Simple modulation and comparator

There is a communication link using simple PSK or FSK in a channel and 
receiving that by a microcontroller with analog comparator. Changing a 
protocol or a hardware is not an option. Now I am trying to make this 
system work through the bandlimited and phase distorted channel.

The problem is the receiver input is behind the comparator. The signal 
gets heavily damaged due to the linear distortion in the channel; and 
this distortion can't be undone.

What could be an optimal way to demodulate such signal? I am thinking of 
  ML approach, however there could be some pitfalls on this way. Do you 
know of any methods applicable to the distorted signals?


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

On Tue, 07 Jun 2011 10:12:48 -0500, Vladimir Vassilevsky wrote:

> There is a communication link using simple PSK or FSK in a channel and
> receiving that by a microcontroller with analog comparator. Changing a
> protocol or a hardware is not an option. Now I am trying to make this
> system work through the bandlimited and phase distorted channel.
> 
> The problem is the receiver input is behind the comparator. The signal
> gets heavily damaged due to the linear distortion in the channel; and
> this distortion can't be undone.
> 
> What could be an optimal way to demodulate such signal? I am thinking of
>   ML approach, however there could be some pitfalls on this way. Do you
> know of any methods applicable to the distorted signals?

Wow.  After all the useful information has been taken away from you by 
the comparator, I'm not sure if you have much chance to make it work.

I take it there's no way you can change the hardware?

-- 
http://www.wescottdesign.com

On Jun 7, 11:12&#4294967295;am, Vladimir Vassilevsky <nos...@nowhere.com> wrote:
> There is a communication link using simple PSK or FSK in a channel and
> receiving that by a microcontroller with analog comparator. Changing a
> protocol or a hardware is not an option. Now I am trying to make this
> system work through the bandlimited and phase distorted channel.
>
> The problem is the receiver input is behind the comparator. The signal
> gets heavily damaged due to the linear distortion in the channel; and
> this distortion can't be undone.
>
> What could be an optimal way to demodulate such signal? I am thinking of
> &#4294967295; ML approach, however there could be some pitfalls on this way. Do you
> know of any methods applicable to the distorted signals?
>
> Vladimir Vassilevsky
> DSP and Mixed Signal Design Consultanthttp://www.abvolt.com

Seems like this is equivalent to a receiver with a 1-bit ADC, like
those often used in GPS. Since your hardware is already very simple,
I'm guessing that you wouldn't be able to highly oversample and filter
to try to squeeze out some extra effective bits of resolution.

Jason

On 06/07/2011 10:01 AM, Jason wrote:
> On Jun 7, 11:12 am, Vladimir Vassilevsky<nos...@nowhere.com>  wrote:
>> There is a communication link using simple PSK or FSK in a channel and
>> receiving that by a microcontroller with analog comparator. Changing a
>> protocol or a hardware is not an option. Now I am trying to make this
>> system work through the bandlimited and phase distorted channel.
>>
>> The problem is the receiver input is behind the comparator. The signal
>> gets heavily damaged due to the linear distortion in the channel; and
>> this distortion can't be undone.
>>
>> What could be an optimal way to demodulate such signal? I am thinking of
>>    ML approach, however there could be some pitfalls on this way. Do you
>> know of any methods applicable to the distorted signals?
>>
>> Vladimir Vassilevsky
>> DSP and Mixed Signal Design Consultanthttp://www.abvolt.com
>
> Seems like this is equivalent to a receiver with a 1-bit ADC, like
> those often used in GPS. Since your hardware is already very simple,
> I'm guessing that you wouldn't be able to highly oversample and filter
> to try to squeeze out some extra effective bits of resolution.

Not really.  The receivers with the 1-bit ADCs work well in environments 
where the ADC is way oversampled, and the SNR at the ADC is very low. 
That SNR is recovered with coding gain.

If the ADC in this case is getting mostly distorted signal and can't 
have noise injected or be sampled much faster (which I had in mind when 
I asked if he can change the hardware at all) then you're kind of out of 
luck on that front.

I suppose there's a route if the distortion isn't too bad, by looking at 
the timing of the sign changes at the comparator and trying to 
reconstruct the original signal.  It seems a very hard row to hoe, and 
fragile, if it's even possible at all.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html