Hi Matthew
When you say TDMA, what are you referred to , there are dozens of
established TDMA protocols.....
if you are talking generic, all you need to do is ensure that you can
deterministically do things.... and have a microcontroller with plenty
of timers and interrupts (be be prepared to do nested stacked
interrupts) - like the STM32F4....
On 26/02/2015 5:04 PM, Matthew Pitts wrote:
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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--
-
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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