Re: Direct Sampling frequency limit in r 1628 onwards #sdrsharp #spyserver


Martin - G8JNJ
 

Hi Joanne,

They are not as bad as you may think.

Two examples.........

Some time ago I tried running a pair of Dongles in direct sampling mode to cover two frequency ranges 0-14.4MHz and 14.4-28.8MHz. I used two instances of PC-ALE and HDSDR (with CAT to HDSDR enabled to control it). Then used it to scan the HF-Link ALE channels. It worked quite well, the only limitation being the amount of PC resources being consumed. 

The number of stations logged was greater than when I used an SDR Play RSP1 instead of the Dongles.

Today I tried an experiment logging FT8 signals on 15m.

I used and RTL SDR V3 in direct samping mode, an RTL 8020T2 dongle in normal quadrature mode and my KiWi SDR all fed from the same antenna.

There wasn't much to choose between them in terms of the number of stations logged and the RX S/N. However it wasn't particularly scientific as I had to swap between the different radios at different times, and the propagation changed over the test period.

Tomorrow I may try and get all three running concurrently but logging to PSK reporter with different callsigns. I think this would be a more realistic test.

The problem using the RTL SDR V3 with Spyserver in quadrature mode above 14.4MHz is that it has an internal diplexer feeding the Direct sampling port and Quadrature tuner port. So anything using the Quadrature tuner port between 14.4MHz and 30MHz is attenuated. This is another reason why I'd prefer to be able to select the receiver mode when using Spyserver.

I also find that if I use another dongle without the diplexer, and select the Quadrature tuner port on these frequencies, I frequently find that I have to put an attenuator in circuit as the strong broadcast stations on 15 & 19MHz cause ADC clipping and intermod even when the RF gain is set to zero. 

Regards,

Martin - G8JNJ




 

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