Re: Baseline

jdow
 

Insert a large attenuator - as much as 30 dB. Of course, really low frequencies will suffer. A sharp high quality 450 kHz LPF will do wonders.
{^_^}

On 20200325 07:41:29, Allan Isaacs wrote:
I do not have any adjustments Sigi, just Gain which has 4 settings: 0, -12, -24 and -30dB; no visual gain. The SDR is the Andrus Mk1.5 which runs over my Ethernet.
Two gain settings 0 and -12dB are not useable with a large aerial due to overloading so I have made a low pass filter which now allows me to use 0 and -12db on the VLF band where the aerial is short in respect of wavelength. If I use that aerial for VLF without a filter I get cross mod and spurii from long wave broadcasts.
73 Allan G3PIY
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*From:*main@SDR-Radio.groups.io [mailto:main@SDR-Radio.groups.io] *On Behalf Of *Siegfried Jackstien
*Sent:* 25 March 2020 13:43
*To:* main@SDR-Radio.groups.io
*Subject:* Re: [SDR-Radio] Baseline
does he have "visual gain" also?!? (depending on used hardware the gui or frontend is different)
here in my pluto settings i have gain (manual) and visual gain (shifts the whole shebang up or down)
with the funcube dongle visual ain is in 5 db steps (simon can you make that a bit finer in 1 db steps like in pluto with a slider?!?)
no need to set cutoff lower to 160... he just have to shift that up with visual gain
dg9bfc sigi
Am 24.03.2020 um 21:05 schrieb Simon Brown:

Allan,

I could reduce the cut-off below -150dBm but I don’t really want to – these signals do seem to be very weak.

Simon Brown, G4ELI

https://www.sdr-radio.com

*From:*main@SDR-Radio.groups.io <mailto:main@SDR-Radio.groups.io> <main@SDR-Radio.groups.io> <mailto:main@SDR-Radio.groups.io> *On Behalf Of *Allan Isaacs
*Sent:* 24 March 2020 18:33
*To:* main@SDR-Radio.groups.io <mailto:main@SDR-Radio.groups.io>
*Subject:* [SDR-Radio] Baseline

I'm looking at signals below the broadcast band so I made a low pass filter which is set to around 110KHz to avoid cross modulation etc. from broadcasts.
It gives me an attenuation of 40-50dB at 190KHz and broadcast stations are so strong they are easily audible even with 50dB attenuation from the low pass filter..
I now notice my Andrus behaves a little differently with the filter in place. I cannot see baseline noise, just a flat line if I increase the gain to 0dB or -12dB..
Previously, any gain setting of greater than -24dB resulted in cross modulation of VLF signals so the filter is doing its job.

My question:  Is it possible to reduce the display cut off  below -150dBm which is position of the flat line?
I can add -160dBm using View, Scale, Low but the flat line stays at -150dBm
If I set the gain to say -24dB or -30dB from 0dB the baseline noise becomes visible but signal to noise is poor. Increasing the gain to -12dB improves S to N and to 0dB improves S to N even more but both settings give me a flat line as in the attached picture. Am I missing a setting or is it a limitation of an AD converter?
Allan G3PIY

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