Softrock Dynamic Range and MDS Redux
- I have done more reading and it seems that "MDS" depends on the eye (ear) of the beholder. The dictionary definition seems to be the "minimum level for a specific decoder to properly read the signal". In the case of CW and SSB the decoder is the human ear so MDS would need to be expressed in terms of a given operator's ability to read the CW or understand the SSB. Apparently the ARRL found this a bit too subjective and for their purposes they said MDS equals the signal level required to double the power out of the speaker for a given bandwidth.
A signal generator output that results in double the power at the speaker as compared to noise alone seems to display at about 20 dB above the visible noise floor in HDSDR.
I tried with Rocky and I find it requires the approximately same signal levels as HDSDR to double the audio power at the speaker. Rocky however displays the resulting signal as a spike approximately 10 dB above the displayed noise floor.
With my ear I found I could still hear the CW signal 10-12 dB below the ARRL definition of MDS. The resulting display on HDSDR was about 8-10 dB above the average noise floor and about half a division in Rocky. In both programs I could still "see" a signal on the baseline below the point at which I could hear it.
All of this dramatically affects the numbers we are throwing around when we discuss dynamic range. The "noise floor" defined by ARRL is 20 dB higher than the displayed noise floor and 10-12 dB higher than the audible noise floor. For a given level at which an interface goes into overload this could still give three wildly different values for "dynamic range".
Example: An audio card displays gross overload symptoms at -10 dBm. The ARRL noise floor is -105 dBm. The "audible MDS" is -117 dBm. The visible noise floor is -125 dBm.
Is the dynamic range:
115 dB (visible)
107 dB (audible)
or 95 dB (ARRL standard) ????
A 20 dB "grey area" seems a little imprecise to me. Does anyone have some further enlightenment on this?
Warren Allgyer - W8TOD
- Good thought Victor.
Crosstalk between the generators is a good possibility since they are on the same Silicon Labs 5338 EVB. So, of course, I set up a test.
Generator A was set for 7.050 and Gen B for 7.055 and both fed through the hybrid, then through a total of 31 dB of fixed/variable pad to the Softrocks.
Alternately disconnecting Gen A and B I measured B= -26 dBm and A= -25 dBm. With A disconnected I measured -116 dBm on 7.050 and with B disconnected I measured -98 dBm on 7.055. So A crosstalks into B at -90 dB and B into A at -72 dB. Not great but at least I know what I have.
I do not have a second variable attenuator so I put fixed 20 dB pads on the output of both generators and reduced the variable/fixed combo after the hybrid to 11 dB. The readings were identical. So the crosstalk is on the board and not in the hybrid.
I also checked the IMD3 products at 7.045 and 7.060. The levels were very comparable in both cases, down about 70 dB from the main signals. In both cases the IMD3 products stepped linearly with the main signals until I reached an input level of -25 dBm which is the op amp overload point and they skyrocketed.
What does this prove? Maybe something, maybe not. It is possible the IMD3 products generated by the onboard crosstalk are so high they mask the true IMD3 generated in the radio.
In order for that to happen the "real" IMD3 product would have to be at -96 dBm or less when the main signal is at -26. If it is at -96 and hidden then that would make the intercept point at +44 dBm, an almost unheard-of number. But not impossible.
Somewhere in these posts there is a reference to tests done on the FST3253 mixer that quoted test numbers in this range.
My bottom line on this is that the IMD3 performance of the Softrocks receiver, at least prior to the op amps, is so good that it is irrelevant.... or that the third order intercept test itself is irrelevant for this breed of receiver, as is maintained by the ARRL handbook.
As usual, probably a lot more information than anyone really wants to know. But you gave me a real fun morning Victor!
Warren Allgyer - W8TOD
--- In firstname.lastname@example.org, "victor" <victorkoren@...> wrote:
> Warren, I bet that your two generators make intermodulation between them so they generate the distortion signals, and that's the reason that the intermod level stays the same (compared to the generators signal level) when you change the level by using a variable attenuator at the receiver input. If you have an additional variable attenuator, do the test again but now put the variable attenuators in series with each generator output before the hybrid coupler and change them together to change the signal level at the receiver input. You will see that at lower signals (higher attenuation) the distortion will go down faster because the isolation between the generators will increase.
> Victor - 4Z4ME
> --- In email@example.com, "warrenallgyer" <allgyer@> wrote:
> > Nick (and others who contributed to this very interesting discussion):
> > I finally got back to my workbench and decided to tear into this issue again. I set a two channel signal generator (homebrew Si5338 referenced to GPS) to generate two signals, 5 kHz apart, at about -30 dBm through a hybrid coupler and a step attenuator. The third order products were readily visible on the baseline about 55 dB down.
> > I tried to confirm the 3X principle where the IMD3 products increase at 3 times the rate of the fundamental but, in my case, they did not. A 3 dB increase in the fundamentals resulted in about a 3dB increase in the IMD3 products as well. It seems to be linear, not 3X!
> > Back to the handbook to check my methodology and I come upon this little nugget on Page 25.32 in the "Receiver Dynamic Range" measurement section:
> > "The third-order intercept is generally not a valid concept for software-defined receivers (SDRs) that do not use an analog front end. Some SDRs do not use a mixer but feed the signal from the antenna directly to an analog- to-digital converter (ADC). ADCs usually do not exhibit the 3 dB per dB relationship between signal level and third-order products, at least over major portions of their operating range. Comparing third-order dynamic range measurements of an SDR and a conventional analog radio may give misleading results."
> > SO...... it seems that we cannot use the third order intercept principle to establish the upper number of the dynamic range for the Softrocks.
> > I am at a bit of a loss. It seems that the practical upper number for the Softrocks seems to be the level at which the op amps start to clip which seems to be in the -12 to -20 dBm range, and that 3rd order intercept is a fairly useless number in these cases.
> > Any thoughts?
> > Warren Allgyer - W8TOD
> > > > On the two-tone intercept: I think I understand the methodology. IMD3 products increase at 3x the rate of the two input signals. So if you measure the product, remove 10 dB of attenuation, measure again... you get two points on a line.
> > >
> > > Actually you only need one point because you know the slope of the line. But two (or more) improves the accuracy of the measurement.
> > >
> > > +30dBm is where my measured 3rd order line meets the 1st order line on my graph.
> > >
> > > > Do you, or anyone, understand the rationale for designating 2/3 of the resulting range as the maximum signal level? Is it just arbitrary or is there some very quantifiable effect on the reception of desired signals at that level?
> > > >
> > > It's not an arbitrary designation. The overload level is defined as that level where the 3rd order products are at the same amplitude as the MDS. It follows that the Two Tone Dynamic Range is 2*(Pi-MDS)/3. There is an algebraic explanation in one of the Appendices to "Solid State Design for the Radio Amateur". It can also be derived graphically.
> > >
> > > > As a number that is replicable and comparable between receivers I can see its' value. I would like to understand though what a signal 100 dB above MDS would sound like compared to 96 dB.
> > >
> > > In this case with one such signal it would sound 4dB louder if you switched the AGC off. But with two such signals the 3rd order products would now be 12dB above the Noise Floor (3 x 4dB) i.e. at -103dBm in my Softrock at 5MHz. Would you hear this? Quite possibly, if the two signals were outside the passband and one of the third order products fell within the passband.
> > >
> > > HTH
> > >
> > > 73 Nick G3VNC
> > >