Re: Increasing the usable dynamic range of the RXTX 40/30/20
I am on very unsteady ground here but plunging ahead anyway. Your patience is greatly appreciated.
I read the article and I am not sure it directly applies to the FST type switch. In the case of the FST I believe we can treat the LO as a perfect square wave despite its' ugly appearance on a scope. This because the FST switch goes from full off to full on at the threshold level on the rise and fall of the LO signal, rather than according to the slope in a multiplying mixer. I think the rise time does not reflect to the output. In my understanding this is why the switches make such clean mixers. IMD3 intercept levels for the switch test at +41-42 dBm ( http://martein.home.xs4all.nl/pa3ake/hmode/ ), well above that of the typical op amp/soundcard.
What do you think? Am I all wet?
Warren Allgyer - W8TOD
--- In email@example.com, Milt Cram <w8nue@...> wrote:
> Hi Warren,
> I would generally agree if we had pure square wave drive and perfect
> switches. However, the rise and fall times of the LO are not
> insignificant, and the switches themselves are not perfect.
> This link--
> gives a brief analysis of the switching type mixer.
> 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