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Calibration

Page history last edited by David-Taylor 9 years ago Saved with comment

Frequency calibration

 

Using the SDR# software:

 

On the Configure dialogue (on SDR# press the Configure button at the top), set the frequency correction to the best value.  On my system it's -2.1 on the FDCPP.  To calibrate yours for HF, try calibrating against a local BBC FM station, or major airport ATC control tower or approach.  Tweak the calibration whilst watching the display, and set the correction such that the received signal is centred at the nominal frequency.  It should be obvious as soon as you try it.  Note that the Swap I & Q box needs to be checked for the FCD and FCDPP.

 

If you have an FM station which is mainly speech, or has really quiet periods, you can use the spectrum display of e.g. SDR# zoomed in close so that the display covers a range of about +/- 10 kHz, and watch for the spectrum peak during a period of silence.  Using the example below (an original FCD looking at BBC Radio 3 in Central Scotland):

 

 

you can estimate that the frequency is 1.4 kHz low at 92.1 Mhz, so that's about -15 ppm.  If using a DVB-T dongle, the error is likely to be considerably greater - as high a +/- 100 ppm.

 

If you are in the USA you might use the National Weather Service broadcasts which are on 162.400 to 162.55 MHz from over 1000 transmitters.  There is a list of frequencies here, and tables of frequencies versus locations here.  If you don't know your local transmitter frequency, you have to hope that your unit is not more than 150 ppm out, but that's unlikely.  Thanks to Dan Egan and Fred Hillhouse Jr for that information.

 

The major ATC ground stations (not necessarily the vehicles or aircraft) at major airports should be within 1 kHz of the correct frequency.  For more accurate frequency alignment, you could use an HF standard frequency transmission (e.g. the RWM signals on 4.996, 9.996 and 14.996 MHz) when you calibrate for zero beat i.e. the same audio frequency when switching between LSB and USB modes (is this true for all SDR programs?), or use a VHF or UHF beacon transmitter which you know to be correct (e.g. maintained with a GPSDO).  Note that the temperature coefficient of the FCD may make calibration closer than about 1 part per million (ppm) pointless for general shack use.

 

Eddy, PE9GHZ, notes: Calibration in each SDR program is done separately (actually you calibrate the software for the (small) TCXO offset that each dongle has.  Calibrate to the highest possible frequency available to you, e.g. a local UHF beacon on 70 or 23cm, then all lower frequencies are also correct.  Once you know the correct offset (ppm value) of your device for one SDR program, you can apply this offset in other SDR programs too.

 

Some frequencies for the UK (from: this page), accuracy not vouched for!  Be aware that some stations may have intentional offsets, so take as many readings as you can and disregard any outliers.  There's another list here, but again of unknown accuracy.

 

Location
Frequency MHz
Airports covered
London Main 135.37 Amsterdam, Brussels, Dublin, Glasgow, London LGW, London LHR, London STN, Manchester, Paris CDG
London North
126.6
Blackpool, East Midlands, Leeds Bradford, Liverpool, London LGW, Manchester, Newcastle, Isle of Man, Teeside
London South
128.6
Birmingham, Bournemouth, Bristol, Cardiff, Jersey, Luton, Norwich, Southampton, Southend
Scottish
125.725
Aberdeen, Belfast BFS, Edinburgh, Glasgow, Inverness, London LHR, Prestwick, Stornoway, Sumburgh
Dublin
127.0
Dublin, Shannon, Cork, Belfast BFS, Glasgow, Prestwick, Manchester, London LHR, London LGW

 

From DJT's own listening, and I'd love to add others....please let me know of accurate stations.

 

Edinburgh ATIS
131.350 MHz
Edinburgh

 

Be aware, though, that some ground stations may operate with an offset to moderate interference between stations nominally on the same frequency, a system kniwn as CLIMAX.  This will only apply to 25 KHz stations at the moment.  It would be helpful if anyone has a list of these offset ground stations.  This doesn't apply to aircraft senders so, ironically, they may provide a more accurate average frequency.  Back in 2007 there were 136 UK stations operating with offset (see this document), but the current situation is unknown. 

 

You should be setting the frequency offset to zero, and just using the PPM value to adjust the frequency scale of the dongle.  If most stations appear correct (BBC main FM stations, most ATC ground and most aircraft), but just the odd station appears a few KHz off, likely the offset is intentional, and not a fault of the dongle or your calibration.  I haven't seen corrections of more than about +/- 120 ppm needed, though.  Remember the dongles are consumer devices, not lab precision instruments!  The FUNCube Pro/Pro+ should be much better.

 

Sigi, DG9BFC notes: And another hint ... in many (not all) SDR software you only see the calibration after you changed the L.O. frequency (tune up and down again 1 or 10 kHz ... it does not matter how much ... just a change up and down ... and then you see the result of changing the ppm value.  In some SDR software you see the result direct (example SDR#), but in HDSDR and others you have to do the up-down-tuning to see the change.

 

Using an up-converter

 

It goes without saying that if you are using an up-converter, you will need to know the exact frequency of the conversion, and it is not sufficient to calibrate the FCD alone.  Calibrate the FCD first, and then the FCD/converter combination.

 

Warm-up

 

If you are using one of the TV receiver USB sticks, keep in mind that the initial frequency error will change as the unit warms up.  Give it a few minutes before performing a calibration.  This effect is less with the original FUNcube Dongle, and substantially less with the FUNcube Dongle Pro+.

 

HF

 

Alan, G4ZFQ, comments: The obvious ones [HF frequencies] are WWV, BPM at 5, 10, 15, 20 MHz and RWM at 4.996, 9.996, 14.996MHz. And CHU at a few odd frequencies.  Most major broadcasters are usually very close but that's not always true.

 

For other devices

 

For other devices such as the RTL DVB-T dongle, you can use the "kalibrate" program, see:

 

  http://rtl-sdr.sceners.org/?p=193

  Linux source: https://github.com/steve-m/kalibrate-rtl

  Windows : http://rtlsdr.org/files/kalibrate-win-release.zip

 

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