[linux-dvb] fmd1216 integration

Hartmut Hackmann hartmut.hackmann at t-online.de
Mon Mar 12 00:43:32 CET 2007


Thomas Pinz schrieb:
> 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
I wrote the code in saa7134-dvb. The FMD1216 has an integrated SAW filter
for DVB-T, the data sheet says 36.13MHz. This is strange because the
data sheet also recommends a tuning step size of 167 kHz (4 MHz / 24),
so it should be 36.167MHz....
When i started writing it, there was no dvb-pll module. And the reason
why i didn't move to dvb-pll yet is that this module has no means to
control the RF AGC. Philips recommends different setting for analog and
I don't know how big the impact on the performane is but i think its good
to stay as close a possible to the recommendations of the manufactor.


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