Re: [Discuss-gnuradio] MSF/DCF/RBU Time Station Receiver Implementation

[Iain Young, G7III]

Hi Lou,

You Wrote:

On 15/05/14 23:28, madengr wrote:

> Iain Young, G7III wrote
> > I lashed up a quick Goertzel filter at 250Hz, and re-tuned so I was
> > 250Hz off from MSF. Hacked up some code to actually decode the
> > output of GRC to the timecode bits for MSF. It decodes perfectly,
> > even without parity checking!
>
> Thank you for the post.  I was experimenting with WWVB reception also and
> will have to try this for NDB DX reception.  What advantage does the
> Goertzel filter have as opposed to just using a narrow band FIR?  I see you
> still have to specify the 250 Hz offset in the filter, so I assume it
> doesn't help with detection if you drift off frequency.

I'm not actually sure what advantages if any the Goertzel has over a
narrow-band FIR, although there is a post in the archive from Marcus
suggesting 15% CPU.

It was mainly an arbitrary decision on my part. I had been thinking
about how to detect particular tones some time ago, and discovered the
Goertzel filter. Some of the other reasons why I selected it this
time were:

a) I could be off frequency from DC when looking at complex
waterfalls (I often write my flowgraphs in this way) I guess an
extra Frequency Translating FIR filter could be used

b) I could hear the signal more easily while developing

c) RBU was going to need something like this, and it seemed easier to
start with a single tone (carrier). Not that I'm getting much further
with RBU at the moment...

d) It also essentially decimates. Out of the Goertzel as shown is a
1kHz sample rate (48k/48). For the radio clocks that was more than
sufficient, and meant less data needed to be stuffed down the pipe and
processed at the other end.

(see http://permalink.gmane.org/gmane.comp.gnu.radio.general/7128 for
how it actually works)


You are correct that the Goertzel filter doesn't help you keep lock if
you drift off frequency. It's whole reason for being is to detect a
particular tone.

In being off frequency by the same amount the Goertzel, of course, it
"sees" the carrier as the expected tone, and lights up.


73s

Iain

PS:and with that comment about detection if you drift off frequency,
you now have me thinking if I could re-implement using some kind of PLL.
Gagh, another project, but I need to find some simple examples first