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
<mailto: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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sponsored by Intel and developed in partnership with Slashdot
Media, is your
hub for all things parallel software development, from weekly thought
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more. Take a
look and join the conversation now. http://goparallel.sourceforge.net/
_______________________________________________
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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
------------------------------------------------------------------------------
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sponsored by Intel and developed in partnership with Slashdot Media, is your
hub for all things parallel software development, from weekly thought
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look and join the conversation now. http://goparallel.sourceforge.net/
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