994 is a good chip...... solves a great many problems. You'll need some
baseband gain I expect (opamps) driving the relatively noise ADCs on the
STM, but that should be OK.
On 26/02/2015 9:26 AM, Daniel Mundall wrote:
Hi Glen,
We haven't dropped the CMX994, David has just been feeling out other
options. I just got my dev boards for the CMX994 and 971 today so I'll
be running some test with them over the next few days.
Daniel
Daniel Mundall
On Wed, Feb 25, 2015 at 2:20 PM, glen english <[email protected]
<mailto:[email protected]>> wrote:
My 5 cents
I agree with Helmut's analysis. Additionally, with such a low
IIP3, the
radio will be absolutely riddled with intermods and essentially
unusable
in a city.
NE602 is good for a cordless telephone, or a well protected 2nd IF
mixer
function.
By modern standards they are a low performer.
The radio is certainly fine in the lab as a demonstration aid.
aim for an IIP3 of > -10dBm for a cheap radio.
For this design, I'd suggest an IQ sampling because it can be done
effectively by the STM32 ADCs.
IQ sampling, will in general be a lower power consumption solution
as it
doesnt have high speed ADCs and high speed downsampling filtering
(fpga) .
There will need to be considerable gain between the ADC and the
antenna.
Say sample rate 256kHz, SNR 60dB, say full scale of 0dBm, Noise
floor =
-60 -54 = -114dBm (Hz)... a noise figure of around 60 dB....
Likely needs to be a double conversion receiver-
so this would maybe be portioned as 12dB pre mixer gain, active mixer
IIP maybe +6dBm, low noise post mixer , IF filter say easy available
21.4 filters, some more gain, and then use a CMOS switch SDR style IQ
mixer with some more low noise opamps before feeding your STM32 ADCs.
Keep the noise floor in the 1st half as low as possible, enabling
you to
really heap on the gain in the baseband opamps .... as heaping on the
gain at the baseband wont risk a feedback/instability issue.
of course, you could use a CMX994 and a couple of opamps and feed your
STM32 dual ADC.
I have some CMX994s on order.
g
On 25/02/2015 12:46 PM, David Rowe wrote
> 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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