[Lowfer] SDR for LF QRSS?

[Garry and Linda Hess]

Lowfers,

Is anyone successfully using a software-defined radio (for example, the 
RFSpace SDR-IQ) for QRSS reception at low frequencies?

Although such radios respond down to nearly DC, I don't see how QRSS can 
work, unless one is blessed by VERY quiet conditions, or the SDR has 
something equivalent to the typical 3 kHz sideband passband of an analog 
radio. Such selectivity could be implemented in an SDR IF but isn't, at 
least in the relatively inexpensive ones I know of.

This is not an overload issue. That can be addressed by (relatively 
wide) filters ahead of the SDR. For example, a 130 kHz high-pass filter 
to protect against Loran-C overload and a 540 kHz low-pass filter to 
protect against AM broadcast overload. That would allow one to tune from 
2000 meters to the top of the LF spectrum. But, when a single lightning 
burst occurs, the relatively wideband SDR front-end puts out lots of 
energy to the PC demodulation software even though it does not 
overload/clip. That results in a bright line across the waterfall. To 
many static bursts with time and the waterfall is useless. Yet, under 
those same useless conditions running ARGO off the audio output of an 
analog radio yields fault-free waterfalls. Am I missing something about 
the AGC/blanker settings of software like SpectraVue or Spec Lab that 
can prevent this? I do implement 5 Hz BPFs with Spec Lab to process 
NDBs. Perhaps running a second waterfall after that filtering is needed.

73,
Garry, K3SIW, EN52ta, Elgin, IL