Re: Increasing the usable dynamic range of the RXTX 40/30/20
- I completely agree with this, especially in the case of a normal receiver where you have rf stages and multiple mixers which can introduce distortion. The dynamic range softrocks receiver itself, at least in the testing I have done, normally far out-performs the audio card it is connected to.
The reason audio attenuation is attractive to me for the RXTX is that it does not require dealing with the attenuator in the transmit path and it requires no modification to the radio itself.
Attenuation to within a few dB of the noise floor will require band by band adjustment no matter where the attenuation takes place. I don't think it matters where we do it if we accept that the primary culprit is the audio card. Most likely I would set the input gain for the band with the lowest noise and not bother with changing it for the others.
I guess what I am saying is that 20 dB of attenuation is needed in the Softrocks line. Whether you choose to do it in the RF or the audio domains does not appear to matter much. And the most convenient place to do it may indeed be in the audio line cord.
Please freely disagree.
Warren Allgyer - W8TOD
--- In firstname.lastname@example.org, "Alan" <alan4alan@...> wrote:
> The FIRST thing must be to reduce the antenna input.
> This reduces signal levels and overloading, whatever causes it.
> The levels of the lowest signals must be only just above the receiver noise. This is why any good receiver has an attenuator.
- 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