Seems there are some possibilities for running recent Pi 4’s at higher CPU clock speeds.
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Interesting, but i think not necessary for flight feeding 
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I had to overclock my Pi4 to 1.75GHz to get stable MLAT results when using the AirSpy Mini at the 20MHz bandwidth mode. And even with that overclock, I can’t have my UAT SDR running on that same Pi without it loosing MLAT sync. Eventually I’ll try bumping up the overclock a bit more to see if I can get it all working on the same Pi4. So I definitely see why some amount of overclocking is useful with flight feeding.
Sure, for that particular case. Not used that often
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That’s just strictly a USB issue with a 2nd device … (USB2 can only handle 480 MBit/s on the whole bus, which includes all ports)
If overclocking helped with this i’d be very surprised.
Try removing the overclock, see if you get MLAT issues.
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I’ve never had mlat sync problems with the airspy mini at 20MHz whether the pi is overclocked or not.
There is a quirk with the RPIs implementation of USB. If USB2 devices are used, there is only one bus and that is limited to the 480MBit/s as wiedehopf stated above. If the airspy could use USB3.0, then you may be able to run both dongles at once. Maybe the RPI5 will fix this limitation.
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It is the second USB2 device that is causing MLAT issues.
Sorry, I wasn’t being specific earlier.
The 2 SDR devices is definitely a USB bus issue with regard to MLAT, and is present regardless of clock speed.
Without an overclock MLAT works fine at 12MHz w/ just the airspy, at 20MHz MLAT seems to come and go (seemingly sporadic) and the preamble filter drops. At 12MHz the preamble filter is locked at 60, but with the standard clock & 20MHz the preamble filter drops to high 40’s low 50’s. When overclocked to 1.75GHz the preamble filter is pretty constant at 59.x and no MLAT issues.
I did not spend much time investigating further since the small overclock fixed it and I already moved the UAT dongle to a Pi3.
Yeah i suppose you might need to reduce -C if the mlat-client isn’t getting enough CPU time.
Overclocking can of course also help … but it’s certainly not the only solution for 20 MSPS.
If you have the newer C0 stepping processor (newer stepping came with my 4G Pi4 a couple months ago after I broke a Pi), you can get the same 1800 MHz with Buster. Just add arm_boost=1 to /boot/config.txt. And leave force_turbo=1 for ADSB purposes.
It’s an easy way to check which stepping you have. If you have B0 stepping, then arm_boost=1 doesn’t change to 1800 MHz.
e.g.
#uncomment to overclock the arm.
#arm_freq=1500
arm_boost=1
force_turbo=1
#over_voltage=2
Voltage increases from this.
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