[linux-dvb] Asys P7131 Hybrid: DVB out of range

Manu Abraham abraham.manu at gmail.com
Sat Sep 8 12:44:03 CEST 2007


Trent Piepho wrote:
> On Thu, 6 Sep 2007, Manu Abraham wrote:
>> Trent Piepho wrote:
>>
>>> At least for many demods, the frequency range supported is determined by
>>> the tuner.  One could have a chip that combines the demod and tuner
>>> functions, or a system where the demod is somehow involved in tuning and
>>> has some limit.  But that is not the case here.
>> Fmclk > 2 * Fsmax * Viterbi
>>
>> Doesn't apply to a tuner
> 
> Of course! Why didn't I see that?  You've made it perfectly clear now.
> 

It is the mother of all calculations, for any Digital communication
device (Nyquist Sampling Rate)

Calculations generally are derived off this. The "2 * " is generally
used in the case of the complete spectral width, ie, you can assume that
case to be valid with a ZIF tuner and not otherwise.

Generally it is classified that the bandwidth of a ZIF tuner is twice
that of a normal tuner.

> Using this formula, what is the allowed range for a tda10046 demod with a
> TDA827X tuner?  How about with a TDA827XA tuner?


I don't have the 827x specs (but i will explain what i can, time being
short as well)

Generally, there is a case of confusion when the tda10046 is used:

* the difference being an optional down-converter.

ie, there will be 2 cases "basically"

* Fsmax = 6/5 * IF1 (Up sampling: 36.15/36.125MHz)
* Fsmax = 4/5 * IF1 (Down sampling: 43.75/57MHz)

Additionally,

specifically for the TDA10046, it reduces to

Fsamp = 55 Mhz as an *absolute* maximum (whatever configuration it might
be). But then this might not be at the Nyquist rate also, in some cases
then.

With the builtin PLL for the ADC clock,
Fsamp = (M / N * P) * Fquartz
where:
M = Clock Multiplier
N = a Clock divider ratio
P = a Clock divider ratio

With default M, N, P values with a default quartz crystal of 4MHz
(according to the specifications, it can go upto 27MHz), the default
sampling frequency becomes 55 Mhz
So as you see in the case of the high IF values, the tda10046 cannot do
Nyquist sampling at the extreme end, but generally this limitation is
not applicable in most cases.



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