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microe

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How to remove chattering in FSK RF receivers
Sep 09^th, 2009, 10:53pm

In an FSK receiver with analog base band (IF filter, FSK demod, data filter, data slicer), when there is no signal, the output will chatter due to noise. What is the typical way to remove this chatter?

I saw some loss of signal detection circuit in fiber optical receiver using RSSI to detect the signal strength and compare it to a given voltage to decide if it is signal or noise. Due to the gain and noise variation in the front end, the noise voltage at RSSI output (power converted to voltage after RSSI) will vary from part to part.

One way to deal with the random variation is to give a lot of margin to the threshold voltage at the cost of degradation of sensitivity. Is there any way to minimize the degradation on sensitivity? Is there any paper or books on this topic?

Thanks!

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biff44

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Re: How to remove chattering in FSK RF receivers
Reply #1 - Sep 13^th, 2009, 9:02am

I am curious...what exactly do you want it transmit when there is no analog input?

For regulatory reasons, you would want it to go to the center of the frequency channel it is assigned to and send out a CW tone. That CW tone will have some measurable phase noise on it too.

If you did not want it to transmit, I guess you could have a analog input level monitor that turned off the transmitter when an input was missing.

Most digital FSK transmitters, by regulation, need to have a randomizer on the input data to insure that a psuedorandom bit pattern is generated when there is not data to transmit. That keeps the "center of mass" of the transmitted energy in the middle of the channel, not all on one side or the other of the channel width.

Rich
Maguffin Microwave LLC
www.MaguffinMicrowave.com

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microe

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Re: How to remove chattering in FSK RF receivers
Reply #2 - Sep 13^th, 2009, 1:04pm

Hi Rich:

Thanks for your reply.

The problem I have is, when the signal is gone or weaker than the sensitivity, the receiver picks up the noise and gives false data at data out, that is the chattering. My question is what is typically used in RF receivers to solve this problem.

It is in the receiver side, not the transmitter.

Thanks.

XF
biff44 wrote on Sep 13^th, 2009, 9:02am:

I am curious...what exactly do you want it transmit when there is no analog input? �

For regulatory reasons, you would want it to go to the center of the frequency channel it is assigned to and send out a CW tone. �That CW tone will have some measurable phase noise on it too.

If you did not want it to transmit, I guess you could have a analog input level monitor that turned off the transmitter when an input was missing.

Most digital FSK transmitters, by regulation, need to have a randomizer on the input data to insure that a psuedorandom bit pattern is generated when there is not data to transmit. �That keeps the "center of mass" of the transmitted energy in the middle of the channel, not all on one side or the other of the channel width.

Rich
Maguffin Microwave LLC
www.MaguffinMicrowave.com

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biff44

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Re: How to remove chattering in FSK RF receivers
Reply #3 - Sep 14^th, 2009, 8:29am

You said analog baseband, but now are talking about data. If you are sending data, the standard way to stop what you are seeing is to use a header that is recognizable by your receiver. No header...no output data.

If you are sending something like video, you might be able to look for the big dynamic range (synch pulse), and squelch if you do not see it.
Rich
Maguffin Microwave LLC

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microe

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Re: How to remove chattering in FSK RF receivers
Reply #4 - Sep 14^th, 2009, 8:42am

Hi Rich:

Thanks for your suggestion.

When I said data, I meant the output of the baseband. As you can see from my first post, the baseband I meant includes data filter and slicer.

I like your idea of header detection. The limitation is that the receiver will work only with the predefined header, unless the receiver is programmable.

I prefer more generic analog squelch circuit, which looks at the RSSI output and decides if there is anything higher than noise floor. But due to the random variation at RSSI output and front end noise, it seems require some onchip calibration to record the noise floor. I am trying to see if there is any existing product or publication on this topic.

Thanks.

XF

biff44 wrote on Sep 14^th, 2009, 8:29am:

You said analog baseband, but now are talking about data. �If you are sending data, the standard way to stop what you are seeing is to use a header that is recognizable by your receiver. �No header...no output data.

If you are sending something like video, you might be able to look for the big dynamic range (synch pulse), and squelch if you do not see it.
Rich
Maguffin Microwave LLC

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ACWWong

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Re: How to remove chattering in FSK RF receivers
Reply #5 - Sep 15^th, 2009, 2:36pm

Hi Microe,

This is an interesting topic. I've seen and used the methods of header/preamble/sync word detection and analog squelch.

Calibration of RSSI on noise (for example by diconnecting on chip antenna switch) is one way to get the best sensitivity if using analog methods. Also deliberate DC offset in the limitter (e.g by size mismatch in ltp) can ensure no chattering under no input signal condition, but again care must be taken to ensure this is sufficent over PVT... that is the DC is just more than the noise so not to impact the sensitivty due possible reduction in SNR. It can also lead to other complications like limitter output duty cycle error when signal is present which may or may not be important, but in some cases doing this method maybe better than RSSI calibration.
Also perhaps a de-glitch circuit somewhere in the analog backend can help as it acts to eliminate chatter within the deglitch circuit delay.

Hope this helps. good luck.

Cheers
aw

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microe

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Re: How to remove chattering in FSK RF receivers
Reply #6 - Sep 15^th, 2009, 6:30pm

Hi AW:

Thanks a lot for your information! I will follow your suggestions.
BTW, is there any publication or product datasheet/appnote that you are aware of on this topic?
Thanks!

XF

ACWWong wrote on Sep 15^th, 2009, 2:36pm:

Hi Microe,

This is an interesting topic. I've seen and used the methods of header/preamble/sync word detection and analog squelch.

Calibration of RSSI on noise (for example by diconnecting on chip antenna switch) is one way to get the best sensitivity if using analog methods. Also deliberate DC offset in the limitter (e.g by size mismatch in ltp) can ensure no chattering under no input signal condition, but again care must be taken to ensure this is sufficent over PVT... that is the DC is just more than the noise so not to impact the sensitivty due possible reduction in SNR. It can also lead to other complications like limitter output duty cycle error when signal is present which may or may not be important, but in some cases doing this method maybe better than RSSI calibration.
Also perhaps a de-glitch circuit somewhere in the analog backend can help as it acts to eliminate chatter within the deglitch circuit delay.

Hope this helps. good luck.

Cheers
aw

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ACWWong Community Fellow Offline Posts: 539 Oxford, UK	Re: How to remove chattering in FSK RF receivers Reply #7 - Sep 16^th, 2009, 12:37pm Hi Microe, Sorry I don't know of any particular published work or apps note on this issue... all I could suggest is googling it or looking at apps notes about FSK transceiver/receivers. cheers aw
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microe Junior Member Offline Posts: 16	Re: How to remove chattering in FSK RF receivers Reply #8 - Sep 16^th, 2009, 5:25pm Hi AW: I will do the research. Thanks! Regards, XF ACWWong wrote on Sep 16^th, 2009, 12:37pm: Hi Microe, Sorry I don't know of any particular published work or apps note on this issue... all I could suggest is googling it or looking at apps notes about FSK transceiver/receivers. cheers aw
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loose-electron

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Re: How to remove chattering in FSK RF receivers
Reply #9 - Oct 2^nd, 2009, 9:32am

Agree with AW - the areas of squelch, hysterisisis or intentional offsets may all be applicable. Your circuit architecture should be looked at with the question - "when the signal amplitude gets too low what can I do to suppress random tranisients on the output?

It could be as simple as applying a defined offset someplace, or as complicated as using a log amplifeir and RSSI system to determine loss of signal.

If you have an AGC system, you might monitor the gain control, and when the gain hits maximum, digitally suppress the detector output.

Lots of possibilities.

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Jerry Twomey
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