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  2. A simplistic explanation (long)

Re: A simplistic explanation (long)

sdwillingham
 

Hi Roy, Gary, Jim, and others:

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.

Cheers,
Scott

--- In ultralightdx@..., "Roy" <roy.dyball@...> wrote:


Thank you Gary for taking the time to carry out the tunning procedure. I
also achieved similar results but with different values of RSSI S/N for
my local stations.

It is interesting that result 3 is higher (6dbu in your case) than
result 2 with the coil in exactly the same position. I believe if you
tried the same thing on a conventional radio (non Si4734) result 2 and
result 3 would stay the same.

I think this highlights the need to steep off and back on to frequency
whenever you make any position adjustments to the loopstick coil because
as shown it is possible to have two different readings with the coil in
exactly the same position.

Using the method of stepping off and back on frequency before noting
signal strength I am finding that on the low end of the MW band my
highest signal readings are with the coil slightly to one side of
centre. While at the high end of the band my best signal readings are
with the coil slightly from the end maybe less than a quarter of an
inch. I find my best overall results are with the coil at the end this
seems to be consistent for me with different coils and different rods.

I am eagerly awaiting my 7.5" and 4.0" X .5" Amidon 61 rods and bundle
of assorted Litz wire to add more empirical results to the group.

Cheers Roy.




--- In ultralightdx@..., D1028Gary@ wrote:

Hi Roy,

The local station signal strength (RSSI) and signal to noise ratio
(S/N)
readings have been completed on the local station here, as you
requested.

On the PL-310 with the 7.5" Amidon ferrite bar and 81-turn Slider
coil,
with the coil at the maximum AM sensitivity position (554 uh), our
local
station KSUH-1450 has a signal strength (RSSI) reading of 80 dBu, and
a signal
strength (S/N) reading of 25 db. When the coil was shifted to one
quarter
inch from the far end of the ferrite bar, the RSSI reading changed to
72 dBu,
and S/N was the same at 25 db.

After moving the frequency to a weaker station and then immediately
changing back to KSUH-1450 (at the same coil position), the RSSI was
78 dBu, and
the S/N was still 25 db. The RSSI shifted between 79 dBu and 78 dBu a
couple of times before settling at 78 dBu, in this final test (as is
typical
when the Pl-310 is changed to a new station).

I hope this information is helpful to you, Roy.

73, Gary
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