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RTL 2832u-R820T SDR as a Spectrum Analyser ?

Discussion in 'General Technical Questions and Answers' started by SV1LJU, May 17, 2013.

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  1. SV1LJU

    SV1LJU Ham Member QRZ Page

    RTL 2832u-R820T USB DVB-T : less than 20 Euros dongles are easily found through internet .

    It is not new that these dongles may be converted to a 20MHz-1.7GHz SDR replacing their driver by those of special Freeware developed by MIT people.

    The project is simple:

    1.....Order a RTL 2832u-R820T USB DVB-T for less than 20 Euros ( mine costed 12 Euros

    2.....Install the supplied drivers (application SW is not required since it will not be used for watching TV)

    3.....Dowload SDRSHARP (SDR#) Freeware from especially for windows download unzip and run install.bat. All SDR# files will be downloaded and installed.

    4.....Plug the USB DVB-T in to the computer.

    5.....Run the file zadig.exe located within the SDR# installation folder. Click on its screen menu OPTION and List All Devices, Select your RTL2832u USB. Take care to not select any other RTL device like WIFI or sound card already installed in your computer because you may ruine its drivers. Then click Install WCID Driver.

    6.....Run SDR# application, select RTL SDR USB as input device and click Play. That's all.

    As you will immediately realise you just got:
    • An SDR radio with the following specs:
      • Frequency range 25 KHz to 1.7GHz
      • Sensitivity (noise floor) -114dbm ( installing the initial SW the noise floor will be much higher than -114dbm but the device is really working at such sensitivity I will let you know later how to fix the screen offset power level. This is because the SDR# supports many other radios and FFT offset level is preset very high to work even with practically "deaf" ones)
      • Instantaneous reception BW selectable from 25 KHz to 2.4 GHz if you computer is fast.
      • Modulations AM, SSB, DSB, FM, (USB, LSB, CWUSB, CWLSB, NBFM, WBFM actually the demodulation filter is fully adjustable up to 100ds of KHz).
    • A radio Scanner with spectrum and waterfall displays. Additional SDR# plugin freeware needed for that, with programmable auto scan and log capability. I mention that to let you thing that a 750 Euros SDR-14 with SPectravue application SW doesn't.!!! The performance of DVB-T and SDR-14 is not comparable but I just mention the poor SW application of the SDR-14.
    • ADSB radio, and monitor installing additional Freeware.
    • An accurate Frequency Meter 20KHz to 1,7 GHz which can be calibrated setting the frequency correction of the R820T through the SDR# SW. And with a range of -30 up to -114dbm! Above -30dbm R820T intermodulation starts. But frequency can still measured efficiently. And in case that you burn it, you just loose 20 Euros. Actually it does not require to be connected, approaching just a little sniffer wire will measure.
    • A Spectrum analyser as we will see later.

    At this point I will shear my experience on this device and strongly advice you to get one or more the sooner possible.

    • SDR# Software To my view is the best basic all in one SDR-scanner-logger SW application. A really one screen basic SDR - Spectrum & waterfall, Frequency logger, and IQ or audio out pout recorder application, almost all the functionality is visible and accessible through its main screen. If you are familiar with Spectrum LAB application then SDR# is the absolute opposite.
    • I have tested other versions of DVB-T tuners but the R820T is the best to my opinion because:
      • It has wide and with better resolution manual gain adjustment range which is very useful (vital i would say) for those wide open without front end filters receivers.
      • It has broader Frequency Range.
    • I didn't even think to try an UP conversion front end to receive HF bands, as may others did because reception in HF is based in front end filtering, however using the DVB-T at the 1st IF (HF band up conversion) of regular HF radios, is really an excellent project where the front end of the radio is combined with this tiny SDR. However the reception BW is limited to the BW of the 1st IF filter or the front end passive filters depending where you will decide to take the IF from (before or after the 1st IF filter. My view is that adding this tiny SDR to an old high performance HF radio is for a broad spectrum view just to survey the stations. So I would rather take the IF before the filter. SDR# frequency shift will allow you to offset the HF radio local oscillator for correct frequency display.
    • Strong signals will intermodulate (anything above -30 dbm). You will immediately notice the IMD products by moving (draging) the frequency left or right but reducing the R802T gain will take care of the problem. I strongly recommend not to set auto AGC neither on the tuner R820T nor on the RTL2832.
    • Best broad band scanner antenna for your RTL SDR is a small discone antenna. I live at 3.5 Km visual contact of the broadcasting antenna park of Athens, with several hundreds of KW from the FM and the UHF TV stations. Constructing a discone antenna with lower frequency at 700 MHz was a nice solution offering nice reception above 80 MHz up to 1.7 Ghz,
    • You can definitely use it as Frequency Counter on CW AM, SSB, FM signals but on pulsed emissions the FFT SW is inadequate for detailed spectrum analysis.
    • Finally USING the DVB-T as Spectrum Analyser Is feasible with nice Frequency Response +/_ 1.5 dbm between 25 and 600 MHz with some limitations on pulsed signals. Monitoring the air surveillance radar of the area at 1.2 GHz you will not be able to measure the PRF nor the PW like Spectrum Analysers do.

    Calibrating the DVB-T, there are two ways to achieve this. Either you get any DVB-T and calibrate it your self or you get the same mentioned above and skip the calibration relaying on the measurement results I provide here.

    Frequency calibration You just use a portable VHF/UHF radio or tune the SDR to the APRS frequency 144.800 or any known frequency station and adjust the R820T ppm offset, through the frequency correction screen.

    Amplitude Calibration

    If you don't intent to use calibration equipment then you just follow these simple steps:
    • Edit the SDRSharp.exe.Config file located in the SDR# folder as follows:

    <?xml version="1.0" encoding="utf-8" ?>
    <section name="frontendPlugins" type="System.Configuration.NameValueSectionHandler,System, Version=, Culture=neutral, PublicKeyToken=b77a5c561934e089" />
    <section name="sharpPlugins" type="System.Configuration.NameValueSectionHandler,System, Version=, Culture=neutral, PublicKeyToken=b77a5c561934e089"/>
    <appSettings file="">
    <clear />
    <add key="fftOffset" value="-97.0" /> ​Only on this line change the pre-set value to -97.0
    <add key="automaticIQBalancePasses" value="5" />
    <add key="pllPhaseAdjM" value="0.0" />
    <add key="pllPhaseAdjB" value="-1.75" />
    <add key="deemphasisTime" value="50" />

    • Set the R820T BW at 0.25 MSPS.
    • Uncheck the RTL and Tuner AGC at the configuration screen and set manualy the gain to 29,7db

    If everything is ok you will see the noise floor at -114dbm and the DVB-T is set exactly as the one which gave the results that follow bellow. If not you need to check if the DVB-T dongle is the same with the one tested here. Inject a -30dmb signal anywhere bellow 600Mhz and varying the level, and the tuner gain check if the specific dongle gets IMD products at -30 dbm. Then note the indicated power level on the SDR# screen and compensate the fftOffset value above accordingly to read -30dbm.

    If you intent to calibrate it then you probably have the knowledge.

    For that you need equipment covering the entire frequency range 20-1700MHz. Some of them is mandatory to be calibrated, others not. Several combinations may be used:
    • A signal generator with a calibrated attenuator and a calibrated power meter.
    • A calibrated signal generator with a calibrated attenuator.
    • A signal generator and a calibrated spectrum analyser.
    • A calibrated noise source and a calibrated attenuator.

    The measurements bellow are the result of various setups many of them overlapping-duplicating.
    • Sweep Generator S3330 Nordmende 3-860 MHz, 0 dbm DC transfer standard, and external attenuator calibrated from 0 to -80 dbm.
    • Calibrated generator HP 8614A 800-2400MHz with its attenuator calibrated from 0 to -120 dbm
    • Calibrated spectrum analyzer HP141T/8555A section 10Mhz to 18 GHz with its attenuator calibrated up to -110dbm.
    • Calibrated Noise Source dc to 45OMHz at -80,1 dbm
    • Calibrated Noise Source dc to 45OMHz and attenuator calibrated from 0 to -80 dbm
    • Sweep generator S3330 and SDR-14 radio as power meter which is not calibrated but agrees nicely with the rest of the devices only on the direct input 0-30 MHz.

    The chart shows that the DVB-T had been tested at two levels -40 and -80 dbm and is performing very well up to 600 MHz. Then some front end low pass filter is probably affecting the results. AT 800 MHz is -3 db deviation witch is normal for a UHF usb dongle trying to avoid nearby GSM or WIFI signals. May be trying to locate and bypass the filter is a good idea. Additionally to the frequency response measurements at -40 and -80 dbm, the linearity of the power level display has been measured at 28, 150, 500, MHz and found to be accurate above through -110 dbm to -30dmb at 10 db increments.

    Any duplication results, on other USB DVB-t dongles would be much appreciated?
    Last edited: May 17, 2013
  2. K4AX

    K4AX XML Subscriber QRZ Page

    These sticks have taken up a lot of my time recently. I've never been much into VHF/UHF but I like using them on HF. My current fave setup is using one with my FT-857 as a second receiver and spectrum display being tuned into the first I.F. of the rig.
    I've also using one with a the Hamitupcoverter v1.2 and that works pretty well. DC to ghz all mode receiver that can be crammed into a tiny box.
  3. KA9JLM

    KA9JLM Ham Member QRZ Page

    Nice information. Thanks for posting the info.

    A little comment on; "You can definitely use it as Frequency Counter on CW AM, SSB, FM signals but on pulsed emissions the FFT SW is inadequate for detailed spectrum analysis."

    I do not think the stability of these devices make it useable as a Frequency Counter for any mode. Close enough for government work maybe ?

    For the price, They are a fun play toy.

  4. SV1LJU

    SV1LJU Ham Member QRZ Page

    Dear friend those sticks have a crystal oscillator visible if you open the stick and the SDR# software application RTL dongle configuration screen has frequency correction adjustment as ppm over a known signal... I tested the accuracy with HP8614 and freq counter and if the RTL dongle frequency correction is set then the accuracy of the dongle is much ...much adequate for amateur use. Other wise you can tune it to a VHF radio you trust and set the frequency correction. I use to set the correction with a 100 MHz comb generator i have.

    Thanks for the comment
  5. KA9JLM

    KA9JLM Ham Member QRZ Page

    I understand what you are saying , it is close enough for government work.

    Temperature drift is what I am talking about.

    If you have a Spectrum Analyzer or frequency counter to cal with every time you use it then it works just fine.

    Many spectrum Analyzers do not make good frequency counters either.

    It all depends on what your tolerance needs to be. For the money, you can not beat it.

    Have Fun Everyone.
  6. K4AX

    K4AX XML Subscriber QRZ Page

    I've thought about making a oven for mine, if I ever get around to putting it in some sort of box. I keep my place pretty cold as I don't seem to have a central nervous system according to my female friends. So every time the stick gets powered up I have about a 5 minute wait for it to "warm up" to it's frequency. I double check it to WWV usually. After it's warmed up, it stays dead on the carrier. When first powered on it drives upwards in frequency about 500hz. It's not been a big deal for me as most of the time it just stays running with HDSDR on a second monitor on my computer 24 hours a day.
  7. SV1LJU

    SV1LJU Ham Member QRZ Page

    I confirm your experience and playing with the latest low price stick form china which realy overheats the frequency slide is much lower than generating problems in demodulating NBFM or AM with preset frequencies.

    What I would also like to note is that I found great diferences on intermodulation behavior between varius sticks. The R820 tuner has greater manual gain adjustment range and more steps the worst was some FC0013.

    Last week I made my HF 37 MHz upconverter with the SA602 mixer-oscillator chip. My experience was that the dongle reception performance was worst due to powerful AM broadcasting signals of the area and the 2 MHZ open bandwidth. The situation gets better reducing the samplig rate of the RTL2832 and the bandwidth.

    Also conected the dongle after the mixer of a RACAL MRR5 radio that I have, just after the first IF mixer (34.5MHz), before and after the 5Khz BPF.

    Next step is to play with the RTL2832U direct input, exactly as the SDR14 radio works , direct analogue to digital convertion. In such a case i intent to connect it just after the second IF 1.5 MHz of the MRR5 radio . I expect significantly reduced bandwidth but the benefit will be that I will have LSB demodulation which was NOT an option for the Greek army procurement!!!

    Using the direct sampling input of the RTL2832 I expect to receive frequencies higher than the sampling rate based on harmonics reception like with the SDR-14 direct input. Nice explanation is available at taking in to account that the SDR14 sampling rate is 66MHz and the maximum instantaneus BW is 33.3MHz (Niquist). So lets see what will be the RTL2832 direct sampling BW. Any experience on that is welcomed.

    Keep playing.
  8. KD0CAC

    KD0CAC Ham Member QRZ Page

    This looks like a good hack ,
    Where's Sue :)
  9. AA7EJ

    AA7EJ Ham Member QRZ Page

    many thanks for the article. Just ordered mine.
    Checked the e-bay and got few laughs.
    The "spec" is hilarious!
    I think they will run out of them soon since they are smartly advertized as "cheap", at least in US market. Just followed another “ham discussion “ here and observed "it is cheap so I'll buy it even if I don’t need it". At least this goup of hams appeared to be that kind of buyers.

    73 Vaclav
  10. SV1LJU

    SV1LJU Ham Member QRZ Page

    After all its a hobby and any one who does not intent to acquire an expensive spectrum analyzer or scanner receiver, can satisfy more than his curiosity playing with it.
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