On Jun 23, 6:17 am, Paul Keinanen <keina...@sci.fi> 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
and decimated by a factor of 48 to produce the 280 Ksamples/s output.
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
I'm missing something ... assuming no aliasing problems why is
a higher sampling rate necessary for recovering the audio and RDS?
Reply by John●June 23, 20072007-06-23
On Jun 23, 11:42 am, Jerry Avins <j...@ieee.org> 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.
> 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?
Reply by Randy Yates●June 23, 20072007-06-23
> On Jun 23, 6:17 am, Paul Keinanen <keina...@sci.fi> 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
> and decimated by a factor of 48 to produce the 280 Ksamples/s output.
> Looking at:
> 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
% 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."
%%%% <firstname.lastname@example.org> % 'Yours Truly, 2095', *Time*, ELO
Reply by ●June 24, 20072007-06-24
On Fri, 22 Jun 2007 13:51:36 GMT, John Ferrell
>On Thu, 21 Jun 2007 17:44:13 -0700, email@example.com 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"
Reply by Alex Gibson●June 25, 20072007-06-25
<firstname.lastname@example.org> wrote in message
> 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
> I'm not looking to use a FPGA at this time.
> -- John
On Sat, 23 Jun 2007 11:56:21 -0700, email@example.com wrote:
>The IF is 1.7 MHz which is sampled using a 13.6 MHz clock by the
>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
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.
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.