Junction Noise

[Martin Czech]

:::Could someone give some general rules for "transversal" filters?
:::If I am using the term correctly?
:::
:::This occurs in tapped binary shift registers where multiple taps are summed
:::to get a multi-level output voltage.
:::
:::This also has frequency selective effects related to the number of taps,
:::the tap weights and clocking frequency?

Yes. Especially the later is interesting, it means Fc control via
clock frequency.

:::So it is a FIR filter? Short of going and buying some books on DSP, are
:::there any useful rules of thumb, or handy references?

This is a FIR filter, but a very special one, with fixed levels "1" and
"0". But we could assume that we have a linear tapped delay line
and a strange input signal of "1" and "0" instead. So normal
FIR theory applies.


Book: Rabiner & Gold. the holy bible of DSP.

In "Art of Electronics" a nice 32 tap design is shown.
The basic idea is that the tap weights are really the impulse response
of the filter, and this is related to the frequency response via 
Monsieur Fourier's theory.

The problem with FIR filters is that we only obtain zeros.
That means: we need quite a lot of zeros to keep the response down
in the stop band(s), i.e. filter length is usually > 64 or 128
or so. OTOH FIR filters have wonderfull properties, eg. phase
linearity. All partials traveling through such a filter will arrive at
the same time.
And: there are optimal filters, ie. a theory that prooves that no other
FIR filter with given length will fit better to a given spec
in some sense.

I've toyed arround with such filter optimisation software
(Remez exchange algorithm).
If you want, I could easily compute some impulse responses/
frequency responses.
If think low pass and band pass are especially usefull.

Even if we have no shift register feedback at all, sampling
analog noise and filtering it could still be usefull.
This time alias and qunatisation noise is very much wellcome!

m.c.