## GQRX - unexpected plot when changing frequency range

Main forum to discuss RTL-SDR related topics.
KevinJones
Posts: 9
Joined: Mon May 04, 2020 4:38 pm

### Re: GQRX - unexpected plot when changing frequency range

alanzfq wrote:
Sat May 09, 2020 7:07 am
You do not mention powers, how much power is transmitted? How much power is sent to the RTL?
Is 10mW the maximum before damage is done? It is a very big signal compared with normal antenna levels microvolls, microwatts.
Sorry for the late reply, I have not been getting email notifications (even though I should according to my preferences.)

As I don't know exactly the input to the amplifier, I can only guess at the power transmitted. However I will try to make a reasonable guess based on the following:

If I connect the SDR directly to the same donor antenna the amplifier (repeater) is using, I am unable to distinguish a signal in the frequency range (860 - 894 MHz) hence it is a very weak signal. Thus the reason for using an amplifier, so I can try and detect these signals. So it seems safe to assume then that the input to the amplifier when connected to this same antenna is less than -80 dBm, would it not? (Even if it is just white noise.)

If we go with that, lets calculate some link gains:

The gain of the amp has been measured to be 52 dB. The output is connected to an antenna with about 8 dB max.

-80 dBm + 52 dB + 8 dB = -20 dBm

So I will assume the output from the amplifier, including the output antenna gain is less than -20 dBm.

The input of the SDR is connected to a half-wave dipole 2 feet away.

Lets assume maximum antenna gain of the dipole to be 2.15 dB.

Free space path loss @ 870 MHz @ 2 feet = 27 dB.

-20 dBm + 2.15 dB - 27 dB ~= -45 dB

So I'm figuring at the very most -45 dBm at the input of the SDR.

Please tell me if this treatment seems reasonable to you (taking into account that I can only guess at the strength of the very weak signal I am amplifying.)

alanzfq
Posts: 140
Joined: Thu Oct 04, 2018 11:18 am

### Re: GQRX - unexpected plot when changing frequency range

>So I'm figuring at the very most -45 dBm at the input of the SDR

Very most? I'm not going to look at the maths but -45dBm is 1.25mV into 50 ohms which is a BIG signal.

Alan

KevinJones
Posts: 9
Joined: Mon May 04, 2020 4:38 pm

### Re: GQRX - unexpected plot when changing frequency range

alanzfq wrote:
Wed May 13, 2020 8:30 am
>So I'm figuring at the very most -45 dBm at the input of the SDR

Very most? I'm not going to look at the maths but -45dBm is 1.25mV into 50 ohms which is a BIG signal.

Alan
I'm not getting the point you are making.

-45 dBm is still -45 dBm or .032 uW. The datasheet lists max input power to be 10 dBm or 10 mW.

Are you saying 1.25 mV input voltage is too much for the R820T?

The R820T datasheet[1] only shows maximum input power; AFAICT it doesn't list maximum input voltage. Is there something you know about this that isn't shown in the datasheet? Or some way I'm not looking at this correctly? Something I'm not taking into consideration?

alanzfq
Posts: 140
Joined: Thu Oct 04, 2018 11:18 am

### Re: GQRX - unexpected plot when changing frequency range

As I said 10dBm is a really big signal. The data sheet does not say but it probably represents the maximum before damage is done to the tuner.
What I do know is that antenna signals are measured in microvolts. 1250 microvolts is far more than normally picked up by an antenna. I suggest reducing it, certainly set the gain to a very low figure.
Have you any feel for the receiver? Have you tried FM broadcast signals?
Alan

hotpaw2
Posts: 143
Joined: Sat Jan 14, 2017 11:07 pm
Contact:

### Re: GQRX - unexpected plot when changing frequency range

The bigger the input signal, the more likely the spectrum is to show spurious signals (intermodulation products, etc.). That's because semiconductor devices are non-linear components. They are only approximately linear enough for a range right above the smallest signals detectable. Near the maximum allowed before frying the input, the data is probably all garbage.

The main reason for the popular need for AM and FM bandstop filters is to reduce the total RF input signal level enough to prevent garbage from showing up in lots of other frequency bands. The alternative is to buy really expensive SDRs with 16-bits or more of true dynamic range. Other more expensive SDRs have input attenuators to reduce the RF input level to something more appropriate. An RTL-SDR only has 8-bits. So you have to keep the input levels and RF gain low enough to not saturate the 8-bits (yet another serious non-linearity that will produce garbage data).

Best practice is to reduce the input level to just above the minimum required to detect real signals, and that level should be calibrated against a known signal slightly above the noise floor. Put a 50 ohm dummy load on your antenna input and see what the noise floor level looks like. If you still see lots of strong spurious signals, your RF input level or RF gain is way too high. Maybe try adding RF input attenuation until you can barely see the noise floor.