[hpsdr] Further Observations on the PowerSDR Noise Blanking Process

[Graham / KE9H]

Mike:

The way that most noise blankers work is that they turn off or gate the 
RF signal
path during the period that an interfering noise impulse is present.

The narrower filters associated with the demodulated signal will "heal"
or smooth out the short hole in the RF signal, improving the ability
to receive dramatically, in the presence of impulse noise.

The bad news is that another way to describe a circuit that turns an RF
signal path on and off in time with a control signal is to call it a Mixer.

So the gating process that protects the downstream parts of the receiver
from the loud impulses, also creates sidebands or mixing products at the
noise repetition rate around all of the signals in the band. If the noise
repetition rate is somewhat random, then the mixing sidebands are also
random noise, and you start to see the noise floor rise across the band,
worse in the immediate vicinity of the stronger signals.

It is hard to fool Mother Nature, and get something for nothing.

--- Graham / KE9H

==

On 4/19/2011 10:38 AM, David McQuate wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> Mike,
>   Your surmise is correct.  Noise blanking is performed before the 
> bandwidth filters are applied.  This is the same order as is usually 
> done in analog radios.  In reducing the bandwidth, the filters slow 
> the rise & fall times of the signal, "smearing out" noise impulses, 
> making them harder to remove.  And yes, noise processing is usually 
> done in the time domain.
> 73,
> Dave
> wa8ywq
>
>
> On 4/19/2011 8:04 AM, Mike F wrote:
>> ***** High Performance Software Defined Radio Discussion List *****
>>
>> I made a posting couple of weeks ago on a noise floor phenomena where 
>> the noise floor across the displayed band varied up and down with the 
>> presence or absence of strong in-band signals.  Phil Harman explained 
>> that it had to do with the NB and NB2 functions in PowerSDR.  At that 
>> time I had it in mind that these functions might be making the noise 
>> worse.
>>
>> Recently the noise floor has been quite high at my QTH (S5) and I 
>> have determined the following.
>>
>> As I turn the NB and NB2 functions on, the noise floor drops 
>> significantly.  When a strong signal within the band increases 
>> amplitude the noise floor noticeably increases, dropping when the 
>> signal decreases or goes away.  I don't believe that the noise floor 
>> ever gets as high as it is without the NB-NB2 functions turned on, 
>> but it does vary.
>>
>> Based on these observations I would surmise that the noise blanking 
>> is applied to the whole 192kHz (in my case) bandwidth (as a time 
>> domain function???) and if there is a significantly large signal 
>> within the band, the blanking threshold/algorithm becomes less 
>> sensitive to the lower amplitudes.
>>
>> Can anyone out there confirm or deny my suppositions?
>>
>> Thanks.
>>
>> Mike Fager, K7SR
>>
>>
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