Rx888 Mk II and v3.0.27

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jdow
Posts: 800
Joined: Mon Aug 10, 2020 8:17 pm

Re: Rx888 Mk II and v3.0.27

#11

Unread post by jdow »

Paul - aliasing is probably the reason for the limitation. Aliasing artifacts would make a 70 MHz ham band potentially unusable. And 70 MHz exactly would be unusable with 140 MHz sampling in any case. It's in a sampling null.

{o.o}

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ukdx
Posts: 41
Joined: Wed Nov 18, 2020 7:52 am

Re: Rx888 Mk II and v3.0.27

#12

Unread post by ukdx »

Thanks for the explanation Joanne, I can't really complain about what it does cover given the price and the HF performance isn't far off what my S3 achieves at six times the cost.

jdow
Posts: 800
Joined: Mon Aug 10, 2020 8:17 pm

Re: Rx888 Mk II and v3.0.27

#13

Unread post by jdow »

I've never contemplated this before but it might sort of work. And I'm close to bed time here so I'm not going to sit down and puzzle out how it could be made to work. Sample at 70 MHz and receive from 35 MHz to 105, probably with an unfortunate region right at 70 MHz. You would need a good hardware filter for that frequency range, well, more likely something like 40 to 45 MHz at one end and 90 to 95 at the other using a very sharp filter and hi-Q elements. I think puzzling this out might drive a person to drink. But, a perhaps misguided intuition insists it might be a go. But nothing can save at least one of the specific bands you want to monitor other than something like a 180 MHz sample rate. (0 to about 80 MHz with reasonably sharp front end filtering plus a BC FM filter.

{o.o}

jdow
Posts: 800
Joined: Mon Aug 10, 2020 8:17 pm

Re: Rx888 Mk II and v3.0.27

#14

Unread post by jdow »

Now that I have played with one a little, it looks like 135 Msps might be the right way to run this fool thing. If I run it at 128 Msps I get a significant frequency error. I got it working with HDSDR (PEEUUWWW what trash!) and a very old SDRSharp that still ran EXTio interfaces. (HDSDR trims the extio interface cutting out most of it. SDRSharp shows the whole thing.)

When running it on all three machines I have that support it I get approximately 7 to 9 ppm error. If I knew the exact sample rate and could talk Simon into using it then I could use any sample rate. As it is 135 MHz is pretty much guaranteed to be exact IF the internal standard is 27.0000 MHz. I bet there are a lot of other frequencies that might work out exactly with the clock generator. I figure to try this tomorrow on both the RX888 and RX888 MK II I purchased.

It needs some interesting care and feeding, it appears. (And, no, 70 MHz ham band is right out. 67.5MHz is the Nyquist frequency. 50 MHz should be fairly decent.)

{^_^}

jdow
Posts: 800
Joined: Mon Aug 10, 2020 8:17 pm

Re: Rx888 Mk II and v3.0.27

#15

Unread post by jdow »

Nope - and I realize I am being obnoxious about this; but, my vendor claimed a 0.5 ppm VCXO. Betcha there is no V gozinta for the VCXO. I had to fuss with the RX888s' internal sample rate synthesizer's control to make it happy. Finding that was annoying.

It's also annoying that SDRC does not allow for the higher sample rates of the MK II. (For me HDSDR and an old SDRSharp I made work both work find at 140 Msps on this machine.)
{^_^}

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