[linux-dvb] fmd1216 integration

Thomas Pinz dc2rpt at gmx.de
Sun Mar 11 18:06:36 CET 2007


Hi, 

On Sunday 11 March 2007 09:40, Trent Piepho wrote:
> In dvb-pll, the frequency offset, which is the IF frequency the tuner will
> output at, is set to 36.213333 MHz.  It looks like this code was written
> about two years ago by Patrick as part of the reverse engineered cxusb
> driver.
>
> In saa7134-dvb, there is some totally different code from programming
> fmd1216 tuners, and it uses an IF frequency of 36.13 MHz.
>
> So, why the difference?

Usually a (DVB-T) Demod has an capture area of +/- 1MHz relative to the 
programmend IF and also in most Demods the IF could be moved with certain 
register settings. 

I don't know the FMD1216, but in many designs you have a third element which 
determines the IF frequency, the SAW filter between the tuner and the demod. 
So depending on which SAW-filter you are using you choose also an certain IF. 
Epcos for example sells filters with 35.1, 35.825, 36, 36.125, 43.75 MHz and 
much more. So if you find a SAW-filter on your device, maybe have a look an 
the datasheet and you will find the right IF.
For example, the X6872D which could be found in some DVB-T sticks, has a
center frequency of 36.125MHz. (And 6.9MHz bandwidth) 

If the IF is shifted around 100kHz bit to the center frequency of the 
SAW-filter and you have a good reception, you wont see much degration in the 
quality because there are some provision. But if the conditions will go 
worser, this would make your quality degration a little bit more worse. 

Kind regards, 

Thomas Pinz



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