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SSB Weaver method filtering

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gbugh

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I want to use a 1.2KHz low pass filter as part of a Weaver method SSB design.

Kind of like this: http://lea.hamradio.si/~s53mv/zifssb/block/b03.gif

I see 8th order and 10th order low pass filter chips available but I'm not sure which type is best, i.e. Elliptic, Bessel, Butterworth, Linear Phase, etc.

Which would provide the clearest voice audio? Linear Phase is best, right? Are any of them good enough?

I only want a switch capacitor type because I want to varying the cut off frequency to hear how it affects the audio out, in case I decide I like some other corner frequency more than 1.2 KHz. After I'm sure what cut off frequency I like I can make a non switched filter if I notice any problem with the switched design.
 
Last edited:

Here the most important specification of the filter is hih sideband rejection.Therefore elliptic filters are more convenient.
 

What about receiver target infrequences?
If they are below 1GHz ( especially @ the short wave range 0.1-30MHz LNA determinately not necessary, sometimes should be attenuator ) LNA really is under big question. Strong intermodulation :-? .
 

Well I did say kind of like that diagram.

It is for 0.1-30MHz and will have an LNA, kind of. I was going to use a magnetic loop antenna with a target Q of 400 and voltage sense across the tuning cap with 2 JFET source followers

Once I get a receiver working and move on to a transceiver then some of that may change. I have to figure out how to transmit with the same loop and not distort the signal from the clamping diodes protection circuitry on the receive circuitry.

This project is to help me learn RF design, going where all other RF designers have gone already, unencumbered by experience, LOL.
 

The clearest voice audio would be given by the filter with minimum Group Delay.
Bessel type has lowest group delay but a good compromise between group delay and rejection is Butterworth type.
 

Since you are recommending something different than Big Boss, I've been studying and trying to understand the tradeoffs better.

I know I need a steep cut off so I would need a higher order filter to cut off as fast as an elliptic filter.

But I'm getting confused about the various types of filters. I see 3 different things promoted in the various text I'm coming across:

constant group delay
minimum group delay
linear group delay

From what I'm reading, it seems like "constant group delay" and "linear group delay" mean the same thing.

I see Group Delay defined as: "a measure of the time delay of the amplitude envelopes of the various sinusoidal components of a signal through a device under test, and is a function of frequency for each component."

I can't tell if this definition is talking about:

how much low frequency components are delayed relative high frequency components (spreading apart of the various frequency components)

or

how much the whole group is delayed relative the various frequency components of the filter input signal. (delay of the whole group relative the original input)

Regardless, when I read up on "Linear Phase", I see: "Since a linear phase (or generalized linear phase) filter has constant group delay, all frequency components have equal delay times."

So if I understand correctly a Bessel filter is a "linear phase" low pass filter but at Digikey.com they have still another category of switched capaitor filter called "Linear Phase" so these ought to also provide clear audio, yes?

For example the 10th order LP filter LTC1569 ought to work well for clear audio,
https://www.digikey.com/product-detail/en/LTC1569CS8-6#PBF/LTC1569CS8-6#PBF-ND/891094

though being more expensive and not as sharp of a cut off as an 8th order elliptic filter like the MAX7403
https://www.digikey.com/product-detail/en/MAX7403CSA+/MAX7403CSA+-ND/1513320

- - - Updated - - -

I didn't say that right.

Its not that the Bessel filter has constant group delay, its that it has low group delay like you were saying.

Since there is low delay there is less time for the spreading or smearing of the various frequency componenets, right?

But if a filter has more delay but that delay is linear, that would be ok also for low distortion, right?

OK, sorry if I'm thinking out loud too much.
 

gbugh said:
Since you are recommending something different than Big Boss, I've been studying and trying to understand the tradeoffs better.

I know I need a steep cut off so I would need a higher order filter to cut off as fast as an elliptic filter.

But I'm getting confused about the various types of filters. I see 3 different things promoted in the various text I'm coming across:

constant group delay
minimum group delay
linear group delay

From what I'm reading, it seems like "constant group delay" and "linear group delay" mean the same thing.

I see Group Delay defined as: "a measure of the time delay of the amplitude envelopes of the various sinusoidal components of a signal through a device under test, and is a function of frequency for each component."

I can't tell if this definition is talking about:

how much low frequency components are delayed relative high frequency components (spreading apart of the various frequency components)

or

how much the whole group is delayed relative the various frequency components of the filter input signal. (delay of the whole group relative the original input)

Regardless, when I read up on "Linear Phase", I see: "Since a linear phase (or generalized linear phase) filter has constant group delay, all frequency components have equal delay times."

So if I understand correctly a Bessel filter is a "linear phase" low pass filter but at Digikey.com they have still another category of switched capaitor filter called "Linear Phase" so these ought to also provide clear audio, yes?

For example the 10th order LP filter LTC1569 ought to work well for clear audio,
https://www.digikey.com/product-detail/en/LTC1569CS8-6#PBF/LTC1569CS8-6#PBF-ND/891094

though being more expensive and not as sharp of a cut off as an 8th order elliptic filter like the MAX7403
https://www.digikey.com/product-detail/en/MAX7403CSA+/MAX7403CSA+-ND/1513320

- - - Updated - - -

I didn't say that right.

Its not that the Bessel filter has constant group delay, its that it has low group delay like you were saying.

Since there is low delay there is less time for the spreading or smearing of the various frequency componenets, right?

But if a filter has more delay but that delay is linear, that would be ok also for low distortion, right?

OK, sorry if I'm thinking out loud too much.
Click to expand...

Since you don't receive HiFi audio signal, don't worry about Group Delay.It may be a trouble for wideband receiption..
Your bandwidth is only 1.2kHz... So, it's nothing...
If you've been receiving HiFi audio signals or wideband video signal etc. in this case group delay can be a problem because of very different delay time for each frequency point and it creates fading or echo or moire effects on audio/video signals.But your case is SSB,,

In these radios, crystal filters or ceramic filters are used and they have very poor group delay performance but never mind.Sideband rejection is much more important.
 

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