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Page history last edited by David-Taylor 7 years, 10 months ago Saved with comment

Reception Issues


Why is the spectrum display limited?


Q: I have a FUNcube Dongle Pro+ working just fine with SDRSharp, but am curious as to why the spectral display span is limited to about 250 kHz? 


A: The IQ data coming from the dongle's A/D converter has a 192 kHz sampling rate in the FCDPP.  This means you only can see signals +/- 96 kHz from the centre. (from: Ray  WB6TPU)


Note that the TV dongles have a typical maximum usable sampling frequency of 2.4 MHz, allowing a display approaching +/- 1.2 MHz.



How to set the frequency offsets so that the display is correct


Q: I was wondering which offset is the best to use to set SDR# to the correct frequencies.  There are two offsets, one is the ppm setting in the configure options for the USB dongle and the other is called shift within SDR#


  • If I use the one in software will it always be correct or would it drift at other frequencies?
  • Also would any stored frequencies will need to be edited otherwise the frequencies will be off in the database?

At the moment I have managed to get the frequency displayed correctly for a ham repeater on 2m band, but if i was to tune to a frequency at 900 MHz would this be off the correct reported frequency because of the shift?  (from:  diddyuk12)


A: Thou art agonizing something simple into something very complex.  The ppm setting is the tool for correcting for errors in the crystal in the dongle.  That appears to stay nice and constant for dongles I know of.  The "Offset Tuning" button in the Configure dialog is a clever way of moving the "zero frequency bump" out of the centre of the spectrum range the receiver can receive.  This makes the natural tendency to tune to the middle of the spectrum display work right.  The Shift checkbox and edit box are there for when you are using a frequency converter in front of the dongle, perhaps to gain HF coverage or use a down converter to gain some higher frequency coverage.  It should be set according to the frequency converter's settings and crystals.  (Joanne, W6MKU)



What standard frequencies can I use?


There are note on some suitable off-air frequency "standards" here.


For RTL dongle users, you can also use tools which calibrate against local GSM stations both for Linux and Windows:







What is that central spike?


Please see this discussion.

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