sdr dongle freq accuracy ?

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gmham
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sdr dongle freq accuracy ?

Post by gmham » 30 Jan 2020, 18:48

How accurate in freq are these cheap sdr dongles ? Reason I ask is that on my own dongle the Noolec sdr covering 28 mhz upwards anything on 28mhz is bang on freq ie. a ssb signal transmitting on say 28.500 is exactly showing as 28.500 but on say a 144mhz ssb a signal on 144.300.000 it has to be tuned to 144.300.800 to resolve the signal. Is this down to the sdr dongle being slightly off which I believe there is a setting within the software to compensate for such an error ?
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Re: sdr dongle freq accuracy ?

Post by ch25 » 30 Jan 2020, 19:44

In SDR sharp there is freq correction. 55 in my case.
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Re: sdr dongle freq accuracy ?

Post by gmham » 30 Jan 2020, 22:02

Okay perfect thanks so much. So from what I understand you are saying in your case your dongle is 55HZ out and in mine 800Hz ie. again mine is reading 144.300.800 for a signal tx on 144.300 ? If I change the freq correction within settings like you say will this effect all freqs as again all seems perfect lower down in freq ie . On 28mhz etc ..better just keeping it a short it is and adjusting where necessary?
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Re: sdr dongle freq accuracy ?

Post by ch25 » 30 Jan 2020, 22:45

55ppm, not Hz.
No idea. I use mine only on 2/70 and correction seems to be the same. I have cheapest RTL avalaible :)
Is yours direct sampling or with upconverter since you can listen 14MHz?
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Re: sdr dongle freq accuracy ?

Post by gmham » 30 Jan 2020, 23:12

144mhz mate not 14mhz. Thanks again.
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Re: sdr dongle freq accuracy ?

Post by 26mb04 » 31 Jan 2020, 00:16

I seem to remember a certain level of correction is normal. It varies from one device to the next, there's no fixed amount.

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Re: sdr dongle freq accuracy ?

Post by milly » 01 Feb 2020, 12:53

Ancient DVB-T television/radio dongles need a calibration adjustment. This should be calculated when the device has warmed to operating temperature.

However:
Most nooelec (and other 'new') dongles marked for general scanner type use usually don't need that and also won't exhibit much temperature drift. I have several dozen of various types and none need any adjustment for the things I do.

Taking this a step further - is a 0.05544% error actually significant in your application? If it is then you are WAY out of hobby equipment precision in your needs.
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Re: sdr dongle freq accuracy ?

Post by gmham » 01 Feb 2020, 15:07

Okay thanks no problem was just interested to find out how accurate they were. Must admitt for the money they are superb.
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Re: sdr dongle freq accuracy ?

Post by Andy » 14 Feb 2020, 14:44

Hi gmham. I've only just seen this thread, sorry for dragging it up.
Two points. The frequency inaccuracy of these dongles is down to an error in the reference xtal, which can be trimmed in software. If the error is (say) 100 Hz at 28 MHz it will be about 500 Hz at 144 MHz ie it goes up proportionally with frequency.
But the other point is - how do you know your 144.300 MHz signal is bang on? Or your 28 MHz one, come to that? Amateur transceivers have exactly the same problems with their reference xtals, and they are rarely smack-on. The only way to set these things up properly is with lab-spec sig-gens and dedicated reference sources.
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