[Discuss-gnuradio] Re: Welcome and brief update

[Dave Emery]

On Sat, Jan 11, 2003 at 03:00:18PM -0800, John Turner wrote:
> Hi Folks,
> 

> I believe the following topology represents a good value performance
> tradeoff, between $500 to $1000 unit cost.
> TV Tuner offering 50 to 860 MHz range and 60dB dynamic range performance A
> channelized output of 40 MHz.
> ADC converter operating at 70 Msps, the idea is that the IF is undersampled.
> Digital Downconverter as a sample rate converter
> Sample rate to USB converter
> I believe the Downconverter and the USB interface could be built into an
> FPGA.
> 

        I hate to inject a note of reality, but isn't the objective of
any receiver design (what we are talking about here I think) to receive
some specific signal or set of signals ?

        Specific signals impose specific requirements on a SDR receiver.

        Some are narrowband (tens or hundreds of hz at most), some
moderate width (2.1, 3,2, 5 khz, 11.2 khz, 15 khz,  20 khz, 30 khz),
some relatively wideband (50khz, 200 khz, 350 khz, 1.25 mhz), some quite
wideband (6 mhz), and some very wideband (18 mhz, 27 mhz, 36 mhz, 50
mhz, 100 mhz...)

        And some are found in environments with nearby signals in the
passband of the tuner at comparable levels within say 20-40 db (CATV
signals on a cable for example or signals on a C or Ku band satellite
transponder) whilst others (weak signals at close to the noise floor on
HF or VHF (2 meter weak signal work) may be up to 110 db or more  below
the strongest signals in the passband of the tuner.

        Some wideband signals (ATSC TV) take quite a bit of processing
horsepower, whilst others (FM broadcast) may be able to be demodulated
with simpler less cpu intensive algorithms.

        In general for narrowband signals LO phase noise and phase
and frequency stability is important - reciprocal mixing makes all
signals downconverted by a LO as bad in respect to frequency and
phase stability and noise sidebands as the LO is.   So for narrowband
work one needs really clean stable synthesizers.   And long term
frequency accuracy and tuning resolution may become important too
with narrow band signals.

        And clean, low phase noise LOs are important with weak signals
in the presence of strong signals too, as the phase noise sidebands from
a noisy LO downconverting nearby strong signals can land on top of the
weak barely discernible signal and cover it up.

        But this has to be balanced off against cost (crummy LOs are
cheaper) and wide tuning range and quick lockup.   

        TV tuners are designed to work with wideband signals where LO
stability and phase noise has not been a major issue but cost and quick
tuning is.   And TV tuners have generally not be required to work all
that well in very high dynamic range environments, though they have
gotten a lot better in recent years.

        And of course for many receiver applications there are other
considerations too - overall noise figure of course determines minimum
detectable and usable signal, and for a lot of real world applications
maximum sensitivity is vital to getting useful results as at least
some of the signals of interest are quite weak.

        And very critical in modern rf environments (particularly at
VHF and low UHF and increasingly at higher frequencies too) is dynamic
range (3rd order intercept).   Many actual signals in the modern world
are found in dense rf environments with powerful nearby transmitters
on adjacent channels and if ones rf front end overloads or generates
significant intermodulation products one may be unable to hear a signal
of interest at all even if it is well over the noise floor.

        And for any SDR there is a tradeoff between analog filtering
ahead of the A/D and use of an A/D with high dynamic range (lots of
bits and few mixing products).  Fast, high dynamic range A/Ds cost
more (but are coming down in price fast), whilst super high dynamic
range A/Ds exist for narrow band signals that are very cheap.

        And finally, of course, for wideband signals one often cares
about the amplitude flatness and phase linearity of the tuner passband.
If it isn't flat more complex demodulation and equalization algorithms
may be required to correct for delay or amplitude errors induced by
the tuner.
        
        So simply designing an abstract architecture ignores some
of the considerations involved with real signals and if one wants
to obtain useful and even competitive performance one has to decide
what sort of things one is targeting and what one is not.

        For example, if some GNU radio hardware is directed primarily at
cheaply allowing a PC to tune in FM broadcast and NTSC or perhaps
especially ATSC TV signals, one hardware architecture may suffice,
whilst if one intends to use it as a useful scanner or communications
receiver another may be needed.  And if one intends to use it to decode
GPS or other specialized signals highly specific requirements may exist
in order to get useful results at all.  And obviously if one intends to
build a device that will interoperate with existing systems, one needs
to try to meet minimum acceptable performance standards for devices used
with those systems.

-- 
        Dave Emery N1PRE,  address@hidden  DIE Consulting, Weston, Mass. 
PGP fingerprint = 2047/4D7B08D1 DE 6E E1 CC 1F 1D 96 E2  5D 27 BD B0 24 88 C3 18