# Bfsk

**URL:** <https://www.ruby-forum.com/t/bfsk/233099>\
**Category:** GNU Radio\
**Created:** [November 17, 2014, 8:10pm UTC](https://www.ruby-forum.com/t/bfsk/233099 "2014-11-17T20:10:31Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\
**Post date:** [November 17, 2014, 8:10pm UTC](https://www.ruby-forum.com/t/bfsk/233099/1 "2014-11-17T20:10:31Z")

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Would someone please recommend a book that steps through the  
implementation  
used in GNU Radio for the Frequency Mod and Quadrature Mod blocks? I’ve  
looked at the source code and I have seen posts trying to explain what  
sensitivity and gain are, but my lack of background knowledge is making  
those sources not very helpful.

Really appreciated,  
Rich

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**Author:** ![reback](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/b5ac83/32.png) [@reback](https://www.ruby-forum.com/u/reback)\
**Post date:** [November 17, 2014, 10:21pm UTC](https://www.ruby-forum.com/t/bfsk/233099/2 "2014-11-17T22:21:44Z")

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Hi Richard,

I suffered through understanding the quadrature mod/demod block settings  
a  
while back. I provide some links to that information below. I provide  
a  
formula for the settings for both digital and audio applications. As  
you  
can see (from the reading), the settings can be derived from  
samples-per-symbol and modulation index or deviation and baud rate. Or  
just peak deviation and sample rate for audio.

The quadrature demod gain is really just a gain, btw. If you set it  
right,  
your demodulated audio will have unity gain, which is nice/necessary for  
most of the clock synchronizers.

Thread:  
[http://lists.gnu.org/archive/html/discuss-gnuradio/2013-07/msg00058.html](http://lists.gnu.org/archive/html/discuss-gnuradio/2013-07/msg00058.html)  
Wiki Summary:  
[http://gnuradio.org/redmine/projects/gnuradio/wiki/SignalProcessing](http://gnuradio.org/redmine/projects/gnuradio/wiki/SignalProcessing)

Very Respectfully,

Dan CaJacob

On Mon, Nov 17, 2014 at 2:09 PM, Richard B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)

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**Author:** ![reback](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/b5ac83/32.png) [@reback](https://www.ruby-forum.com/u/reback)\
**Post date:** [November 18, 2014, 2:31pm UTC](https://www.ruby-forum.com/t/bfsk/233099/3 "2014-11-18T14:31:40Z")

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If you’re interested in the workings of the CPM modulators (but it seems  
like Dan’s response might be more appropriate), a very complete  
description can be found in “Anderson, Aulin and Sundberg; Digital Phase  
Modulation”, as quoted in  
[http://gnuradio.org/doc/doxygen/classgr\_1\_1analog\_1\_1cpm.html#ab9e2018801fc5c8a82c02226a9cadceb](http://gnuradio.org/doc/doxygen/classgr_1_1analog_1_1cpm.html#ab9e2018801fc5c8a82c02226a9cadceb)

M

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<div class="post-metadata">

**Author:** ![reback](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/b5ac83/32.png) [@reback](https://www.ruby-forum.com/u/reback)\
**Post date:** [November 19, 2014, 6:28pm UTC](https://www.ruby-forum.com/t/bfsk/233099/4 "2014-11-19T18:28:36Z")

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On Mon, Nov 17, 2014 at 11:09:14AM -0800, Richard B. wrote:

> Would someone please recommend a book that steps through the  
> implementation used in GNU Radio for the Frequency Mod and Quadrature  
> Mod blocks?

I discuss the algorithm implemented in the Quadrature Demod block in  
this video:

> **[Software Defined Radio with HackRF, Lesson 7 - Great Scott Gadgets](https://greatscottgadgets.com/sdr/7/)**

If you aren’t very familiar with complex numbers, you may want to watch  
this one first:

> **[Software Defined Radio with HackRF, Lesson 6 - Great Scott Gadgets](https://greatscottgadgets.com/sdr/6/)**

> I’ve looked at the source code and I have seen posts trying to explain  
> what sensitivity and gain are

It’s a scaling of the output of the demodulator (or the input of the  
modulator).

Mike