Multichannel receiver (SDR based)

Hi there,

I'm new in the forum. I started reading some book and papers about SDR and
suddenly had some question about a multichannel receiver.
Let's suppose that my receiver is made of an analog RF front end with a
fixed gain, dowconverting the entire band of interest to a low IF freq.
Then, I put an ADC with a sampling frequency high enough so that to
digitize all the channels in the band of interest. The channel selection is
then performed with a standard DDC (NCO, decimation filters...)

1. what does it happen if I have a weak and strong channels? (They don't
have to be necessarily co-channel or adjacent.) 
2. If the analog gain is too high, can the strong channels be properly
demodulated? (ADC overflow?)
3. If the analog gain is too low, can the weak channels be properly
demodulated? (Poor resolution in the ADC?)
4. Does the modulation used affect the receiver performance in this case?
(i.e. it can turn out that it's not an issue for FM signals while it can be
a problem for OFDM)

In a standard receiver (not SDR based), the gain is dynamically adjusted
so that the IF signal takes the whole ADC range, but preventing the
multichannel reception.
My target is to have at the same time:
1. Fixed analog gain
2. Channel selection in the digital domain only

I am not aware of state of the art solutions for multi channel reception
in case of different signal strength. Any clue on that?

Thanks,

Alberto

alberto.fuggetta <alberto.fuggetta@gmail.com> wrote:

> The channel selection is then performed with a standard DDC 
> (NCO, decimation filters...)

This may be a dumb question but what does DDC stand for?

Steve

On Mar 28, 11:18�pm, spop...@speedymail.org (Steve Pope) wrote:
> alberto.fuggetta <alberto.fugge...@gmail.com> wrote:
> > The channel selection is then performed with a standard DDC
> > (NCO, decimation filters...)
>
> This may be a dumb question but what does DDC stand for?
>
> Steve

should be digital down-conversion

On Mar 28, 10:22&#4294967295;pm, "alberto.fuggetta" <alberto.fugge...@gmail.com>
wrote:
> Hi there,
>
> I'm new in the forum. I started reading some book and papers about SDR and
> suddenly had some question about a multichannel receiver.
> Let's suppose that my receiver is made of an analog RF front end with a
> fixed gain, dowconverting the entire band of interest to a low IF freq.
> Then, I put an ADC with a sampling frequency high enough so that to
> digitize all the channels in the band of interest. The channel selection is
> then performed with a standard DDC (NCO, decimation filters...)
>
> 1. what does it happen if I have a weak and strong channels? (They don't
> have to be necessarily co-channel or adjacent.)
> 2. If the analog gain is too high, can the strong channels be properly
> demodulated? (ADC overflow?)
> 3. If the analog gain is too low, can the weak channels be properly
> demodulated? (Poor resolution in the ADC?)
> 4. Does the modulation used affect the receiver performance in this case?
> (i.e. it can turn out that it's not an issue for FM signals while it can be
> a problem for OFDM)
>
> In a standard receiver (not SDR based), the gain is dynamically adjusted
> so that the IF signal takes the whole ADC range, but preventing the
> multichannel reception.
> My target is to have at the same time:
> 1. Fixed analog gain
> 2. Channel selection in the digital domain only
>
> I am not aware of state of the art solutions for multi channel reception
> in case of different signal strength. Any clue on that?
>
> Thanks,
>
> Alberto

I guess you talk about the dynamic range you have at the ADC input.
You can saturate all your bits or perhaps works only on the MSB (quite
difficult in noisy environment, to correctly demodulate.
You should have an AGC circuit before sampling to keep you in the full-
dynamic range of ADC, define the AGC time response in your system.
Do you do quadrature sampling on IF ? or undersampling ? or you have I
and Q in baseband ?

On Mar 28, 11:18�pm, spop...@speedymail.org (Steve Pope) wrote:
> alberto.fuggetta <alberto.fugge...@gmail.com> wrote:
> > The channel selection is then performed with a standard DDC
> > (NCO, decimation filters...)
>
> This may be a dumb question but what does DDC stand for?
>
> Steve

Digital Down Conversion, meaning the oscillator, mixer and filtering
are all down in the digital domain.  These days a digital radio is
built with a broad band analog RF front end amplifier with a mixer to
convert the band to an IF frequency.  This IF is digitized, often in
quadrature, and all further processing done in the digital domain.

The RF front end still requires pre-selectors (front end filters) to
isolate the band of interest.  Otherwise any strong signal source will
overdrive either the RF section or the digital section producing
interference in the band of interest.

Rick

rickman  <gnuarm@gmail.com> wrote:

>On Mar 28, 11:18&#4294967295;pm, spop...@speedymail.org (Steve Pope) wrote:

>> alberto.fuggetta <alberto.fugge...@gmail.com> wrote:

>> > The channel selection is then performed with a standard DDC
>> > (NCO, decimation filters...)

>> This may be a dumb question but what does DDC stand for?

>Digital Down Conversion, meaning the oscillator, mixer and filtering
>are all down in the digital domain.  These days a digital radio is
>built with a broad band analog RF front end amplifier with a mixer to
>convert the band to an IF frequency.  This IF is digitized, often in
>quadrature, and all further processing done in the digital domain.

Thanks.

(Guess I'm not used to prefacing every function I perform in
the digital domain with the word "digital", as this is a bit redundant;
e.g. digital FFT would sound odd.)

So this is a wideband-IF radio with channel selection in DSP.

Steve

alberto.fuggetta <alberto.fuggetta@gmail.com> wrote:

>Let's suppose that my receiver is made of an analog RF front end with a
>fixed gain, dowconverting the entire band of interest to a low IF freq.
>Then, I put an ADC with a sampling frequency high enough so that to
>digitize all the channels in the band of interest. The channel selection is
>then performed with a standard DDC (NCO, decimation filters...)

>1. what does it happen if I have a weak and strong channels? (They don't
>have to be necessarily co-channel or adjacent.) 
>2. If the analog gain is too high, can the strong channels be properly
>demodulated? (ADC overflow?)
>3. If the analog gain is too low, can the weak channels be properly
>demodulated? (Poor resolution in the ADC?)
>4. Does the modulation used affect the receiver performance in this case?
>(i.e. it can turn out that it's not an issue for FM signals while it can be
>a problem for OFDM)

>In a standard receiver (not SDR based), the gain is dynamically adjusted
>so that the IF signal takes the whole ADC range, but preventing the
>multichannel reception.
>My target is to have at the same time:
>1. Fixed analog gain
>2. Channel selection in the digital domain only

>I am not aware of state of the art solutions for multi channel reception
>in case of different signal strength. Any clue on that?

Now that I understand your terminology:

I have rarely, if ever, seen an application where a fixed-gain
receiver was the right solution.  Even if you can get it to work,
implementing AGC is nearly always a better tradeoff.  Especially
if you're receiving an over-the-air signal (as opposed to a wired
connection, where you could maybe get away with it).  Every wireless 
receiver that I have personally seen the final design for had 
an AGC.  

Furthermore, and perhaps less importantly, the earlier in your signal 
chain you can implement selectivity, the better.  Otherwise, you are 
just creating additional opportunities for out-of-band signals to 
mix into the band of interest.

I am not saying that what you're doing will not work -- I don't
know your requirements -- but you may be over-constraining your
design space by deciding it must be wideband and (especially) 
fixed-gain.

Steve

alberto.fuggetta wrote:


> I'm new in the forum. I started reading some book and papers about SDR and
> suddenly had some question about a multichannel receiver.

[...]

For the variety of reasons, the SDR will never reach the performance 
level of the traditional superhet. That may or may not be important 
depending on the particular application.


> I am not aware of state of the art solutions for multi channel reception
> in case of different signal strength. Any clue on that?

You got to have enough of the dynamic range.
Google "RF digital subsystem", "Near-Far problem". Check the web sites 
of AD, TI, Intersil and other major vendors.



Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

alberto.fuggetta wrote:


> My target is to have at the same time:
> 1. Fixed analog gain
> 2. Channel selection in the digital domain only

It depends. The dynamic range of the reasonable high speed ADC is going 
to be about 60...70dB.

VLV

Vladimir Vassilevsky <antispam_bogus@hotmail.com> wrote:
 
> For the variety of reasons, the SDR will never reach the performance 
> level of the traditional superhet. That may or may not be important 
> depending on the particular application.

I probably agree, but I think you should say what is
fixed and what isn't.  If you have a cheap superhet receiver,
and no holds SDR (high sample rate and dynamic range on
the A/D conversion, etc.) then I would probably believe 
the SDR was better.

-- glen