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
ultralightdx@yahoogroups.com, 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.
> >
>