HFU HF Underground
Technical Topics => SDR - Software Defined Radio => Topic started by: Ray Lalleu on April 29, 2021, 1408 UTC
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The KiwiSDRs display the kHz frequency to the second decimal figure,
but are often quite aside, so the users often report the wrong decimals,
thinking those figures are exact.
Generally this results in no serious consequences, just adding confusion to the reports and identifications. But on the 28th of April 2021, 'Brian' was testing a new DDS for a TX on 5800 kHz, and Texas1xer stated with no doubts that the frequency was 5800.23 on two different KiwiSDRs in Weston. That almost fooled Brian, but at that moment the real frequency was 5800.02 (or maybe 5800.03), nothing bad. After a lot of replies, the things were cleared out, with Brian knowing now how to check exactly his frequency by using the Twente SDR. Soon he was nearer to 5800.00, and we could see on the Twente SDR 5800.005 with a very slow drifting upwards. (At that level of 1/10 of a hertz, the driting can result from varying Doppler effect).
At the end of the thread, 'dave1965' added that the KiwiSDR in N.W.Ireland [not Weston] uses the frequency standard from the GPS system. OK for NW Ireland, but many other KiwiSDRs (and maybe other SDR systems) are not as good as a simpler RX with TCXO. The problem is that many users of remote SDRs are not aware of those discrepancies and believe that the displayed frequencies are the truth up to the latest decimal.
BTW, I use my readings up to 0.01 kHz as clues about the IDs of the stations. But I'm aware that many stations drift more or less, so I just round the frequencies a bit, with added indications ( ++ or + or - or -- ) about the offset.
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This is a good point. You can check the status of a KiwiSDR to see how many active GPS satellites it is tracking, it should be 4 or more. If fewer, the frequency could be off as you noted.
You can also of course check against a time station or known good MW/SW BC station as well, just to make sure the Kiwi frequency is accurate.
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Hi Ray,
The KiWi's at Weston do have their GPS enabled, but by default this is run as a background task and sometimes if the KiWi's are busy the GPS acquisition will be 'paused' until there are less users on the KiWi.
As an experiment I have changed the GPS settings on my KiWi which is co-sited at at Weston, to see if this helps to improve the frequency accuracy.
Please try http://websdr.uk:8060/ and let me know via the owner / admin link that can be found in the yellow tabbed 'stats' pane if you are still seeing significant frequency errors, or any other problems.
Regards,
Martin
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I wish kiwi would display to .000 since you can tune them to .000 or 1 Hz. My Tx is within +- .0001 or 0002 Hz max GPS frequency accuracy.
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I'm trying to wrap my head around worrying about such small frequency differences!
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I'm trying to wrap my head around worrying about such small frequency differences!
One use is on the MW band, where you can use the exact carrier frequency of a station to help ID it. Here's a plot of 1690 kHz from last night for example, produced with my Carrier Sleuth app: https://www.blackcatsystems.com/software/medium_wave_carrier_display_app.html
(https://i.imgur.com/rRFi629.jpg)