Hi Glen,
Wow that would be a big cost savings.
I'm presuming you're saying that I don't need the saw filter if I have a
good BPF.
As nice as dual band would be, there's too many things to think about as it
is for the first version, this adds another problem of matching network for
the PA. I think this will have to get dropped until version 2.
Maybe this is a stupid question for someone that's deep into the RF world,
but would it be possible to just have the BPF as the first thing after the
antenna?
In regards to diversity, do you think that hopping frame by frame between
two frequencies that are like 250khz apart would offer the same advantages
as actually having two continues carriers? Another idea was if I used space
diversity, and use a method where the AP sent the frame out which ever RX
antenna had the best copy on the last RX frame. Either of these schemes
should be doable with an STM32, presuming that they'd help.
Thanks again for your input on all this, you have no idea how much time you
are saving me!
Daniel VA7DRM
Daniel Mundall
On Sun, Feb 8, 2015 at 7:17 PM, glen english <g...@cortexrf.com.au> wrote:
> Hi Daniel
>
> SAW filter post VNA is good idea. Although the high loss of most saw
> filters might degrade the receiver too much.
>
> It will probably be just fine with a simple double tuned circuit input,
> costing about $3 total in high quality inductors (coilcraft SQ series etc
> - high Q and SMT-able) . Remember, most off the shelf transceivers are
> 6-8dB NF. You need at least 20dB attenuation of FM/TV stations, and you
> will get that.
>
> As for the LPF TX - download half a dozen schematics for dual band
> portables from yaesu, icom etc and see what they have done in the PA- LPF
> department for their dual band handhelds- that will give you a bit of an
> idea to what can be achieved.
>
> The filter will need zeros to make spec. Some switch larger filters ,
> some just have zeros. (traps) at harmonic frequencies. Which works fine for
> the ham application. The dual band harmonic filter is tricky , especially
> when the third harmonic of 2m is 70cm ! you might need to switch in
> different filters with PIN diodes or Skyworks PIN switches.
>
>
> g
>
>
>
>
>
>
> On 9/02/2015 1:58 PM, Daniel Mundall wrote:
>
> Glen,
> Wow fantastic! Thanks so much for the info.
>
> The thought of the LNA was to reduce the NF, but that doesn't sound like
> a good idea. The noise figure of the receiver the way they have it setup is
> 4.5db.
>
> The saw filter, can be positioned after the first internal LNA to help
> reduce the NF. In any case it needs an LNA before the saw filter as the
> loss is pretty high. I'm not bias to SAW filters, I'm actually very open to
> any option as far as filtering goes. But there don't seem to be very many
> low cost options out there. You seem to like the Helical filters, would you
> recommend them over saw filters? Also any recommendation for a LPF? We're
> going to have a TX power of around 30dbm.
>
> Our current BOM is like $35, and $20 would be a pretty big chunk, are
> there any lower cost options that would still work well? We're aiming to
> get this out there for less than $150 all things included, hopefully for
> more like $100.
>
> I'm thinking for just getting version 1 on the road to go just single
> band, but if there's some way that's not too complex, dual band would
> always be nice.
>
> Thanks both of you for the warning about harmonics, have to make sure
> they're not too high. I'm thinking that a good LPF should for the most part
> minimize this problem?
>
> Thanks again for your input.
>
> Daniel Mundall
>
> On Sun, Feb 8, 2015 at 5:05 PM, glen english <g...@cortexrf.com.au> wrote:
>
>> Daniel
>>
>> I've had a look at your block diagram, and I read the data sheets - That
>> CMX994 is probably good enough without any sort of aggressive filtering.
>> 3rd order input intercept of around -3dBm is very reasonable. You just
>> need to keep out the FM stations...and pagers.
>>
>> BUT - the chip does really not need that extra LNA.
>>
>> That LNA you have ahead of the SAW filter could spell trouble for in band
>> intermods (that is , strong signals that are in the SAW filter passband)...
>> (unless the SAW has alot of loss)
>>
>> A TX low pass filter is not shown- I assume that is in there - all
>> harmonics need to be below -30dBm , (preferably -36dBm if it is going to
>> connect to a vehicle antenna and run more than 5W)
>>
>> You'll likely need TWO separate low pass filters for 2m/70cm operation.
>>
>> g
>>
>> On 9/02/2015 8:51 AM, Daniel Mundall wrote:
>>
>> Hello Glen and Matt,
>>
>> I've been going back and forth with David here about a very low cost
>> SDR for 2M/70cm and a few concerns have come up. The main one has been
>> dealing with strong interferers at a very near by frequency.
>> Do you guys have any suggestions to to help mitigate receiver
>> overload/desensitization?
>> I've attached a block diagram of the radio we're working on.
>> Here's a link for the datasheet of the RX IC we're using:
>> http://www.cmlmicro.com/DesignSupport/resources/2012/11/23/CMX994-Datasheet
>>
>> What sort of specs should we aim for to have good strong signal
>> performance?
>> Any suggestions will be greatly appreciated!
>>
>> Daniel Mundall VA7DRM
>>
>>
>>
>>
>> ------------------------------------------------------------------------------
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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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>
>
> ------------------------------------------------------------------------------
> 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/
>
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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
>
>
>
> ------------------------------------------------------------------------------
> Dive into the World of Parallel Programming. The Go Parallel Website,
> sponsored by Intel and developed in partnership with Slashdot Media, is
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> look and join the conversation now. http://goparallel.sourceforge.net/
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