FM Filters and Digital Signals?

[John_M]

I suspect that none of our FM Filter manufactures have not tested their filters with Digital Signals. Recieving Digital signals while using an FM filter may very well distort the Digital signal and result in poor reception of these signals. This is a good subject and worthy of attention.

John

 

[gr8amp]

I suspect that none of our FM Filter manufactures have not tested their filters with Digital Signals. Recieving Digital signals while using an FM filter may very well distort the Digital signal and result in poor reception of these signals. This is a good subject and worthy of attention.

John

Care to explain how a FM broadcast band filter can result in distortion of a digital signal? This doesn't make any sense.

 

[N_Jay]

I suspect that none of our FM Filter manufactures have not tested their filters with Digital Signals. Recieving Digital signals while using an FM filter may very well distort the Digital signal and result in poor reception of these signals. This is a good subject and worthy of attention.

John

Care to explain how a FM broadcast band filter can result in distortion of a digital signal? This doesn't make any sense.

That is so much more eloquent a response than what I was thinking. :wink:

 

[John_M]

Have each of you actually tested a FM Filter. Each filter has a slightly different design. Different types of signals can result in wierd
signals at the output of the filter. There are many ways to design filters.
Unless you have personally tested a FM filter with digital signals you should be quiet until somebody does. Don't take anything for granted. I suspect that these filters were tested only with an analog Sine wave.

 

[N_Jay]

Have each of you actually tested a FM Filter. Each filter has a slightly different design. Different types of signals can result in wierd signals at the output of the filter. There are many ways to design filters.

The FM trap filters are simple one and two stage LC arrangements.

They are extremely effective at adding attenuation over a portion of the RF band. They do nothing significant within the desired pass band that would effect the modulation or encoding of any digital signal used in LMR systems.

And YES, I have used all sorts of filters in many radio system both digital (Paging, DVP/DES/Trucking Control/P25) and analog.

Look at it this way. There are hardly any filters in use in LMR that are more critical than those in a duplexer. And duplexers are the same for both digital and analog systems.

Unless you have personally tested a FM filter with digital signals you should be quiet until somebody does. Don't take anything for granted. I suspect that these filters were tested only with an analog Sine wave.

You don't have to "take things for granted" when you understand the engineering that goes into the systems.

 

[John_M]

You have answered my question. I am not familiar with filters used in radio systems. I just wanted to be sure. A seemingly dumb question to you may not be a dumb question to someonelse. As far as taking things for granted N,Jay obviously you are familiar with these filters. When you are familiar you can take things for granted. (In the design and testing stage there is no such thing as taking things for granted.) Right?

 

[gr8amp]

Different types of signals can result in wierd
signals at the output of the filter.

I am not convinced, please elaborate.

Unless you have personally tested a FM filter with digital signals you should be quiet until somebody does. Don't take anything for granted.

Nothing is taken for granted here.

Unless you have personally tested a FM filter with digital signals you should be quiet until somebody does. Don't take anything for granted. I suspect that these filters were tested only with an analog Sine wave.

Define your defininition of a digital signal? Do you mean a square wave (Which, just FYI, is made of a series of sine waves)? I think you are missing the concept of modulation.

 

[PhilJSmith67]

A filter does not change the shape of a modulated waveform at a specific frequency. Don't buy into this "digital signals act differently than analog" concept. EVERY signal is an analog signal. Even the new generation of COFDM and 8VSB HDTV modulation methods result in analog signals. Likewise, there is no such thing as an HDTV or Digital TV antenna. It either picks up a segment of frequencies, or it doesn't.

 

[John_M]

Different types of signals can result in wierd
signals at the output of the filter.

I am not convinced, please elaborate.

Unless you have personally tested a FM filter with digital signals you should be quiet until somebody does. Don't take anything for granted.

Nothing is taken for granted here.

Unless you have personally tested a FM filter with digital signals you should be quiet until somebody does. Don't take anything for granted. I suspect that these filters were tested only with an analog Sine wave.

Define your defininition of a digital signal? Do you mean a square wave (Which, just FYI, is made of a series of sine waves)? I think you are missing the concept of modulation.

I believe that a Digital Signal is not a perfect series of Square Waves nor is it a series of Sine waves. There is something that distinguishes it to be considered a Digital Signal. (A not so perfect
Square wave) The DSP in a Digital Scanner has to be able to distinguish
between an Anolog Voice Signal and a Digital voice signal some how.

As far as wierd signals at the output of a filter. I have seen many strange looking signals from the output of a filter with different types of signals fed into it.

 

[John_M]

Guys I have to retire for the night I have a F'in spittling headache.

 

[Thayne]

WOW


Wooey



Amazing

You got off easy--at least the guy from Michigan and/or Batman didn't shoot at you!

 

[N_Jay]

I believe that a Digital Signal is not a perfect series of Square Waves nor is it a series of Sine waves. There is something that distinguishes it to be considered a Digital Signal. (A not so perfect
Square wave) The DSP in a Digital Scanner has to be able to distinguish
between an Anolog Voice Signal and a Digital voice signal some how.

As far as wierd signals at the output of a filter. I have seen many strange looking signals from the output of a filter with different types of signals fed into it.

I believe every possible waveform is made up of a series of sine waves. (Either that or I wasted 4 years in engineering school). :wink: :wink:

 

[astrodanco]

EVERY signal is an analog signal

You bet it is. This is actually rather important to realize. We live in an inherently analog universe. Digital is an abstraction. Usually a first order abstraction, but sometimes even further removed.

 

[John_M]

I must have also wasted my 4 and 1/2 years. BSEET 1989.
(sometimes I really think so) Its been a while since I have worked with circuits about 3 years. This website brings back memories, some good memories. To tell you the truth I have no desire to reenter the field. My boss told me when I started working at Kodak: This job isn't for everybody. (in my opinion it wasn't for anybody) After about 3 years I found out just exactly what he was talking about. Kodak layed off 1500 people in 1999 I was one of the unfortunates. Actually it was a blessing. In 1982 Kodak here in Rochester employed over 60,000 people, as of Jan 1 2005 Kodak now employs 17,000 people. Every year around Christmas there seems to be another layoff. Digital Photography is the main reason. I never worked with RF technology so please bare with me. Iv'e worked with all kinds of circuit board test equipment though, and I have seen alot of strange signals on Oscilloscopes from filtering circuits and other circuits even if you guys don't believe me. Most of the voltages I dealt with were +-15 and +-5vdc. Ive also worked with 120ac to wire relays and other instrumentation.

"This job is not for everybody" Willard Rhoads 1992

 

[John_M]

EVERY signal is an analog signal

You bet it is. This is actually rather important to realize. We live in an inherently analog universe. Digital is an abstraction. Usually a first order abstraction, but sometimes even further removed.

You do not think like this when using designing logic circuits.
Everything is either a 1 or a 0.

N,Jay remember Fourier this must ring a bell!


I still think you guys are alright.


John.

 

[N_Jay]

EVERY signal is an analog signal

You bet it is. This is actually rather important to realize. We live in an inherently analog universe. Digital is an abstraction. Usually a first order abstraction, but sometimes even further removed.

You do not think like this when using designing logic circuits.
Everything is either a 1 or a 0.

N,Jay remember Fourier this must ring a bell!


I still think you guys are alright.


John.

Yes, Fourier (ouch my brain hurts) Tranforms!

BSEET We won't get in to the whole argument about what is and what is not an engineering degree. :twisted: :wink:

 

[PhilJSmith67]

You do not think like this when using designing logic circuits.

You are absolutely correct when designing logic circuits.

However, we are talking about RF here; specifically, modulated signals filtered through a simple RC, LR, and LCR circuit. The rules of the analog world are the only rules that apply. These circuits do not distinguish between analog and digital signals. They might augment the amplitude of one or more ranges of frequencies, but the modulated signals themselves are not augmented. In the scope display above, if the signal we want is outside the notch, the signal will come through in its entirety.