SDRplay RSP2:
It goes remarkably low:
- High-Z input: 1 kHz – 30 MHz
- SMA antenna ports A & B: 1.5 MHz – 2 GHz
- So the overall quoted coverage is 1 kHz – 2 GHz.
For your VLF work, the important part is the High-Z input. It has a nominal 1 kΩ balanced input and a 30 MHz low-pass filter. SDRplay specifically designed this input for low-frequency reception.
So, for example, your 20 kHz VLF signals are comfortably within its specified range.
One caveat: although specified down to 1 kHz, sensitivity falls off substantially at the very lowest frequencies; an ARRL test noted declining sensitivity below about 100 kHz.
For your VLF loop, I can also explain exactly how I would connect your loop to the RSP2 High-Z terminals, including whether you need impedance matching.
RSPduo:
The SDRplay RSPduo is better than the RSP2 for very low frequencies.
- Overall coverage: 1 kHz–2 GHz
- Tuner 1 SMA (50 Ω): 1 kHz–2 GHz
- Tuner 2 SMA (50 Ω): 1 kHz–2 GHz
- Tuner 1 High-Z input: 1 kHz–30 MHz, balanced 1 kΩ.
Importantly, SDRplay says the RSPduo’s low-frequency performance was improved over the RSP2. It has a 0–2 MHz low-pass filter on the 50 Ω inputs, and the High-Z input has selectable 0–2 MHz or 0–30 MHz low-pass filtering. SDRplay specifically says the High-Z port should give the best LF/MW performance and be superior to the RSP2 High-Z input.
So for your ~20 kHz VLF work, I’d choose the RSPduo High-Z input rather than an RSP2.
It should also be a very interesting receiver for your VLF loop because the 1 kΩ balanced input is much closer to the sort of interface you’d want than a conventional 50 Ω SDR input.