Direct Sampling

Main forum to discuss RTL-SDR related topics.
Post Reply
geeds
Posts: 3
Joined: Sat Feb 22, 2020 2:58 am

Direct Sampling

Post by geeds » Fri Mar 27, 2020 11:27 am

Could someone please explain the difference btw direct sampling and quadrature sampling with the RTL-SDR V3
How come when I'm looking at the FM band with CubicSDR or SDR#, with direct sampling off I get the local FM stations but with direct sampling on (Q branch) the FM channels disappear and AM channels from Asia show up.
I have a good filter to stop local AM (0-2mHz) and an LTE (850mHzLP) filter.
My fm filter doesn't seem to have much effect with direct sampling. Whats going on???
I'm also having trouble finding a linux spectrum analyser that does direct sampling down to 0mHz. Anyone have any recommendations?
thanks

Quadrature Sampling
fm-quad.png
fm-quad.png (173.38 KiB) Viewed 1091 times

Direct Sampling - Q
fm-QBranch.png
fm-QBranch.png (289.26 KiB) Viewed 1091 times

alanzfq
Posts: 112
Joined: Thu Oct 04, 2018 11:18 am

Re: Direct Sampling

Post by alanzfq » Fri Mar 27, 2020 4:55 pm

Perhaps you need to read up on what you bought?
Direct sampling is used for reception below 24MHz
Alan

geeds
Posts: 3
Joined: Sat Feb 22, 2020 2:58 am

Re: Direct Sampling

Post by geeds » Sat Mar 28, 2020 5:31 am

Thanks for the advice.
From the description it says "the raw I/Q data on the RTL2832U chipset could be accessed directly,"
IMO so that the 24mHz restriction on the R820T2 is bypassed. It doesnt mention anything about not going over 24mHz.
That aside, Could someone tell me what I'm seeing? Is it a reflection? Something to do with the 28.8mHz osc?
Is it in the cable or a artifact of the hardware/software?

alanzfq
Posts: 112
Joined: Thu Oct 04, 2018 11:18 am

Re: Direct Sampling

Post by alanzfq » Sat Mar 28, 2020 8:34 am

https://www.rtl-sdr.com/buy-rtl-sdr-dvb-t-dongles/

Please read this

HF Direct Sampling Mode - Listen to 500 kHz - 24 MHz with direct sampling. Simply connect an HF antenna to the SMA port, and choose the Q-branch mode. There is a built in 25 MHz low pass filter, but additional HF filters may be required for optimal performance (e.g. to attenuate strong MW AM).

The RTL is cheap, performance not amazingly good. Strange effects can be observed when not operated with care.

Alan

geeds
Posts: 3
Joined: Sat Feb 22, 2020 2:58 am

Re: Direct Sampling

Post by geeds » Sat Mar 28, 2020 9:14 am

Ok, it seems to just repeat every 28.8mHz.
I'll accept that and move on
cheers

radiolistener
Posts: 75
Joined: Thu Jan 25, 2018 10:48 pm

Re: Direct Sampling

Post by radiolistener » Wed Apr 08, 2020 12:43 am

Direct sampling and quadrature is different mode. When you use direct sampling you will get the data from ADC directly connected to antenna (actually through some filter). RTLSDR ADC works at 28 MHz. So, with RTLSDR direct sampling mode you will get just 14 MHz bandwidth. All frequencies above 14 MHz is mirror frequencies for 0...14 MHz range. Also it is merged with aliases above 14 MHz which is mapped to 0...14 MHz range.

When you use quadrature mode, it uses RF tuner before ADC. This RF tuner shift down frequencies to 0...14 MHz range. For example when you tune LO = 104 MHz it means that you will receive 104 MHz as 0 Hz frequency, 105 MHz as 1 MHz, etc.

So, when you use direct sampling, it don't shift frequency down and this is why you're listen different frequencies. In short, just don't setup LO above 14 MHz in direct sampling mode, because it is useless. When you setup LO=15 MHz, you're actually listening for 14+14-15=13 MHz...

You can use aliases in order to listen stations above 14 MHz. In order to do that just tune LO to 28-F frequency. For example in order to listen for 15 MHz you can tune LO = 28-15=13 MHz. In order to listen 16 MHz, you can tune LO = 28-16=12 MHz, 17 MHz = 11 MHz, 18 MHz = 10 MHz 19 MHz = 9 MHz, etc...

But note, that you will hear 13 MHz and 15 MHz simultaneously, this is why you're needs to use bandpass filter on the RTLSDR input in order to reduce interference.

Post Reply