here in Beijing to check at what level the op amps went into clipping:
Radio Band OP Amp Saturate
dBm (WSPR Center Freq)
RXTX 160 160 -14
RXTX 80/40 80 -15
RXTX 30 -11
30/20/17 20 -10
RXTX 15 -19
15/12/10 12 -18
The answer is "anywhere from -10 to -19 dBm". So even if I could get the sound card range shifted up I was still limited by the op amps.
The other problem with "raising the floor" is it is ugly down there! With my attenuator cord in place I was able to move the Realtek and the iMic card floors up to match the antenna noise. With the 24 bit X-Fi I was able to get it up within 10 dB of the noise. But with the iMic and especially with the X-Fi I found that the noise, spikes, and spurs were so prevalent in the bottom 10-30 dB of the card that I really did not want to see them. In real life the antenna noise hides a multiple of these digital sins. The X-Fi useable noise floor, once you get above the noise and spikes, is not a significant improvement over that of iMic and not as good as the internal 16 bit Realtek.
So, again in real life, I am bounded on the top end by the op amps, let's say -17 dBm on 40 meters, and by the noise floor on the bottom end, normally about -110 dBm. That gives me 93 dB of useable dynamic range which is perfectly accommodated by a 16 bit card.
I do confess to having an E-MU 0204 on the way by courier from the US. It should be here next week. It will be interesting to see if the lower 20 dB will be useable and I am justifying the purchase on the basis that it will be a lab tool rather than operational.
Closing the book on this one for now
Warren Allgyer - W8TOD