Re: [softrock40] Re: Increasing the usable dynamic range of the RXTX 40/30/20
- On 2/24/2013 7:27 PM, warrenallgyer wrote:
> Hi MiltHi Warren,
> I am over my head in attempting a technical explanation. I have looked at the output spectrum and it does produce some interesting products but they are just what you might expect; the products of the incoming RF and either the fundamental or the harmonics of the LO.
> The FST simply passes the incoming RF signal unaltered or blocks it completely, switched at the LO rate. I believe the low IMD is the result of the mixer not altering the level of the signal so non-linearities cannot creep in.
> Of course such a mixer has products at each of the harmonics of the oscillator as well and these are the responsibility of the RF filter in the front end and the audio filter on the backend to remove. However the linearity of the entire chain is not affected by any active components until the op amps.
> That is my understanding. There are much smarter people on this board who can affirm or refute it.
> Warren Allgyer - W8TOD
> --- In email@example.com, Milt Cram <w8nue@...> wrote:
>> On 2/24/2013 4:14 PM, warrenallgyer wrote:
>>> Good points all..I remain doubtful about the mixer itself being the problem at those levels. It is nothing more than a switch and should be good out to at least the supply rail.. But I agree that your evidence points to something in the RF domain and it seems your solution was effective. The was an interesting post earlier in this thread that discussed the contribution of toroid cores to IMD3. If not the mixer or the cores, then we are down to bad solder! :-)
>>> Warren Allgyer - W8TOD
>> Hi Warren,
>> Why do you say that you think the mixer will not produce IMD? The
>> switching mixer is an extremely nonlinear device. A true "mixer"
>> multiplies two sine waves and produces only sum and difference
>> frequencies. In the switching mixer, you are multiplying a sine wave
>> (the input) with a square wave (that is rich in harmonics). I believe
>> that there will be many spurious outputs when a very strong undesired
>> signal is present. I haven't analyzed the entire spectrum of outputs,
>> but I'm sure it is a mess.
>> 73, Milt
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.
gives a brief analysis of the switching type mixer.
- 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