Hi Daniel
Yes.. you can improve the demod to cope with multipath, but for a flat
fading channel like 2m or 70cm, it does not matter how good your demod
is if you have no signal when you stop at the traffic lights..... that
is why the system needs some sort of diversity. It doesnt matter what
type, space, frequency, site just some type. Unless you go for a linear
radio (which would be a no no for compatibility and upgrade and all
sorts of reasons) you are pretty much limited to some sort of dual site
TDM (single frequency) or dual frequency single site.
circular polarisation can go about 50% of the way and will make a big
difference... but in most cases you wont have much choice of existing
antennas.
glen
On 16/02/2015 2:33 PM, Daniel Mundall wrote:
Hello Glen,
Thanks for your thoughtful email, and I totally agree, I'm probably
not experienced enough to make a radio yet.
But I am familiar with SPI and have used it on other projects
FlexScada. (http://flexscada.com/)
As for the C-Bus, no where could I find it mentioned that it was SPI,
but by the looks of it it seemed to be, which was why I was asking
about it.
And as you suggested, I am starting with the dev boards connected to
some STM32's once that's all working maybe move to a single PCB.
Anyway, I appreciate your patience and time answering my questions.
Daniel VA7DRM
PS. I've been doing over the air testing with David's Octave GMSK
modem, we're having some fun with multipath now. :) Nothing that can't
be fixed, but it's helping us prefect the demod. Multipath fades
driving seem to be anywhere from 5-20Hz. Interesting how on the fringe
the fading is much less (I assume that when the path losses are higher
there are just less paths even making it to the RX.)
Daniel Mundall
On Sun, Feb 15, 2015 at 7:09 PM, glen english <g...@cortexrf.com.au
<mailto:g...@cortexrf.com.au>> wrote:
Hi Daniel
Cbus/SPI is easy, you dont need a library BUT
- but... if you need to ask about "what is SPI/Cbus, do I need a
lib ?" then you do not, in my opinion, have enough experience to
make this radio work properly..... these are very basic things and
skillsets you MUST have.
I'll be honest , while I think your endeavours are great, I dont
think you have the skillsets to execute a PRODUCT, without a lot
more rubber on the road..... Certainly, this is a very good way to
learn about the technology, but if you are planning to build and
sell a radio, or have a radio club project, you need much much
more understanding and experience to be able to execute such an
endeavour successfully. Please dont take this the wrong
way....Maybe I have misunderstood your intentions..
Get yourself the eval board, or get some chips and build your
own... you will need to learn everything and understand everything
about each building block. This probably means bulding and
prototyping each block of your radio one by one, and connecting it
all together before even thinking about integrating it into one
housing....
There will be many issues for you to overcome that are not
documented anywhere.
What do I mean ?
>>>>>I mean that the datasheets for a chip (this CMX chip and
others) , and the application notes only ever have about HALF of
what you really need to get the thing working. The other half is
required to be learnt by the designer ! this requires alot of work
and intuition.
DC offsets are no issue....they are servoed out.
I am certainly happy to continue to answer questions adlib.
cheers
On 16/02/2015 1:28 PM, Daniel Mundall wrote:
Thanks, Glen.
I agree we want to keep things simple.
Since you've used this IC, maybe you could answer two of David's
concerns. Is C-Bus a problem to interface to, or is it just
something like SPI? Are there already lib's out there?
And also has DC offset been an issue with your devices?
Thanks,
Daniel
Daniel Mundall
On Sun, Feb 15, 2015 at 6:19 PM, glen english
<g...@cortexrf.com.au <mailto:g...@cortexrf.com.au>> wrote:
Daniel , External VCO might be 80 cents.. but then it is
something that has to be tracked in production.
Don't set lofty goals for your 1st version, just use the
internal VCO.
Don't promise too much..
Over deliver.
David's route is different and not comparable- you are
talking about a standalone receiver.
A crystal filter, for this application type and performance,
in my opinion, is to be avoided .
They are expensive, need specific tuning/ production checks
to get the shape right (otherwise you are wasting your time) .
There is a place for crystal filters in receivers , but I
can't think why it would be here.
glen
On 16/02/2015 1:08 PM, Daniel Mundall wrote:
Hello Glen,
Thanks so much for your email!
I've been wondering which way to go with that, how much cost
do you think an external VCO adds to the BOM?
The dev boards I ordered should be here soon and I'll have
to play around with that.
David on the other hand is playing with a much more simple
approach, if he can get it to work well, we may go that
route. He has a basic mixer and xtal filter and then dealing
directly with IF.
Anyway thanks again for the warning.
Daniel
Daniel Mundall
On Sun, Feb 15, 2015 at 12:24 AM, glen english
<g...@cortexrf.com.au <mailto:g...@cortexrf.com.au>> wrote:
Hi Daniel
I think the CMX994 is a good choice for your radio,
overall. But don't
expect anything magnificent out of the receiver for 1st
2nd and 3rd
adjacent channels though- this should be strictly
assumed as a portable
radio receiver, or 'average' mobile radio receiver.
The reason for this is that the internal VCO is rather
noisy, and as an
integer N loop with a small step size, you are limited
to quite narrow
PLL bandwidths. (<2 kHz) This has the effect of not
correcting for the
rather noisy VCO away from channel.
An external VCO would improve it, and external LO again
improve it.
BUT !
it is more than adequate for a basic receiver that can
'do all' at a
good price.
You would not do better for the same cost.
I am using this chip on another project, but with an
external local
oscillator- a DDS . (Not a portable application) (and
at a higher cost)
footnote
The advantage of an integer-N PLL is that is is
relatively free of PLL
generated spurs, whereas a fractional N PLL can operates
at loop
bandwidths, typically for this application , up to
100kHz, which often
translates into a very good adjacent phase-noise
performance- at the
expense of many (calculable) spurs. The integer N PLL
will also often
have much lower power consumption.
regards
glen english VK1XX
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au mobile :+61 (0)418 975077 <tel:%2B61%20%280%29418%20975077>
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&
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au mobile : +61 (0)418 975077
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