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Re: [Discuss-gnuradio] Re: Welcome and brief update

From: John Turner
Subject: Re: [Discuss-gnuradio] Re: Welcome and brief update
Date: Sat, 11 Jan 2003 21:34:49 -0800

Occam's Razor
"one should not increase, beyond what is necessary, the number of entities
required to explain anything"

It sounds as if there are a number of highly diverse applications being
developed. The topology I put forward was meant to form a platform roughly
consistent with wideband radio receivers of the hobbyist variety, refer to
ICOM and Kenwood units. These radios are suitable for general monitoring of
signals. Their typical radio specifications are in the 50 to 60db range,
i.e. 60 dB selectivity and image rejection, 50 dB intermodulation rejection.
The radios typically demodulate AM/FM/SSB, some even include analog TV
tuners, but they are not intended for commercial applications. The ultimate
goal of the hardware being to allow a wide spectrum of applications to be
explored without undue cost or effort.

I made a general assumption that the host PC has limited processing
capabilities, i.e. limiting the maximum effective sampling rate of 500 to
1MSPS. I'm curious, what can a PC realize in terms of processing benchmarks
at sampling rates of up to 20MSPS? And are there typical formats that are
desirable FM/AM/SSB, IEEE 802.11, Bluetooth, GPS, OFDM, CDMA.

I wouldn't dismiss CATV tuners out of hand. They have come along way with
the advent of inexpensive frequency synthesis and the requirements of DOCSIS
cable modems. The conversion phase noise is specified as dBc/Hz instead of
residual FM (close to 90dBc/Hz at 10kHz offset). The conversion phase noise
of the tuners will limit the demodulated C/N of the signal, my guess is to
around 40dB. This may limit the performance of phase sensitive systems that
require high C/N ratios but these are pretty rare. Phase noise also has the
effect of limiting adjacent channel selectivity performance (aka reciprocal
mixing). The phase noise mentioned is consistent with 50dB selectivity at 25
kHz offsets. As for intermodulation and dynamic range they are required to
function in a multicarrier environment, an 860 MHz cable plant has 110
carriers, and the tuners have intermodulation products typically down 55dBc.
The third order intercept point approaches that of mobile grade LMR
equipment. Again, this performance is consistent with the hobby level
wideband receivers.

As for signal filtering, careful selection of the sampling frequency to
match the selectivity of the tuner eliminates spurious responses. At the ADC
input the signal would be wideband, i.e. 6 MHz. since the bandwidth is wide
relative to the expected signal bandwidth the effects of amplitude and phase
would be minimal. Note: The ADC would need to match the dynamic range
requirements, i.e. 12 bits is sufficient.

The incorporation of an FPGA allows features such as filtering at the sample
rate and digital down conversion to be realize to balance the processing and
data transmission requirements. Note: As USB 2.0 takes hold there may be a
migration path to allow higher sample rates.

Again, this isn't meant to be the ultimate in receiver / tuner performance.
My interpretation from the postings is that a simple and flexible hardware
platform is required.


----- Original Message -----
From: Dave Emery <address@hidden>
To: <address@hidden>
Sent: Saturday, January 11, 2003 8:21 PM
Subject: [Discuss-gnuradio] Re: Welcome and brief update

> 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
> > channelized output of 40 MHz.
> > ADC converter operating at 70 Msps, the idea is that the IF is
> > 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
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