[R-390] DSP IF

Thu Apr 21 22:26:56 EDT 2005

Cecil,

We built something like this at work (at a higher IF frequency 45 MHz!!).
I'm no expert but..

I think you have to sample the IF with an outboard A/D converter first
before sending it into the DSP to keep the costs down. If you want to talk
about a digital IF processor in an outboard box, the intermediate frequency
spectrum (of the R390A) is first digitized by an analog-to-digital converter
(ADC) into a slower digital data stream; this contains all of the signals
present in the IF. Nyquist says we need to sample at twice the frequency of
our 455 kHz IF and most converters can easily do this. Even low cost Sigma
Delta converters (like the one in your sound card) get close to being able
to do this. There may be some Sigma Deltas around which can do 1 MSPS or
better.

The digitized IF signals are then translated to baseband by something called
a Digital Drop Receiver or Digital Drop Converter  (DDC). This could be a
chip or it more likely is code running in an FPGA or DSP core.
Downconversion is accomplished by digitally mixing the intermediate spectrum
of frequencies with a sinusoidal waveform generated by a synthesizer. The
baseband output can be thought of as the R390A receiver's 4th IF, which is
selected from the DDC's other output frequencies by a very efficient
brick-wall digital filter. It is at this stage that the outboard circuit can
crank in some serious selectivity. Remember, we are talking about
programmable gate arrays or pure software in a DSP for all of these stages
and functions.

The DDC's baseband output is fed to a DSP (or into another section of the
DSP or FPGA which is actually acting as a DSP), which performs signal
demodulation. The DSP's output is then converted from the digital domain
back into the analog domain by a digital-to-analog converter, amplified, and
is made available to Cecil via a bigass tube amplifier and a speaker.

Mike Murphy   WB2UID

----- Original Message ----- 
From: "Cecil Acuff" <chacuff at cableone.net>
To: "Bob Camp" <ham at cq.nu>; "Dave Maples" <dsmaples at comcast.net>
Cc: "R-390 HF Receiver List" <r-390 at mailman.qth.net>
Sent: Thursday, April 21, 2005 8:15 PM
Subject: Re: [R-390] DSP IF

> The thoughts of an  455 Khz IF based DSP filter system has been rolling
> around in the back of my head for quite some time.  An outboard accy. of
> that type would be very popular considering the number of radios that use
> the 455 Khz IF.  I might try sending the IF output from my R-390A to the
> antenna port on my Icom 756 Pro II and use the Pro as an
IF/Processor/Demod
> and see how things stack up...
>
> My understanding is that DSP processors that go up to 455 Khz are quite
> expensive and mostly smoke and mirrors at this point.  You'd have to down
> convert to something lower....
>
> Cecil...
> ----- Original Message ----- 
> From: "Bob Camp" <ham at cq.nu>
> To: "Dave Maples" <dsmaples at comcast.net>
> Cc: "R-390 HF Receiver List" <r-390 at mailman.qth.net>
> Sent: Thursday, April 21, 2005 6:32 PM
> Subject: [R-390] DSP IF
>
>
> > Hi
> >
> > The idea of putting a DSP IF processor on the IF output of an R390
sounds
> > like an interesting project. The nice thing about the 390 is that you
can
> > get at both the IF output *and* the AGC chain without modifying the
radio
> > at all. Given the high performance RF section and the mechanical filters
> > the result should be very competitive.
> >
> > Of course this gets into the general direction of witchcraft and the
like
> > ....
> >
> > Enjoy!
> >
> > Bob Camp
> > KB8TQ
> >
> >
> >
> > On Apr 21, 2005, at 5:22 PM, Dave Maples wrote:
> >
> >> All: Hmmm...why stop there?  Get a large touch-screen display so the
> >> radio
> >> can REALLY be "man-sized", and follow this with a hue-changing routine
> >> where
> >> the panel, knobs, and display fade from color to color.
> >>
> >> Add DSP to correct distortion in the audio chain.
> >>
> >> Possibilities are endless...
> >>
> >> Dave WB4FUR
> >>
> >> -----Original Message-----
> >> From: r-390-bounces at mailman.qth.net
> >> [mailto:r-390-bounces at mailman.qth.net]On Behalf Of Barry
> >> Sent: Wednesday, April 20, 2005 5:49 PM
> >> To: r-390 at mailman.qth.net
> >> Subject: Re: [R-390] Crayola front panels
> >>
> >>
> >> Don't paint the front panel at all.  Mount motor drives on all front
> >> panel
> >> controls with the appropriate serial/parallel interface to a computer
> >> program that looks like the front panel.  Put the R390x right up next
to
> >> the
> >> antenna to eliminate any feedline loss.  Run the controlling wires from
> >> there to the computer.
> >>
> >> Change the color any way you want, any time you want.
> >>
> >> Barry(III) - N4BUQ
> >>
> >> _____________________________________________________________
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> >>
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> >
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