Kiwi SDR

[NJQA]

Some good discussions of some Kiwi SDR limitations and potential solutions here:

http://ka7oei.blogspot.com/2019/08/revisiting-limited-attenuation-filter.html

https://ka7oei.blogspot.com/2018/06/a-limited-attenuation-high-pass-filter.html

https://ka7oei.blogspot.com/2018/02/managing-hf-signal-dynamics-on-rtl-sdr.html

[Josh]

Conveniently it also has info on limitations of the RTL SDR;

"This has the disadvantage that the sample rate is about 28.8 MHz meaning that signals above 14.4 MHz will be aliased.  For example, a signal at 21.25 MHz will also appear at (28.8 - 14.4 = ) 7.55 MHz.  This can usually be mitigated by the addition of band-pass filtering around the frequencies of interest if "fixed frequency" operation is expected."

And also;
"With 8 bits one can only attain an overall dynamic range of about 48dB (the actual amount is actually harder to calculate owing to oversampling, thermal and circuit noise, external noise, etc.)  The problem arises from the fact that a "weak" signal at, say, 160 meters may be on the order of 1 microvolt (-107dBm) but a nearby AM broadcast transmitter may be presenting a signal that could be 500 microvolts (-53dBm) or even much more!  In our example, we can see that this could pose a signal difference of 54dB - greater than the range that can be represented using an 8 bit converter.  In other words, assuming a 48dB dynamic range of our A/D converter, if we adjusted our levels so that a 1 microvolt signal (-107dBm) just barely registered on the A/D, any signal(s) that were 48dB above this (-59dBm) would "max out" our converter - again, ignoring oversampling, etc."

https://ka7oei.blogspot.com/2018/02/managing-hf-signal-dynamics-on-rtl-sdr.html

[kris]

   Thanks for this thread! Now I am aware of the work of my RTL SDR.  And what's the difference between the dynamic range of an AC / DC converter 8 and 16 bits?
I got the final impulse to build an input module containing an adjustable attenuator, bandpass filters and an amplifier as soon as possible. The ATT has been in my daily use for years, time for more.

[ChrisSmolinski]

   Thanks for this thread! Now I am aware of the work of my RTL SDR.  And what's the difference between the dynamic range of an AC / DC converter 8 and 16 bits?

Night and Day 

[Ray Lalleu]

   Thanks for this thread! Now I am aware of the work of my RTL SDR.  And what's the difference between the dynamic range of an AC / DC converter 8 and 16 bits?

Night and Day 

To get the dynamic range of an ADC, take the number of bits minus 1, then multiply by 6 and this gives the range in dB.
So for 8 bits, it's 42 dB (very poor), then for 16 bits, it's 90 dB (quite good).
The only way to get some decent results in HF with a 8-bit ADC  is with a good preselector. Or use a traditional receiver with an IF output. With an RTL donkey, thinking to pick up the 45 MHz IF often used in synthesized SW designs, so no need of an external upconvert.

edit : I forgot the possible effects of dithering, i.e. adding some 'calibrated' noise to the signal before sampling and converting into a digital stream. I don't know much about that, but I smell that it can give only a few bits more than the native ADC output, and maybe at the price of a better FPGA processing ?

BTW, that topic could be renamed SDR limitations or something like that,
and moved in the SDR forum.

[Pigmeat]

Anything about knobs?

[Josh]

"Anything about knobs?"

I have word an inexpensive usb knob for RTL and the like is in the works.




Now as to ADC dynamic range, yes going from 8 to 16 bits is like night and day. Then add dithering wich increases dynamic range to a slight degree but reduces sensitivity, dithering really is a means to end dsp spurs than increase dynamic range.

A neat trick called Decimation can be used to enhance dynamic range at the cost of bandwidth of a given tuned signal, giving a 12bit adc the virtual dynamic range of say 14bit or a bit more, the more narrow your bandwidth you can go, the higher the processing gain and dynamic range can be arrived at by decimation. The 12bit sdrs can get up around 80dB dynamic range with decimation.

The 8bit RTL donkeys have a effective tracking filter with is why they can survive in the harsh V/UHF environment, but this feature is unusable on hf and is some 100s of kc wide anyway. If you use a downconverter to feed hf sigs to a RTL running in its native V/UHF you can use the filter to help keep out of band sigs down.

The Icom Pro series used a 24bit 144dB range ADC by downconverting HF to an IF in the audio range, 36Kc as I recall. Sadly, the chip has an outstanding dynamic range of 144dB while the analog radio parts feeding that chip are around 90dB dynamic range, so you don't get the benefit of huge dynamic range, however you do get the benefits of ADC conversion and all that entails to sdr.

An ADC that can be hooked to hf directly is on the order of a hundred dollas or more for the chip alone, hence manufacturers are loath to employ them. Direct ADC conversion from rf is becoming the standard as time goes on and prices drop, however. The Icom 7300 is a direct sample hf rig and on the used market I see them under $900.

Prolly the best treatise on decimation and sdr dynamic range I've seen;
https://www.qsl.net/z33t/dynamic_range_eng.html

https://training.ti.com/sites/default/files/docs/TIPL%204701%20-%20Sampling%20rate%20vs%20Data%20rate%2C%20concept%20of%20decimation%20DDC%20and%20interpolation%20DUC.pdf