Hello Glen,
I totally agree with the flat fading, at the moment it appears that we are
getting anywhere from 10-30db of attenuation during fades, but when we are
within say 5km of the TX I still have around 10db during the worst fades.
So I think there's more at play than just no signal. Which is kind of
pointing towards a demod bug, likely to do with the rapid amplitude or
phase shifts. Seems to do fine if the fades are slow, like with say
walking. At the moment moving samples on the edge (35km away) of the
coverage seem to be demoding better than the ones are are say 1km away.
I've been scheming about diversity: And my current idea is dual TX chains
but closely spaced say 10khz apart, and separated by say 10λ horizontally.
Another idea is to use cross popularization instead of horizontal space.
The data going into these channels could be interleaved and some redundancy
added, or it could be an exact copy of the other chain.
I think it would be nice to stay within a single 25 or 15khz channel if
possible, makes spectrum planing way easier, and for that matter filtering
on the RX side.
With the above method there would still be a weak link with the uplink,
you're dealing 3db less power, but we should be able to regain that with
either digitally combining the two RX antennas using an FEC method, or by
using something more complex like MRC(which I'd like to avoid).
At the end of the day it would only be an extra chain at the repeater which
doesn't add much cost in the big picture.
Anyway plan to get a test setup for this done up in the next day or two so
I can see how much the horizontal space diversity helps. See how many of
the errors are correlated.
Do you have any suggestions about this idea, or other test I should run
while I'm doing them?
Oh and by the way, do you know anything about turnstile antennas configured
as horizontal polarized omni antennas, are they any good? I've been
thinking of building one for testing H-pol. (
http://www.g8seq.com/vhf_antennas.php )
Thanks,
Daniel VA7DRM
Daniel Mundall
On Mon, Feb 16, 2015 at 1:59 AM, glen english <g...@cortexrf.com.au> wrote:
> 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>
> 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>
>> 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>
>>> 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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>>>
>>>
>>>
>>>
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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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>> --
>> -
>> 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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> --
> -
> 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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