SDR - Bandwidth

Main forum to discuss RTL-SDR related topics.
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SDR - Bandwidth

Post by FlyMario » Thu Sep 06, 2018 5:54 pm

I have been playing lately with SDR and I am so confused. How is it that an SDR can see signals several MHz above and below the primary frequency? Is there some site or Reference that you guys know of that can maybe explain this? I play with GNURadio and it seems that one source could see a wide range.

I was always under the impression that there was an oscillator that would tune to the frequency you desired in radios.

Thanks for any information you can provide!


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Re: SDR - Bandwidth

Post by w5dxp » Fri Sep 07, 2018 1:55 am

I bought a $10 Kindle ebook that has a wealth of information and I recommend it for questions like yours. ... he+RTL-SDR

If you go there and click on "Look inside", you may find the answer to your question.

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Re: SDR - Bandwidth

Post by hotpaw2 » Mon Sep 10, 2018 4:55 pm

Even an analog AM radio has a non-zero bandwidth, usually around 10 kHz wide. An SDR is designed and built with much wider filter bandwidths than your typical analog AM/SW radio. An SDR also uses much higher sample rates (240 ksps to 3.1 msps in the case of a typical RTL-SDR) compared to typical audio output DACS at 44.1 or 48 ksps.

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Re: SDR - Bandwidth

Post by snn47 » Wed Oct 03, 2018 5:55 am

To help you get an idea what to look for

There are different types of receiver designs, with and without selectivity

1. a single resonant circuit tuned to the the receive frequency and a dector diode works for AM-BC if BC stations are separated sufficiently

Bandwdith and therefore receiver RF selectivity to suppress adjacent signal is very limited and won't work on much higher frequencies.

2. Solution 1, you convert the input frequency range to one or more intermediate frequencies (IF) with the Local Oscillator (LO). At the IF you have one or more filters, where you can suppress adjacent signals by filtering. IF range start at 455 kHz in cheap AM radios to about 70 MHz, the wider the signal the higher the IF needs to be.

3. you can use direct conversion and apply AD and a lot of software instead of hardware. For SSR, CW a AD in a soundcard is enough, in SDR you need video AD or better.

While you are limited with filters by the bandwidth, you are less limited with receiver designs using AD-conversion designs (>520 MHz in newer SA), but still you are limited by the available AD resolution which can be used to maximise bandwidth or sensitivity never both.

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Re: SDR - Bandwidth

Post by alanzfq » Thu Oct 04, 2018 11:43 am

An SDR does not have to be broadband. For reception of a single signal a low sample rate and say 24 bit Analogue to Digital Converter (ADC) resolution could make a very good narrowband SDR receiver.
A wideband SDR needs an exceptionally good ADC to get really good reception.
Fundamentally there is not a lot of difference between a SDR and a conventional receiver. The display simply shows the signals present at the mixer/ADC and enables you to select one. However the display may well be too wide and you have to zoom in to make that easy.
After the mixer, or ADC, the software defined part takes over. Instead of a single channel IF/filter/detector all is done in software. With a wideband SDR some software programs can provide a number of different channels, all with individual filtering and detectors. The ADC sampling rate and performance of the computer make the limits.
By today's standards the RTL is not wideband, nor does it have a good ADC but even so produces quite good results.

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