Re: Increasing the usable dynamic range of the RXTX 40/30/20
- One counterexample to this statement is the Softrock mixer noise level at high frequencies (>20MHz). This noise is the limiting factor of the dynamic range on the low end in this case, and not the soundcard performance.
I can only offer anecdotal evidence for the strong signal end of the dynamic range, because I didn't do any measurements. But in my experience, there are no visible IMD products (they are below the real-life antenna noise floor) right up to the opamp stage clipping happening. This means that in real life you see the limitation of the Softrock components well before you see limitations of the soundcard. A visible issue on the high end is the image suppression (again, usually a Softrock LO/mixer stage limitation).
And when I tried to use some really crappy souncards, the limitations were in a form of spurious peaks that do not depend on the input at all, or in a form of uneven frequency response. I guess technically you can call these issues "dynamic range issues", but this would be misleading.
--- In email@example.com, "warrenallgyer" <allgyer@...> wrote:
> Now I have to disagree Alan.
> I believe measurement of IMD3 products is one of the key factors for determining the dynamic range of a receiver. The higher the intercept point for IMD3, the higher the dynamic range.
> I will maintain that the dynamic range of a Softrocks receiver is limited the parameters of the sound card and has nothing to do with the mixer performance. This assumes of course there is no other failure or fault in the build.
> I am very sure I have not seen evidence to the contrary... either in my tests or in the responses on this board. If I missed something then I will gladly yield on this.
> Warren Allgyer - W8TOD
- 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