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 >>>>> >>>>> ---------------------------------------------------------------------------- >>>>> -- >>>>> Dive into the World of Parallel Programming The Go Parallel Website, >>>>> sponsored by Intel and developed in partnership with Slashdot Media, is >>>>> your >>>>> hub for all things parallel software development, from weekly thought >>>>> leadership blogs to news, videos, case studies, tutorials and more. Take a >>>>> look and join the conversation now. http://goparallel.sourceforge.net/ >>>>> _______________________________________________ >>>>> Freetel-codec2 mailing list >>>>> [email protected] >>>>> https://lists.sourceforge.net/lists/listinfo/freetel-codec2 >>>>> >>>>> >>>>> ------------------------------------------------------------------------------ >>>>> Dive into the World of Parallel Programming The Go Parallel Website, >>>>> sponsored >>>>> by Intel and developed in partnership with Slashdot Media, is your hub >>>>> for all >>>>> things parallel software development, from weekly thought leadership >>>>> blogs to >>>>> news, videos, case studies, tutorials and more. Take a look and join the >>>>> conversation now. http://goparallel.sourceforge.net/ >>>>> _______________________________________________ >>>>> Freetel-codec2 mailing list >>>>> [email protected] >>>>> https://lists.sourceforge.net/lists/listinfo/freetel-codec2 >>>>> >>>> ------------------------------------------------------------------------------ >>>> Dive into the World of Parallel Programming The Go Parallel Website, >>>> sponsored >>>> by Intel and developed in partnership with Slashdot Media, is your hub for >>>> all >>>> things parallel software development, from weekly thought leadership blogs >>>> to >>>> news, videos, case studies, tutorials and more. Take a look and join the >>>> conversation now. http://goparallel.sourceforge.net/ >>>> _______________________________________________ >>>> Freetel-codec2 mailing list >>>> [email protected] >>>> https://lists.sourceforge.net/lists/listinfo/freetel-codec2 >>>> >> ------------------------------------------------------------------------------ >> Dive into the World of Parallel Programming The Go Parallel Website, >> sponsored >> by Intel and developed in partnership with Slashdot Media, is your hub for >> all >> things parallel software development, from weekly thought leadership blogs to >> news, videos, case studies, tutorials and more. Take a look and join the >> conversation now. http://goparallel.sourceforge.net/ >> _______________________________________________ >> Freetel-codec2 mailing list >> [email protected] >> https://lists.sourceforge.net/lists/listinfo/freetel-codec2 >> > ------------------------------------------------------------------------------ Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ _______________________________________________ Freetel-codec2 mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/freetel-codec2
