David or Glen,
Do either of you know of a decent TDMA chipset that doesn't require a
proprietary blob to function? Or is this something we are going to have to do
in another microcontroller?
Matthew Pitts
N8OHU
On February 26, 2015 12:53:37 AM EST, David Rowe <[email protected]> wrote:
>Hi Glen,
>
>Yes considering the CMX994 has a built in LO (of reasonable) quality I
>take your point!
>
>I am interested in developing a radio for commercial manufacture, to
>allow roll out of VHF FreeDV systems. The medium term goal is real
>world testing of VHF FreeDV, including TDMA and diversity.
>
>Cheers,
>
>David
>
>On 26/02/15 13:55, glen english 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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