Paul Keinanen wrote:

...

Even with a proper S&H (ten nanosecond sampling and several
microsecond hold times) for decimation, the 280 Ksamples/s sounds a
bit low for FM broadcast detection. The higher order (Bessel function)
sidebands are still quite strong with the modulation indexes used, so
truncating the bandwidth to the 100-140 kHz range might not be a good
idea.

...

The IF passband of an FM receiver needs to be at least 200 KHz for good
quality and -- counterintuitive to me -- I'm told that the capture ratio
improves as the bandwidth increases. Back in the 50s, one premium
receiver -- the first I knew of to use semiconductor diodes in the
detector -- had a half-MHz IF.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

On Jun 23, 6:17 am, Paul Keinanen wrote:
Even with a proper S&H (ten nanosecond sampling and several
microsecond hold times) for decimation, the 280 Ksamples/s sounds a
bit low for FM broadcast detection.

The IF is 1.7 MHz which is sampled using a 13.6 MHz clock by the
AD9874
and decimated by a factor of 48 to produce the 280 Ksamples/s output.

Looking at:

http://en.wikipedia.org/wiki/Fm_broadcasting

I see that FM audio goes from 0 to 53 KHz and RDS is at 57 KHz.
The AD9874 decimation filter should prevent any signal from aliasing
into the bandwidth of interest and the resulting sample rate is
above Nyquist.

I'm missing something ... assuming no aliasing problems why is
a higher sampling rate necessary for recovering the audio and RDS?

-- John

On Jun 23, 11:42 am, Jerry Avins wrote:
Paul Keinanen wrote:

...

Even with a proper S&H (ten nanosecond sampling and several
microsecond hold times) for decimation, the 280 Ksamples/s sounds a
bit low for FM broadcast detection. The higher order (Bessel function)
sidebands are still quite strong with the modulation indexes used, so
truncating the bandwidth to the 100-140 kHz range might not be a good
idea.

...

The IF passband of an FM receiver needs to be at least 200 KHz for good
quality and -- counterintuitive to me -- I'm told that the capture ratio
improves as the bandwidth increases. Back in the 50s, one premium
receiver -- the first I knew of to use semiconductor diodes in the
detector -- had a half-MHz IF.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Assuming cochannel signals with small power difference, as the BW
increases the weaker
one will hit threshold (knee in the SNRout vs SNRin curve) first. Does
that explain it?

John

writes:

On Jun 23, 6:17 am, Paul Keinanen wrote:
Even with a proper S&H (ten nanosecond sampling and several
microsecond hold times) for decimation, the 280 Ksamples/s sounds a
bit low for FM broadcast detection.

The IF is 1.7 MHz which is sampled using a 13.6 MHz clock by the
AD9874
and decimated by a factor of 48 to produce the 280 Ksamples/s output.

Looking at:

http://en.wikipedia.org/wiki/Fm_broadcasting

I see that FM audio goes from 0 to 53 KHz and RDS is at 57 KHz.
The AD9874 decimation filter should prevent any signal from aliasing
into the bandwidth of interest and the resulting sample rate is
above Nyquist.

I'm missing something ... assuming no aliasing problems why is
a higher sampling rate necessary for recovering the audio and RDS?

You're looking at the audio baseband bandwidth, i.e., the signal after
being FM demodulated. From your description the signal you're getting
has not yet been FM-demodulated. Thus you need to account for the +/-
75 kHz peak deviation of the FM signal, at a minimum. If you can
swallow up the whole 200 kHz, you gain slightly better performance due
to the admission of more the bessel function sidebands, as others have
noted.
--
% Randy Yates % "Maybe one day I'll feel her cold embrace,
%% Fuquay-Varina, NC % and kiss her interface,
%%% 919-577-9882 % til then, I'll leave her alone."
%%%% % 'Yours Truly, 2095', *Time*, ELO
http://home.earthlink.net/~yatescr

On Fri, 22 Jun 2007 13:51:36 GMT, John Ferrell
wrote:

On Thu, 21 Jun 2007 17:44:13 -0700, wrote:

I'm working on a SDR design using the AD9874 to
digitize the IF producing 280 Ksamples/sec and
I am way behind you guys on the DSP-SDR subject but I appreciate every
scrap of information you share.

I am currently trying to puzzle out the strategy used in acquiring
data from a sound card.

Using C++, it's not that hard. This page of my website,
http://sdeyoreo.tripod.com/id5.html gives the steps and code to
capture sound and fill an array with the samples from the soundcard.
Fell free to ask me more if you want.

I have successfully built the Softrock Lite 40 meter receiver and am
ever so slowly working on a Softrock RXTX Ver 6.2 40 M transceiver.

John Ferrell W8CCW
"Life is easier if you learn to
plow around the stumps"

wrote in message
ups.com...
I'm working on a SDR design using the AD9874 to
digitize the IF producing 280 Ksamples/sec and
am trying to size the DSP. I'm leaning towards
the ADSP-BF532 which is a fixed point DSP rated
at 400 MIPS / 800 MMACS and is available in a LQFP
package. I'd like to be able to handle everything
from decoding standard shortwave SSB signals to
broadcast FM stereo signals (including RDS).

1) Any pointers to information which can be used
to size the DSP in terms of MIPS, program RAM,
and data RAM? Something like a minimum of X MIPS
are needed to do a reasonable job handling
broadcast FM stereo signals (which I'm assuming
require more MIPS than SSB) would be useful
to know.

2) Anyone have a specific DSP they favored for
this type of application? Keep in mind I
need the DSP to be in a package I can handle
using home equipment. It would be nice if the
DSP was flexible enough so that it can be used
for general purpose things such as controlling
the keyboard and LCD instead of adding a
microcontroller.

I'm not looking to use a FPGA at this time.

-- John

Can you use one of the blackfin stamp boards ?
http://blackfin.uclinux.org/gf/project/stamp

Digikey sells the BF537 stamp board for US$226
http://www.digikey.com/scripts/DkSea...440505&Site=US

http://www.analog.com/en/prod/0,2877...DSTAMP,00.html

Could always try and make a network interface for controlling the radio.


I'm just starting to look at what I need for getting a similar set up but
A.M
and ssb only for students to use in a lab to introduce them to the bascs of
sdr.

The lecturers want the students to program different modules ,
different types of modulation , encoding and decoding, set different
sampling rates etc They want the students to be able use both c and matlab.

Looks like we'll be using a c6713 dsk as the base
(thanks to TI providing free software)
I just get to assemble/build/integrate what gets specified.

Someone in the faculty has a couple of lyrtech signalwave boards
which they aren't using(been sitting in their filling cabinet for a few
years), have to see if we can prise them off them.
Then see what it costs to get support (need the up to date software).
http://www.lyrtech.com/DSP-developme...signalwave.php




Alex

On Sat, 23 Jun 2007 11:56:21 -0700, wrote:


The IF is 1.7 MHz which is sampled using a 13.6 MHz clock by the
AD9874
and decimated by a factor of 48 to produce the 280 Ksamples/s output.

With 1.7 MHz IF you are going to have to construct your own LC band
pass filters in front of the AD9874, if 1.7 MHz is the first IF.

If this is the second IF and you have proper filtering (ceramic or
crystal) at the first IF, say 10.7 MHz, a very simple filter at 1.7
MHz should be sufficient. Due to not so spectacular performance of
that chip, I would suggest using a selectable IF filter at 10,7 MHz
(or whatever the first IF is), to even roughly match the required
bandwidth of the transmission.

If the IF filter would be 110-280 kHz wide to allow broadcast FM
reception, using such receiver in Europe to receive the 7.0-7.1 MHz
amateur band when there are very high power international broadcasters
starting at 7.1 MHz, might give quite disappointing results, since
those strong broadcast signals would control the AGC reducing
sensitivity and you still would get a lot of intermodulation products.

It also appears that the quoted dynamic range applies only to certain
AGC settings.

The use of 1.7 MHz IF with a very wide input filter may be problematic
at least in countries that are still using the AM broadcast band
actively, with very strong signals up to 1.6 MHz. On the other hand,
local 1.8 MHz amateur radio signals may be quite strong. In any
superheterodyne receiver, there is always a risk for signal
break-through from the environment to the IF stage at that frequency
range. For this reason, common IF frequencies, such as 10.7 MHz and
455 kHz, are usually excluded from transmitter frequency assignments.

Looking at:

http://en.wikipedia.org/wiki/Fm_broadcasting

I see that FM audio goes from 0 to 53 KHz and RDS is at 57 KHz.

While the modulation index for the RDS signal is quite low, thus
producing only the first order Bessel sidebands at +/- 57 kHz from the
carrier, the stereo difference signal S can have a quite high
modulation index, thus producing the first pair of Bessel sidebands
below +/-53 kHz from the carrier but also producing some significant
second order Bessel sidebands below +/-106 kHz and possibly even the
third order at +/-159 kHz.

Of course, this requires that there is a strong high pitch tone only
in one audio channel to get a huge difference signal.

Paul OH3LWR