Re: [softrock40] Re: Increasing the usable dynamic range of the RXTX 40/30/20
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Subject: Re: [softrock40] Re: Increasing the usable dynamic range of the RXTX 40/30/20
Yes, Milt has it exactly.
At the time I noticed it I was using the EMU, trying Warren's audio attenuator idea.
I realised I was getting distortion at the same RF level.
I see a clean output up to 4V5 pk-pk from the Softrock. The EMU takes that fine with the controls set to minimum input.
When I tried to get a level spectrum at 192 Ksps I had to increase these controls significantly. Then overload occurred at a much
lower RF level.
I did not check this with the other cards I used. My usual input level setting is the lowest possible to get results. But the D44s,
I must check, I use those at the -10dBV setting. Thinking about it that's too sensitive?
Warren, I'd guess you are not far short of the ideal, you probably just need a few dB attenuation before the SR overloads first. I
used http://chemandy.com/calculators/matching-pi-attenuator-calculator.htm and guessed at 10K impedance. But I was lucky and found a
stash of 2% resistors that had the values I needed.
73 Alan G4ZFQ
> Whether the SoftRock op-amps or the soundcard overloads first depends on
> the gain setting of the soundcard. Since the softrock can output barely
> 1Vrms, I would expect that the soundcard should be able to handle this
> unless the gain is set too high. If the soundcard gain is set for 0dB,
> I would not expect the soundcard to saturate. If, however, the
> soundcard gain is set for 20dB, it would most certainly saturate on peaks.
> Most soundcards are designed for "line level" inputs, or something less
> than 1V rms. Unless the soundcard does not have a minimum gain of less
> than 0dB, it would saturate easily with the softrock signal
> 73, Milt
- 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