[Milsurplus] Low Frequency receiver, RBL

[Peter Gottlieb]

The problem with picking up everything is dynamic range.  Trying to pick up 
something near the noise floor when your system is taking in massively strong 
signals is a challenge.  There is always some analog before you get to digital 
bits no matter how soon you do the conversion.

I had a long wire antenna here and it picked up over a volt into 50 ohms 
overall, dominated by local AM broadcast stations.  That's around +10 dBm.  So 
now I want to pick up a weak station at -120 dBm, that means I need at least a 
130 dB dynamic range on my input. To add some headroom let's say you need a 24 
bit ADC with an input which can withstand the high levels and low noise design 
to permit such a noise floor.  To produce a real-life true 24 bit analog system 
is not trivial.  And to get one for HF usage running at 60 MSPS and keep those 
specs would be one impressive engineering feat. Not saying it can't be done but 
why not make the job easier and less costly by doing some filtering and 
pre-processing of the input first?  Filter design has come a long way.

So I was thinking, use an electronically tunable filter and operate wide open 
and do your fast digitization at lower resolution until you want to listen to 
something specific and then your filters center on your area of interest, your 
digitizer may even slow to get higher resolution and better dynamic range, and 
so forth.  OR, you could IF convert and run the digitizer in ultra high 
resolution mode and go down below the noise floor.

Peter



On 9/3/2014 4:05 PM, KD7JYK DM09 wrote:
> :I always thought the best SDR would be one embedded inside a more classical
> : receiver.  Sort of a hybrid approach where the front end has the filtering
> and
> : low noise capabilities and the decoding and analysis are done by DSP and
> other
> : techniques.
>
> I suppose, technically...
>
> I've always envisioned a DC-daylight receiver, or in the case of the Alpha
> RX, 10-14 KHz or maybe wider, it receives everything at once, more than you
> could hear by ear, then use a computer with adequate spectrum analysis
> software to focus on a swath of spectrum.  For example, an RX with 30 MHz
> bandwidth, SEE the entire swath, zero in on a section, maybe 500 KHz wide to
> watch, zero in again to listen to an odd spike every now and then.
>
> In a similar instance, about a decade ago, I had desire to monitor a signal
> on HF that was about 200 KHz wide.  I could hear a few KHz of it, but,
> wanted to see it all.  I had an idea of a direct conversion receiver
> connected to a soundcard of adequate bandwidth, then monitor with software,
> at the time, the best I could get, price-wise, was 96 KHz bandwidth, not
> enough for that project so I dropped it.
>
> Kurt
>
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