Glen,
nt7s has done some characterisation of the si531, has lots of graphs and
things that I don't quite follow (yet!), look for: Si5351A Investigations
in your favourite search engine, or via:
http://nt7s.com/tag/si5351/
May turn your 'potentially awful' into 'actually awful' but I'll let you
comment :)
--
Paul
VK1ATP
On Thu, Feb 26, 2015 at 2:25 PM, glen english <[email protected]> wrote:
> the VHF softrock is clever ...
>
> Si5351 is potentially awful
>
> Si570 is OK. a bit noisy around the carrier +/- 500Hz
>
> ADF4351 is probably your best bet for an IF receiver/LO. only 10 bucks.
> like the Si570, knows how to drink the juice from your battery....no
> free lunch....
>
> Image rejection in practice- using an LMS algorithm is 60-90dB.
>
> but if you design your digital voice algorithm to not have any DC
> component, then you can put it slap bang in the middle and quadrature
> matching matters little.....20dB would be sufficient for the demod....
>
> Of course if you want to go off channel one side and reject the other
> side, 60-80dB is what you need. In practice, you need to work very hard
> on other approaches (IF filters etc) to do be that good anyway on
> adjacent channels, so don't get too worried about it....
>
> the good thing is...IF there is a problem adjacent channel that is
> higher than the image rejection you have, you can always change your LO
> and move that adjacent somewhere else so the image products do not
> affect you....problem solved.
>
> All this makes the integration of the CMX994 a bargain....I understand
> the want though to roll your own. I often have my commercial hat on when
> I am talking about hobby stuff and it gets in the way (it is a big hat).
>
> glen
>
>
>
> On 26/02/2015 1:46 PM, David Rowe wrote:
> > Hi Glen,
> >
> > Thanks you for those suggestion, the VHF soft rock is very neat and the
> > front end might be just what I need.
> >
> > I wonder the if the Si570 might be a better choice that the Si5351.
> >
> > OK so a softrock style IQ SDR stage gives us IF (in our case)
> > selectivity. However what sort of image rejection is possible in
> > practice? Given the phase and amplitude variations of two separate
> > analog channels? For example can it achieve 90dB?
> >
> > Thanks,
> >
> > David
> >
> > On 26/02/15 12:26, glen english wrote:
> >> David
> >> take a look at the soft-rock designs for quadrature demods.
> >>
> >> these are pretty good performers......
> >>
> >> the VHF particularly
> >> this is a CLEVER design and I have built half a dozen...
> >>
> >> http://www.wb5rvz.com/sdr/ensemble_rx_ii_vhf/
> >>
> >> and this one
> >> http://fivedash.com/resource/RXEnsembleIISchematic.pdf
> >>
> >>
> >>
> >> On 26/02/2015 11:51 AM, David Rowe wrote:
> >>> Hi Glen,
> >>>
> >>> 1/ What characteristics should I look for to determine if the S/H is
> >>> good (or bad)? Is there test I should try, like perhaps two signals at
> >>> once in the ADC passband?
> >>>
> >>> The S/H RC time constant has a 3dB point of 7MHz.
> >>>
> >>> I'm not expecting sparkling performance, but would like to experiment a
> >>> bit more to see if it's a usable as a platform for our VHF FreeDV
> >>> experiments.
> >>>
> >>> 2/ Thanks you for the comments on the blog. yes I enjoy explaining
> >>> things, and the blog is an outlet for the teacher in me :-) Learning
> >>> and helping others understand is something about open source that I
> like
> >>> and appreciate.
> >>>
> >>> 3/ Thanks for the IF filter spec. Yep, was going to push for 60dB on
> >>> the next pass. It's those +/- 1MHz points that are critical, as they
> >>> alias on top of my wanted signals. A few kHz either side and the DSP
> >>> will nail it. So its likely to have better +/- 5kHz than +/- 1 MHz
> >>> performance. But I guess every rx has its weak spots where images or
> >>> some equivalent pop up.
> >>>
> >>> Is it possible to get an xtal filter that has 100kHz 3dB BW (or
> whatever
> >>> we need for 2 channel diversity) and is > 60dB down at +/- 1 MHz?
> >>>
> >>> The other option is a few more coupled LC sections, they seem to offer
> >>> about 20dB extra attn per section. Not sure if there is a more
> elegant way.
> >>>
> >>> Cheers,
> >>>
> >>> David
> >>>
> >>> On 26/02/15 08:55, glen english wrote:
> >>>> and..
> >>>> Using the ADC in the STM32 undersampling at 10.5 meg signal at 2 Msps.
> >>>>
> >>>> Are you sure the STM ADC sample and hold is actually any good at 10
> MHz ?
> >>>>
> >>>> 1st adjacent channel performance (signals that get through the crystal
> >>>> filter) will be problematic for the ADC because the SFDR / IMD
> >>>> performance of the ADC where the S&H is poor will be rather awful.
> >>>>
> >>>> Additionally, the stopbands of the first IF filter for a cheap radio
> >>>> have got to be at least 60dB down, otherwise strong off channel
> >>>> frequencies will be aliased down into the baseband... A good radio
> would
> >>>> be 120dB.....
> >>>>
> >>>> keep learning ! Nice work so far.
> >>>>
> >>>> Nicely explained in your blog. you should have been a teacher.
> >>>>
> >>>>
> >>>> cheers
> >>>>
> >>>>
> >>>>
> >>>> On 26/02/2015 9:08 AM, David Rowe wrote:
> >>>>> Hi Helmut,
> >>>>>
> >>>>> Can you please tell me how you worked that out? My next step is to
> add
> >>>>> gain to balance MDS and IP3 but not sure how you obtained those
> numbers!
> >>>>>
> >>>>> Yes the radio as it stands requires another design pass. At this
> stage
> >>>>> it is a "proof of concept" to test the novel features: such as the
> use
> >>>>> of a uC for most of the tuning, and extreme hardware simplicity.
> >>>>>
> >>>>> Thanks,
> >>>>>
> >>>>> David
> >>>>>
> >>>>> On 26/02/15 08:09, Helmut wrote:
> >>>>>> Hello David,
> >>>>>>
> >>>>>> a rough simulation of that SDR design predicts a rather deaf radio:
> Total
> >>>>>> noise figure = 38 dB, MDS=-95dBm @ 12,5kHz System BW, Input IP3 =
> -20 dBm.
> >>>>>> I would suggest at least a 20 dB preamp with high IP3 performance
> in front
> >>>>>> of the ADC. So you will achieve a MDS of -114 dBm. Sure no splendid
> value,
> >>>>>> but will detect some VHF signals, hi.
> >>>>>>
> >>>>>> 73, Helmut, DC6NY
> >>>>>>
> >>>>>> -----Ursprüngliche Nachricht-----
> >>>>>> Von: David Rowe [mailto:[email protected]]
> >>>>>> Gesendet: Dienstag, 24. Februar 2015 22:16
> >>>>>> An: [email protected];
> [email protected]
> >>>>>> Betreff: [Freetel-codec2] Simple SDR for VHF FreeDV
> >>>>>>
> >>>>>> Hello,
> >>>>>>
> >>>>>> To support VHF FreeDV developments I've been working on a really
> simple and
> >>>>>> very open SDR architecture for VHF radio:
> >>>>>>
> >>>>>> http://www.rowetel.com/blog/?p=3884
> >>>>>>
> >>>>>> Glen and Matt - would appreciate you suggestions on appropriate
> input BPF
> >>>>>> filtering and a VHF mixer. Also pls let me know if you see any show
> >>>>>> stoppers in this architecture.
> >>>>>>
> >>>>>> Cheers,
> >>>>>>
> >>>>>> David
> >>>>>>
> >>>>>>
> ----------------------------------------------------------------------------
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> ------------------------------------------------------------------------------
> > Dive into the World of Parallel Programming The Go Parallel Website,
> sponsored
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> for all
> > things parallel software development, from weekly thought leadership
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>
> --
> -
> Glen English
> RF Communications and Electronics Engineer
>
> CORTEX RF
> &
> Pacific Media Technologies Pty Ltd
>
> ABN 40 075 532 008
>
> PO Box 5231 Lyneham ACT 2602, Australia.
> au mobile : +61 (0)418 975077
>
>
>
>
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--
Paul Warren
p dot a dot warren at gmail dot com
pwarren.id.au
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