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]> 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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