Re: A simplistic explanation (long)
Hi Roy, Gary, Jim, and others:toggle quoted message Show quoted text
I've been following this group for a week or so with a special interest in your experiments with radios based on the Si4734 chip. My interest comes partly as a radio enthusiast and casual occasional DXer, but more specifically as one of the designers of the Si4734. I am posting in hopes that I can clarify a few of your questions about the Si4734's operation.
First, the AM front-end of the Si4734 is a tuned-tank circuit with a fairly conventional LNA and quadrature mixer as shown in the block diagram. There is no sampling at the RF signal. The I/Q signals at the (low) IF are converted to digital form where channel filtering, image rejection, demod, and audio processing are performed. The analog front-end circuits are controlled and calibrated by an on-chip microcontroller.
As Roy has pointed out, the loopstick (or air-loop) inductance is resonated with an on-chip varactor, which tunes in small discrete steps. This tuning is done each time the frequency is changed. Roy is absolutely correct that it is vital to repeatedly re-tune the radio as one evaluates different antenna changes. The chip's tuning method actively adapts to the inductance, bringing the tank to resonance regardless of the change. In other words, the varactor value is not set simply by a lookup table corresponding to frequency. The chip measures and peaks the resonance upon each tuning event.
Regarding the inductance range listed in the data sheet: this is a range over which Silicon Labs guarantees _every_ chip will successfully resonate all broadcast-band frequencies. For a particular chip and/or band of frequencies, the range will generally be larger. Since the LC product is inversely proportional to frequency-squared, the chip can successfully tune much larger inductors at 500 kHz than at 1700 kHz.
I have a Tecsun PL-300wt and have ordered a PL-380. I hope to find some time in the near future to open them up and poke at the implementations of the antenna and front-end circuits.
--- In ultralightdx@..., "Roy" <roy.dyball@...> wrote: