Just checking some levels.. for evaluating how many bits does one need.
In general, you set the gain in front of the ADC so that the noise floor
just toggles the low bits.
http://g4fkh.co.uk/wp-content/uploads/2012/07/August-2012-RadCom.pdf has
a nice summary of noise levels..
And, you can look up the curves in the ITU recommendation (which
overestimate the galactic noise, by the way):
at 1 MHz I get manmade noise of 55dBa for quiet, 75dBa for noisy
business area.
at 5 MHz, I get 35dBa for quiet, 55dBa for noisy.
dBa is "noise above thermal noise (kT) for an ideal antenna"
For a 3 kHz bandwidth (35dBHz) and kT of -174 dBm/Hz, we would take the
dBa number and add -139 to get "dBm into the receiver"
35 -> -104 dBm (S4)
55 -> -84 dBm (S7)
75 -> -64 dBm (S9+10dB)
So the ITU numbers seem roughly consistent to what G4FKH measured.
What do people actually observe? If you're heading out into the Salton
Sea in mid winter for Stew Perry, you're probably lower noise than than
the "quiet 5MHz".
Now, when looking at instantaneous dynamic range, you want to make sure
that the sum of the strongest signals doesn't saturate the ADC. A
RTL-SDR has 7 bits ENOB, or roughly 42 dB, but you have to be careful
about RMS vs pk voltage. But taking 40 dB as a working range, that means
that in a middling area (-84dBm background noise in 3 kHz) you could
handle a -44 dBm signal, which is S9+30dB (a very strong signal).
In reality, you'll get more dynamic range - the RTL samples at 2 MHz and
you need, say, 10 kHz sample rate to cover your 3 kHz BW, so in
decimating from 2M to 10k, you pick up 23 dB of dynamic range.
There are other inexpensive SDRs in the $100 range that claim 12 bits
(also at MHz sample rates).
For that matter, an AD9265 (16 bits, 125 MSPS) eval board is $225. the
actual part is $90, so it's pretty pricey, but clearly, one can probably
build an inexpensive sampler with very good performance.
(the challenge with the 100 MSPS kinds of rates is that you'll need
something to filter/decimate.. you might as well get one of those fancy
ADC/DDS/Digital filter chips.
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|