Baseband Noise Level with No Antenna

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kiheiman
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Joined: Thu Jan 31, 2019 8:32 pm

Baseband Noise Level with No Antenna

Post by kiheiman » Thu Jan 31, 2019 8:39 pm

I have just installed a RSP1A receiver. Everything seems to be working good. I was expecting the baseband noise level to go down to somewhere in the range of -110 dbm to -120 dbm, with no antenna attached. On 40M, the noise level is about -100 dbm with or without an antenna. I checked the calibration of the spectrum display with my signal generator. The display is very accurate at -73 dbm. At -10 dbm, the level is about 2 db low which I think is good.

Two questions:

What noise level should I see with no antenna?
Shouldn't the noise level increase when I connect an antenna?

Regards
Clint
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Tech_Support
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Re: Baseband Noise Level with No Antenna

Post by Tech_Support » Thu Jan 31, 2019 10:17 pm

Hello Clint,
The power meter in SDRuno measures the equivalent RF input level, which includes all signals and noise from the antenna AND self generate noise from the receiver. Check that your gain is at maximum (minimum RF Gain reduction), as varying the RF gain slider in SDRuno will effect the equivalent input noise level because it effects the receiver noise figure. Please also note that the power is measured within the demodulator bandwidth and so the wider the selected bandwidth, the higher the noise. Please don't confuse the visual indication of the noise level on the SP1 display with the actual noise floor. The visual indication of the noise floor is the noise power measured within the resolution bandwidth of the FFT used for the SP1 display and will typically be much lower than the bandwidth used by the demodulator.

Sincerely

Tech_Support

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glovisol
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Location: Piedmont, Italy

Re: Baseband Noise Level with No Antenna

Post by glovisol » Fri Feb 01, 2019 1:09 pm

Hello Clint,

You can get a lot of information about noise, antenna noise etc. from these threads:

https://www.sdrplay.com/community/viewt ... f=5&t=3436

https://www.sdrplay.com/community/viewt ... f=5&t=3352

https://www.sdrplay.com/community/viewt ... f=5&t=3723

Cheers,

glovisol

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kiheiman
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Re: Baseband Noise Level with No Antenna

Post by kiheiman » Fri Feb 01, 2019 2:20 pm

Thanks for the info. I should have been more specific. I was using the latest version of the HDSDR software. It has been two years since I have played around with the HDSDR, SDRUno, and SDR Console software (I was using Windows 7). At the time, I preferred the HDSDR software. For my latest testing, I am using Windows 10. It appears that either I have something setup wrong in HDSDR or it is generating a high baseband noise level with no antenna connected. I was able to reduce the baseband noise level to about -110 dbm by dropping the RBW down to its lowest value. However, there is still no difference is the baseband noise level with and without the antenna.

I installed the latest version of SDRUno. Immediately upon fireup, it is reading about -115 dbm baseband noise level on 40M with no antenna. With an antenna, the noise level goes to about -110 dbm. I found the signal level tracking to be very accurate - within 1 db of actual from -90 dbm to 0 dbm.

With my Elad FDM Duo, I see a baseband noise level of about -130 dbm with no antenna and the level goes to -110 dbm with an antenna connected. Do I think the difference between the noise readings on the RSP1A and the Elad Duo is due to the better sensitivity of the Duo receiver.

Regards
Clint
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Tech_Support
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Re: Baseband Noise Level with No Antenna

Post by Tech_Support » Fri Feb 01, 2019 4:15 pm

Hello Clint,
You still have not said what your RF gain settings are. The noise on the RSPs will vary depending upon how the RF gain slider has been set.

HDSDR is a completely uncalibrated system and absolute readings or noise or power are meaningless. They will vary from device to device depending upon its noise level and gain. All you can really compare is the SNR for two different devices. Changing the RBW is meaningless on its own because noise is a function of bandwidth. Any device of a given noise figure will show a noise floor that will increase by 3 dB for every doubling of the RBW. That is because noise power is a function of bandwidth. That is why I recommended using the power meter in SDRuno for comparisons as the bandwidth is defined by the bandwidth of the demodulator.

SDRuno gives calibrated power readings , but the only way to compare different RSPs is when the gain is set to maximum (or RF attenuation set to minimum).

I suggest that you post screen shots to illustrate what you are seeing as without additional information on settings, it is impossible for anyone to help with your questions.

Sincerely

Tech_Support

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kiheiman
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Re: Baseband Noise Level with No Antenna

Post by kiheiman » Fri Feb 01, 2019 6:43 pm

Hi - Sorry I did not give you the info that you needed. In SDRUno, the RF gain is set to max. This app appears to be working correctly for displaying baseband noise levels. Please see two attachments. The baseband noise with and without the antenna is about what I see with other SDR radios. In the past, I have always found HDSDR to also be accurate with signal levels once the S meter was calibrated. This current issue could be related to Windows 10.

Regards
Clint
Attachments
SDRUno without Antenna.jpg
SDRUno without Antenna.jpg (203.15 KiB) Viewed 2138 times
SDRUno with Antenna.jpg
SDRUno with Antenna.jpg (202.73 KiB) Viewed 2138 times
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Tech_Support
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Re: Baseband Noise Level with No Antenna

Post by Tech_Support » Fri Feb 01, 2019 8:01 pm

Hello Clint,
Your power meter indicates a noise power of -123.2 dBm in a 2800 kHz bandwidth, which is an equivalent input noise of -157.7 dBm/Hz. This would equate to a noise figure of around 16 dB at 7 MHz, which compares very closely to the specified NF of 15 dB @ 12 MHz. Technically, one should add the noise power from a 50 ohm source to get a true value for the noise figure, but the error is so small as to be insignificant.

Your noise floor with the antenna disconnected is around 33dB lower than the noise floor from your antenna system, so the RSP1A is adding an immeasurably small amount of noise to your receiving system at these frequencies. So I believe what you are showing is completely correct and the performance is good.

As you reduce the RF gain, you will see the noise floor increase. That is because the NF of the RSP1A is increasing in-line with what you would expect. This will be more obvious with the antenna disconnected as you would need to see an increase of nearly 30 dB to the RSP1A before you would see the noise start to increase with the antenna connected.

As far as HDSDR is concerned, the power reading after calibration will only be accurate at a fixed level of gain, if you have AGC enabled or adjust the RF gain setting, it will immediately throw out the calibration as HDSDR does not automatically correct for changes in the gain in the way that SDRuno does.

Sincerely

Tech_Support

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