Re: Resonance



Thanks for writing up your guide to the Si4734 receiver chip!
I'm personally thrilled to see it on your site. Your "Phil's
Portable Guide" and other articles were very influential to me
when researching AM receivers while we were designing the
Si4734. (History lesson: The Si472x and Si473x are part of our
2nd generation of radio chips following the Si4700, which is an
FM-only receiver.)

The description of AM antenna tuning, however, is not quite
right. The method described (injecting a test signal and
adjusting to maximize RSSI) is workable, but tends to be slow
and have some practical difficulties. For instance, if the
tuning is in error by a large amount, the RSSI gives little
information for correction. That method can also be fooled by
large received signals.

So here's a layman's description of the tuning. The ferrite bar
inductance and the varactor capacitance form a resonant circuit
with reasonably large Q. An analogy to this resonator is the
tuning fork, a mechanical resonator. When one "whacks" a tuning
fork with a mallet, it rings for a while with a damped tone.
The Si4734 whacks on its LC-tank circuit with a switch that
forces current in the inductor. The tank rings for a short
while -- at its resonant frequency -- and the chip measures that
frequency using a counting method. Using iteration with
changing capacitance values, the chip tunes the resonance to its

Some other small comments: 1) On page 2, you describe our clock
method as "radiometric". The term is actually "ratiometric", as
in all the frequencies are related by integer ratios. 2) It
would be helpful to your readers if you added citations for the
documents you have used in compiling your guide.


Join to automatically receive all group messages.