Re: Early results of 7.5" Amidion rod

Gary DeBock

Hi Roy,
Your detailed explanation of the Si4734 control interface (and recommendation to take full advantage of the programming manual) are greatly appreciated, and I'm grateful for your thorough description of how the capacitance value returned by the Si4734 DSP chip can allow us to determine the tuning function of the chip, in its operation with loopsticks of various ferrite mixes and inductances. Thank you very much for patiently explaining all these details to me, in a very understandable description.
In the same way, I'll certainly do my best to patiently digest all the essential information you have provided, in order to have a more thorough understanding of the chip's tuning operation.
I must apologize for the delay in responding to your very helpful post, primarily because of going off in a tangent to attempt creation of a more effective in-cabinet loopstick for the PL-380 model. After receiving various short type 61 ferrite bars from Amidon, a quick investigation of the internal cabinet space in the model convinced me that a messy plastic cutout to accommodate the cylindrical type 61 ferrite bars was impractical. The model does have significant open space around the loopstick slot, but from experience I know that strange side effects can happen in a model when serious "plastic surgery" is attempted. In the absence of a type 61 ferrite bar identical in size and shape to the flat stock bar (apparently unavailable anywhere), the only practical option was to recycle the stock bar with superior 40/44 Litz wire. It was hoped that this ultra-cheap modification would provide a significant AM sensitivity upgrade for the model, but frankly it was a stab in the dark.
The flat design of the stock bar made it ludicrous to attempt the use of a Slider coil for inductance-optimizing experiments (as you probably guessed), so my only coil tuning option was to wind the coil, and fine-tune the high AM band inductance by subtracting coil turns one by one, to unlock the Si4734 varactor operation for maximum AM sensitivity. To my relief this low-tech strategy worked like a charm, with the radio suddenly booming to life on 1700 kHz when I subracted the final Litz wire turn, bringing the inductance down to 433 uh (from an original value of 510 uh). The PL-380 then had great reception from 530-1700 kHz, with an average 4 dBu increase in RSSI readings across the MW spectrum. On LW the sensitivity boost was even more impressive, with 4 NDB beacons audible on the modified model that were completely absent on the stock PL-380.
My second modified PL-380 model was completed this morning, and Steve has also completed a similar experiment, apparently with equivalent results. Sorry again about the delay in responding to your excellent and informative post, but I suppose that one of the congenital quirks of a confirmed tinkerer is that new, bizarre projects are commenced on the spur of a moment :-)
73, Gary         
In a message dated 1/16/2010 11:50:10 P.M. Pacific Standard Time, roy.dyball@... writes:


Hi Gary

The Si4734 is a very clever and complex chip at the digital and engineering level and has many facets. However the designers have been able to remove most of the complexity for the person wanting to use the chip by having the chip respond to commands issued to through a control interface (2wires or 3wires). All of these commands are listed and dealt with in detail in the programming manual. It looks complicated when you first look at it but don't let it scare you away. It is written in such a way that not much digital knowledge is required.

Along with issuing commands you can also get answers as to what is going on inside the chip. These answers come back as simple binary bytes and when converted to decimal just look like a range of numbers e.g. 0-127, in this case this could represent the value of the RSSI or the S/N ratio. You can also request that the chip tell you what value of capacitance it has just used to tune in a station you have just selected. This once again comes back and when converted to decimal represents a number within its range, every time the number goes up by one the capacitance used to tune the loopstick goes up by 0.095pf. So knowing this capacitance we can use the Si4734 chip as a pretty acuate L/C meter one that will take into account the broad range of materials we are using in our loopstick e.g. the type of ferrite.

The discussion with Scott was how to know what the correct single figure inductance is of the loopstick coil over the whole tuning range. At the moment my software is just taking the value of capacitance required to tune in a station at a particular frequency  and using our old faithful formula of one on two pi square route LC to calculate the inductance e.g. if the chip told me the capacitance was 343.3pf when tuned to 520Khz I would know that the loopstick inductance at that frequency was 272.88 µh. When the same coil is tuned to 1710Khz the chip tells me that 20.87pf has been used to tune the loopstick and this equates to an inductance of 415.09 µh. These figures are great and I have been able to use them to tell when the coil is resonating properly within range of the varactor as in the above example.

Scott has now kindly shown me how to equate the two figures (above) that are always different and produce a single figure for the inductance of the loopstick being an accurate representation of the inductance wound on the loopstick. I will now be able to have a single button in my software that when pressed will act like an L/C meter but actually take into account what the chip is seeing.

I advise everybody to have a look at the programming manual just to get an idea what this chip is capable of and how it goes about it. The manual is one of the best written and easy to follow programming manuals I have seen.

Cheers Roy.

--- In, D1028Gary@... wrote:
> Scott and Roy,
> Your discussion of the software commands to the Si4734 commands is very
> impressive, and I wish that I could follow what is being discussed.
> Unfortunately my Navy technical training in digital electronics (early 1980's) was
> very limited, which is probably why loopstick tinkering and relentless
> experimentation is more suitable in my case :-) You both have my admiration,
> and I'm sure that all the group will look forward to a full explanation of
> the Si4734 varactor's tuning process with various experimental loopsticks.
> 73 and Thanks, Gary
> In a message dated 1/16/2010 8:45:07 P.M. Pacific Standard Time,
> roy.dyball@... writes:
> Thank you Scott for the very clear answer about the calculation. I will
> put that routine into my software on a small panel with a text box and a
> command button. When the button is pressed it will tune the set to 520Khz grab
> the varactor reading then tune to 1710Khz also getting the new varactor
> reading, then perform the calculation supplied and finally place the
> inductance result in the text box it should do that in a couple of seconds.
> Maybe you can help with another question. The data sheet for the Si4734
> shows a separate earth for the antenna (RFGND pin 3) on the chip and in one
> part of the sheet it recommends that antenna earth should be separated from
> the system earth. Then in another spot in the data sheet it says that all
> earths should be common. The Tecsun radios have a common ground including pin
> 3. Is there any benefit in utilising this separate RF ground?
> Roy.--- In ultralightdx@ In u--- In ultralightdx@sdwillingham@sdw
> wrote:
> >
> >
> >
> > Roy,
> >
> > The calculation I use is pretty straightforward. The idea is that
> > I assume inductance L is constant across the band and measure the
> > resonating capacitance at two frequencies. I end up with two
> > equations with two unknowns (L and parasitic C) to solve for.
> >
> > So, define:
> > - two frequencies f1 and f2
> > - corresponding Si4734 varactor readings V1 and V2
> > - varactor gain G = change in capacitance per varactor code (0.095 pF)
> >
> > First calculate two LC products:
> > - LC1 = 1 / (2*pi*f1)^2
> > - LC2 = 1 / (2*pi*f2)^2
> >
> > Assuming constant inductance:
> > LC1 - LC2 = L*(C1 - C2) = L * G*(V1 - V2)
> >
> > Therefore L = (LC1 - LC2) / (G*(V1 - V2))
> >
> > The capacitances C1 and C2 can then be calculated:
> > C1 = LC1 / L
> > C2 = LC2 / L
> >
> > Each capacitance consists of a fixed and a variable amount:
> > C1 = Cf + G*V1
> > C2 = Cf + G*V2
> >
> > So Cf can be calculated as Cf = C1 - G*V1 = C2 - G*V2
> >
> > Part of Cf can be considered the 7pF documented minimum Si4734 AM
> > input capacitance. The rest is contributed externally, partially by
> > the circuit boards and wires, partially by the loopstick. In my
> > experiments, the stock configuration has about 8.6 pF external
> > capacitance. Lifting the loopstick leads far from the PCB reduces
> > this to 8.0 pF. Clipping the SW diode reduces this to about 5.25 pF
> > with the loopstick leads routed as stock.
> >
> >
> > Regarding noise from the computer connection. This is a difficult
> > problem for AM reception and a slight problem in FM. I' ve found
> > the noise unacceptable with a desktop compter, but quite a bit less
> > using a laptop with charger _unplugged_. The LCD screen is a
> > problem too. I've been looking at ways to reduce the noise, but
> > there's not a lot to be done at MW frequencies short of an
> > opto-isolated interface. A further problem is that the ground
> > connection to the computer distorts the antenna response
> > (common-mode currents).
> >
> > -Scott-
> >
> >
> >
> > --- In ultralightdx@ --- In ultr --- In ultralightdx@yahoogr
> > > Hi Scott
> > >
> > > Thank you for all your input, we are all privileged to have your
> > > knowledge and experience in this group. Are you able to share how your
> > > calculations work for calculating inductance from starting and ending
> > > frequencies (520-1710 KHz) from the values returned from the varactor.
> I
> > > am already conve rting the capacitance returned to inductance at the
> > > frequency tuned but of course the value of inductance calculated is
> > > different at the top and bottom of the band. I am able to work with
> > > these values but it would be good to know the true averaged value.
> > >
> > > Now that I am almost done with initial loopstick readings I want to
> move
> > > on to winding some toroids to match an incoming long wire antenna. This
> > > will replace the loopstick as I am finding the noise generated by the
> > > computer when using the USB interface on AM is unacceptable. Have you
> > > found that the earth provided from the USB connection from the
> computer
> > > is reducing reception on both AM and FM and is also a source of noise?
> > >
> > > Cheers Roy.
> >

Join to automatically receive all group messages.