Re: A simplistic explanation (long)

Roy <roy.dyball@...>

Hi Scott

Welcome to the group.

Thank you for putting us straight on the operation of the Si4734 and congratulations on your part in designing one of the best little radio chips on the planet. It has been fun trying to work out the operation of the chip with just logic and an old version of the programming manual plus the data sheet.

Ultralight DXing is a fantastic sport and I hope you get to enjoy it from the other side of the chip. Gary has put so much energy into squeezing as much performance as possible from these little radios and has a natural ability to write up the results of his experiments keeping the group informed and spurred on in the quest of more distant radio stations.

I will be interested in your findings on the implementations of the antenna and front-end circuits as this seems to change on each model Tecsun brings out changing the performance.

I have written a small application via USB interface that enables control of the Si4734 in the G8 by intercepting the control lines. I initially used the I2C implementation but soon re-wrote using the 3 Wire (Legacy) protocol because Tecsun for some reason use the 3 wire interface to control the Si4734 and then have a separate I2C bus to read and write the EEprom that saves channel info (could not work that out). If you get a chance the software is here along with a pictorial explanation of how to break into the control lines and connect the USB interface.

I suppose the biggest question the group will have is how can we turn off the AM soft mute. Although this is a great feature for the average user it just works against you in AM DX work. I can turn the soft mute off in my software but lose the change in power down.

The PL-380 I have seems to have the soft mute turned off by default. It has Ver 20 firmware. I notice that Ver 40 firmware has been released and has a less aggressive default soft mute settings according to the data sheet but I have not seen one of these chips yet.

I feel like I could ask you a many questions and hope I can over the course of time.

Cheers for now


--- In ultralightdx@..., "sdwillingham" wrote:
> 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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