Bandaxall Oscillator

On Apr 16, 3:37=A0pm, Kooner <jjkoo...@gmail.com> wrote:
> On Apr 14, 6:47=A0pm,BillSloman<bill.slo...@ieee.org> wrote:
>
>
>
>
>
>
>
>
>
> > On Apr 14, 11:35=A0pm,Kooner<jjkoo...@gmail.com> wrote:
>
> > > Hello,
>
> > > I made aBandaxallClass D Oscillator with a 11mm x 7mm pot core
> > > transformer and it works quite well. The input is 8VDC and the
> > > secondary output is 800V peak to peak.
>
> > > The transformer is very difficult (for me) to wind because the
> > > secondary uses #46 awg wire and it breaks easily.
>
> > I could never do better than 0.1mm OD wire - 42 awg. Professional coil
> > winders can do a lot better - down to 0.2mm wire, around 52 awg.
>
> > > Also, I would like
> > > to use a shorter transformer, around 5mm tall. There are smaller pot
> > > cores but there is no way I could wind them.
>
> > Again, sub-contract the job to a professional. It's not a business
> > that lends itself to automation, so get one-off jobs down isn't too
> > difficult.
>
> > > I'm looking for a SMT off the shelf transformer to use in this
> > > circuit.
> > > A tried a CCFL transformer made by Coilcraft (FL2810-1L)
>
> > >http://www.coilcraft.com/misc/ccflp.html
>
> > There do seem to offer quite a few variants on that theme.
>
> >http://www.coilcraft.com/search/us_search_results.cfm
>
> > > but it didn't work. I believe the winding capacitance is too low. The
> > > secondary is wound on a segmented bobbin as opposed to having entire
> > > layers wound on top of each other.
>
> > That's not likely to be a correct explanation of why it didn't work.
>
> Here's a simulation of the circuit I've been working with.
>
> http://www.flickr.com/photos/58722598@N07/7083763563/in/photostream
>
> http://www.flickr.com/photos/58722598@N07/6937689758/in/photostream/
>
> So far, I haven't had to add a capacitor to the primary to get it to
> work because there is enough reflected capacitance (15pF from 1000
> turns on a pot core transformer).
>
> In the simulation I removed the 15pF on the secondary and added a
> 4.2nF across the primary and got the same results.
>
> I don't know why the circuit didn't work in the lab using the FL2180
> (low winding capacitance) with a capacitance added to the primary as
> shown on Bill'spage :
>
> http://home.planet.nl/~sloma000/Baxandall%20parallel-resonant%20Class...
>
> but I will keep trying.....
>
> > > I tried adding a small cap across the secondary, but it wouldn't
> > > oscillate, I'm guessing the winding resistance on the secondary (300
> > > ohms) is too high which prevents the circuit from oscillating.
>
> > It would have needed to be a very small capacitor with a very high
> > voltage rating
>
> > > I tried adding a cap across the primary, the circuit did oscillate bu=
t
> > > the input current seemed too high and the current waveform didn't loo=
k
> > > right.
>
> > A high winding capacitance is unlikely to be all that helpful.
>
> > If you haven't already read J Williams application notes on the
> > subject (AN45, AN49, AN51, AN55, AN61, AN65 on the Linear Technology
> > web-site) you should do so. Jim Williams never called the circuit a
> > Baxandall class -D oscillator but that's what he's describing.
>
> Bill, I looked through those notes....there is an extra winding on the
> transformer to drive the bipolar transistors, is this circuit the same
> principle as the Bandaxall osc?

Read the original Baxandall paper from 1959, back before MOSFETs had
been invented and some 15 years before they became commercially
available.

http://home.planet.nl/~sloma000/0344_001_Baxandal.pdf

> =A0Also, I've been using mosfets because
> I thought this was necessary since they need to conduct both positive
> and negative current?

Obviously not true, since the classic Baxandall uses bipolar
transistors. MOSFETs are better than bipolar transistors, in that they
don't squeg if the feed inductor (L1 in your circuit) gets too big.

--
Bill Sloman, Nijmegen

Bill Sloman wrote:
> On Apr 17, 9:15 am, Robert Baer<robertb...@localnet.com>  wrote:
>> Kooner wrote:
>>> On Apr 14, 6:47 pm,BillSloman<bill.slo...@ieee.org>    wrote:
>>>> On Apr 14, 11:35 pm,Kooner<jjkoo...@gmail.com>    wrote:
>>
>>>>> Hello,
>>
>>>>> I made aBandaxallClass D Oscillator with a 11mm x 7mm pot core
>>>>> transformer and it works quite well. The input is 8VDC and the
>>>>> secondary output is 800V peak to peak.
>>
>>>>> The transformer is very difficult (for me) to wind because the
>>>>> secondary uses #46 awg wire and it breaks easily.
>>
>>>> I could never do better than 0.1mm OD wire - 42 awg. Professional coil
>>>> winders can do a lot better - down to 0.2mm wire, around 52 awg.
>>
>>>>> Also, I would like
>>>>> to use a shorter transformer, around 5mm tall. There are smaller pot
>>>>> cores but there is no way I could wind them.
>>
>>>> Again, sub-contract the job to a professional. It's not a business
>>>> that lends itself to automation, so get one-off jobs down isn't too
>>>> difficult.
>>
>>>>> I'm looking for a SMT off the shelf transformer to use in this
>>>>> circuit.
>>>>> A tried a CCFL transformer made by Coilcraft (FL2810-1L)
>>
>>>>> http://www.coilcraft.com/misc/ccflp.html
>>
>>>> There do seem to offer quite a few variants on that theme.
>>
>>>> http://www.coilcraft.com/search/us_search_results.cfm
>>
>>>>> but it didn't work. I believe the winding capacitance is too low. The
>>>>> secondary is wound on a segmented bobbin as opposed to having entire
>>>>> layers wound on top of each other.
>>
>>>> That's not likely to be a correct explanation of why it didn't work.
>>
>>> Here's a simulation of the circuit I've been working with.
>>
>>> http://www.flickr.com/photos/58722598@N07/7083763563/in/photostream
>>
>>> http://www.flickr.com/photos/58722598@N07/6937689758/in/photostream/
>>
>>> So far, I haven't had to add a capacitor to the primary to get it to
>>> work because there is enough reflected capacitance (15pF from 1000
>>> turns on a pot core transformer).
>>
>>> In the simulation I removed the 15pF on the secondary and added a
>>> 4.2nF across the primary and got the same results.
>>
>>> I don't know why the circuit didn't work in the lab using the FL2180
>>> (low winding capacitance) with a capacitance added to the primary as
>>> shown onBill'spage :
>>
>>> http://home.planet.nl/~sloma000/Baxandall%20parallel-resonant%20Class...
>>
>>> but I will keep trying.....
>>
>>>>> I tried adding a small cap across the secondary, but it wouldn't
>>>>> oscillate, I'm guessing the winding resistance on the secondary (300
>>>>> ohms) is too high which prevents the circuit from oscillating.
>>
>>>> It would have needed to be a very small capacitor with a very high
>>>> voltage rating
>>
>>>>> I tried adding a cap across the primary, the circuit did oscillate but
>>>>> the input current seemed too high and the current waveform didn't look
>>>>> right.
>>
>>>> A high winding capacitance is unlikely to be all that helpful.
>>
>>>> If you haven't already read J Williams application notes on the
>>>> subject (AN45, AN49, AN51, AN55, AN61, AN65 on the Linear Technology
>>>> web-site) you should do so. Jim Williams never called the circuit a
>>>> Baxandall class -D oscillator but that's what he's describing.
>>
>>> Bill, I looked through those notes....there is an extra winding on the
>>> transformer to drive the bipolar transistors, is this circuit the same
>>> principle as the Bandaxall osc?  Also, I've been using mosfets because
>>> I thought this was necessary since they need to conduct both positive
>>> and negative current ?
>>
>>> - Jason Kooner
>>
>>>>> any help appreciated.
>>
>>>> I've got some thoughts on the subject on my web-site
>>
>>>> http://home.planet.nl/~sloma000/Baxandall%20parallel-resonant%20Class...
>>
>>>> along with a copy of Peter Baxandalls's 1959 paper
>>
>>>> http://home.planet.nl/~sloma000/0344_001_Baxandal.pdf
>>
>>>> which wasn't easy to get hold of. I ended up buying a copy from the UK
>>>> IEE (and got their permission to put it on my web-site).
>>
>>     Hello.....
>>     Did i not say why?
>>     The transformer is made to limit the current for constant conduction
>> (think requirements of fluorescent, of CFL); the turns ratio obviously
>> there to make the required high voltage for initializing the arc.
>
> There's nothing in the Baxandall circuit that explicitly limits the
> output current. The CoilCraft transformer will obviously have a
> relatively high resistance secondary (because it is made with lots of
> turns of fine wire) and will have a highish leakage inductance - which
> the OP could measure by shorting one or other of the coils and
> measuring the residual inductance of the other coil - but I wouldn't
> think that either feature was designed in as an intentional current
> limit.
>
> --
> Bill Sloman, Nijmegen
>
   May i "pete" again (re-pete)? Magic word: TRANSFORMER.

On Apr 18, 3:38=A0am, Robert Baer <robertb...@localnet.com> wrote:
> Bill Sloman wrote:
> > On Apr 17, 9:15 am, Robert Baer<robertb...@localnet.com> =A0wrote:
> >> Kooner wrote:
> >>> On Apr 14, 6:47 pm,BillSloman<bill.slo...@ieee.org> =A0 =A0wrote:
> >>>> On Apr 14, 11:35 pm,Kooner<jjkoo...@gmail.com> =A0 =A0wrote:

<snip>

> >> =A0 =A0 Hello.....
> >> =A0 =A0 Did i not say why?
> >> =A0 =A0 The transformer is made to limit the current for constant cond=
uction
> >> (think requirements of fluorescent, of CFL); the turns ratio obviously
> >> there to make the required high voltage for initializing the arc.
>
> > There's nothing in the Baxandall circuit that explicitly limits the
> > output current. The CoilCraft transformer will obviously have a
> > relatively high resistance secondary (because it is made with lots of
> > turns of fine wire) and will have a highish leakage inductance - which
> > the OP could measure by shorting one or other of the coils and
> > measuring the residual inductance of the other coil - but I wouldn't
> > think that either feature was designed in as an intentional current
> > limit.
>
> =A0 =A0May i "pete" again (re-pete)? Magic word: TRANSFORMER.

"Transformer" may be a magic word in your vocabulary, but it doesn't
seem helpful to impute magical powers to a transformer design that
seems to have been shaped only by the familiar requirement to get a
high turns ratio while minimising the inter-winding capacitance of the
secondary.

If you could tell us why you think that CoilCraft have added something
extra, your contribution might become marginally useful, but at the
moment you appear to be pretending an expertise that you don't
actually possess.

--
Bill Sloman, Nijmegen

On Apr 17, 4:18=A0am, Bill Sloman <bill.slo...@ieee.org> wrote:
> On Apr 16, 3:37=A0pm, Kooner <jjkoo...@gmail.com> wrote:
>
>
>
>
>
>
>
>
>
> > On Apr 14, 6:47=A0pm,BillSloman<bill.slo...@ieee.org> wrote:
>
> > > On Apr 14, 11:35=A0pm,Kooner<jjkoo...@gmail.com> wrote:
>
> > > > Hello,
>
> > > > I made aBandaxallClass D Oscillator with a 11mm x 7mm pot core
> > > > transformer and it works quite well. The input is 8VDC and the
> > > > secondary output is 800V peak to peak.
>
> > > > The transformer is very difficult (for me) to wind because the
> > > > secondary uses #46 awg wire and it breaks easily.
>
> > > I could never do better than 0.1mm OD wire - 42 awg. Professional coi=
l
> > > winders can do a lot better - down to 0.2mm wire, around 52 awg.
>
> > > > Also, I would like
> > > > to use a shorter transformer, around 5mm tall. There are smaller po=
t
> > > > cores but there is no way I could wind them.
>
> > > Again, sub-contract the job to a professional. It's not a business
> > > that lends itself to automation, so get one-off jobs down isn't too
> > > difficult.
>
> > > > I'm looking for a SMT off the shelf transformer to use in this
> > > > circuit.
> > > > A tried a CCFL transformer made by Coilcraft (FL2810-1L)
>
> > > >http://www.coilcraft.com/misc/ccflp.html
>
> > > There do seem to offer quite a few variants on that theme.
>
> > >http://www.coilcraft.com/search/us_search_results.cfm
>
> > > > but it didn't work. I believe the winding capacitance is too low. T=
he
> > > > secondary is wound on a segmented bobbin as opposed to having entir=
e
> > > > layers wound on top of each other.
>
> > > That's not likely to be a correct explanation of why it didn't work.
>
> > Here's a simulation of the circuit I've been working with.
>
> >http://www.flickr.com/photos/58722598@N07/7083763563/in/photostream
>
> >http://www.flickr.com/photos/58722598@N07/6937689758/in/photostream/
>
> > So far, I haven't had to add a capacitor to the primary to get it to
> > work because there is enough reflected capacitance (15pF from 1000
> > turns on a pot core transformer).
>
> > In the simulation I removed the 15pF on the secondary and added a
> > 4.2nF across the primary and got the same results.
>
> > I don't know why the circuit didn't work in the lab using the FL2180
> > (low winding capacitance) with a capacitance added to the primary as
> > shown on Bill'spage :
>
> >http://home.planet.nl/~sloma000/Baxandall%20parallel-resonant%20Class...
>
> > but I will keep trying.....
>
> > > > I tried adding a small cap across the secondary, but it wouldn't
> > > > oscillate, I'm guessing the winding resistance on the secondary (30=
0
> > > > ohms) is too high which prevents the circuit from oscillating.
>
> > > It would have needed to be a very small capacitor with a very high
> > > voltage rating
>
> > > > I tried adding a cap across the primary, the circuit did oscillate =
but
> > > > the input current seemed too high and the current waveform didn't l=
ook
> > > > right.
>
> > > A high winding capacitance is unlikely to be all that helpful.
>
> > > If you haven't already read J Williams application notes on the
> > > subject (AN45, AN49, AN51, AN55, AN61, AN65 on the Linear Technology
> > > web-site) you should do so. Jim Williams never called the circuit a
> > > Baxandall class -D oscillator but that's what he's describing.
>
> > Bill, I looked through those notes....there is an extra winding on the
> > transformer to drive the bipolar transistors, is this circuit the same
> > principle as theBandaxallosc?
>
> Read the original Baxandall paper from 1959, back before MOSFETs had
> been invented and some 15 years before they became commercially
> available.
>
> http://home.planet.nl/~sloma000/0344_001_Baxandal.pdf
>
> > =A0Also, I've been using mosfets because
> > I thought this was necessary since they need to conduct both positive
> > and negative current?
>
> Obviously not true, since the classic Baxandall uses bipolar
> transistors. MOSFETs are better than bipolar transistors, in that they
> don't squeg if the feed inductor (L1 in your circuit) gets too big.
>
> --
> Bill Sloman, Nijmegen





Bill...sorry for the late response.  In Baxandall's paper, fig 9e the
collector current is shown as positive (half sinewave). However, in my
simulations (see above)  and in the lab (where I use mosfets) the
drain current is a full sine wave (positive and negative current). In
the simulation you have on your site I don't see the drain current. Is
it a half sinewave or full sinewave?

thanks,

-jason

On Apr 17, 3:18=A0am, Bill Sloman <bill.slo...@ieee.org> wrote:
> On Apr 16, 3:37=A0pm, Kooner <jjkoo...@gmail.com> wrote:
>
>
>
>
>
>
>
>
>
> > On Apr 14, 6:47=A0pm,BillSloman<bill.slo...@ieee.org> wrote:
>
> > > On Apr 14, 11:35=A0pm,Kooner<jjkoo...@gmail.com> wrote:
>
> > > > Hello,
>
> > > > I made aBandaxallClass D Oscillator with a 11mm x 7mm pot core
> > > > transformer and it works quite well. The input is 8VDC and the
> > > > secondary output is 800V peak to peak.
>
> > > > The transformer is very difficult (for me) to wind because the
> > > > secondary uses #46 awg wire and it breaks easily.
>
> > > I could never do better than 0.1mm OD wire - 42 awg. Professional coi=
l
> > > winders can do a lot better - down to 0.2mm wire, around 52 awg.
>
> > > > Also, I would like
> > > > to use a shorter transformer, around 5mm tall. There are smaller po=
t
> > > > cores but there is no way I could wind them.
>
> > > Again, sub-contract the job to a professional. It's not a business
> > > that lends itself to automation, so get one-off jobs down isn't too
> > > difficult.
>
> > > > I'm looking for a SMT off the shelf transformer to use in this
> > > > circuit.
> > > > A tried a CCFL transformer made by Coilcraft (FL2810-1L)
>
> > > >http://www.coilcraft.com/misc/ccflp.html
>
> > > There do seem to offer quite a few variants on that theme.
>
> > >http://www.coilcraft.com/search/us_search_results.cfm
>
> > > > but it didn't work. I believe the winding capacitance is too low. T=
he
> > > > secondary is wound on a segmented bobbin as opposed to having entir=
e
> > > > layers wound on top of each other.
>
> > > That's not likely to be a correct explanation of why it didn't work.
>
> > Here's a simulation of the circuit I've been working with.
>
> >http://www.flickr.com/photos/58722598@N07/7083763563/in/photostream
>
> >http://www.flickr.com/photos/58722598@N07/6937689758/in/photostream/
>
> > So far, I haven't had to add a capacitor to the primary to get it to
> > work because there is enough reflected capacitance (15pF from 1000
> > turns on a pot core transformer).
>
> > In the simulation I removed the 15pF on the secondary and added a
> > 4.2nF across the primary and got the same results.
>
> > I don't know why the circuit didn't work in the lab using the FL2180
> > (low winding capacitance) with a capacitance added to the primary as
> > shown on Bill'spage :
>
> >http://home.planet.nl/~sloma000/Baxandall%20parallel-resonant%20Class...
>
> > but I will keep trying.....
>
> > > > I tried adding a small cap across the secondary, but it wouldn't
> > > > oscillate, I'm guessing the winding resistance on the secondary (30=
0
> > > > ohms) is too high which prevents the circuit from oscillating.
>
> > > It would have needed to be a very small capacitor with a very high
> > > voltage rating
>
> > > > I tried adding a cap across the primary, the circuit did oscillate =
but
> > > > the input current seemed too high and the current waveform didn't l=
ook
> > > > right.
>
> > > A high winding capacitance is unlikely to be all that helpful.
>
> > > If you haven't already read J Williams application notes on the
> > > subject (AN45, AN49, AN51, AN55, AN61, AN65 on the Linear Technology
> > > web-site) you should do so. Jim Williams never called the circuit a
> > > Baxandall class -D oscillator but that's what he's describing.
>
> > Bill, I looked through those notes....there is an extra winding on the
> > transformer to drive the bipolar transistors, is this circuit the same
> > principle as theBandaxallosc?
>
> Read the original Baxandall paper from 1959, back before MOSFETs had
> been invented and some 15 years before they became commercially
> available.
>
> http://home.planet.nl/~sloma000/0344_001_Baxandal.pdf
>
> > =A0Also, I've been using mosfets because
> > I thought this was necessary since they need to conduct both positive
> > and negative current?
>
> Obviously not true, since the classic Baxandall uses bipolar
> transistors. MOSFETs are better than bipolar transistors, in that they
> don't squeg if the feed inductor (L1 in your circuit) gets too big.
>
> --
> Bill Sloman, Nijmegen




Bill...sorry for the late response.  In Baxandall's paper, fig 9e the
collector current is shown as positive (half sinewave). However, in my
simulations (see above)  and in the lab (where I use mosfets) the
drain current is a full sine wave (positive and negative current). In
the simulation you have on your site I don't see the drain current. Is
it a half sinewave or full sinewave?

thanks,

-jason

On May 17, 6:40=A0am, Kooner <jjkoo...@gmail.com> wrote:
> On Apr 17, 3:18=A0am,BillSloman<bill.slo...@ieee.org> wrote:
>
>
>
>
>
>
>
>
>
> > On Apr 16, 3:37=A0pm, Kooner <jjkoo...@gmail.com> wrote:
>
> > > On Apr 14, 6:47=A0pm,BillSloman<bill.slo...@ieee.org> wrote:
>
> > > > On Apr 14, 11:35=A0pm,Kooner<jjkoo...@gmail.com> wrote:
>
> > > > > Hello,
>
> > > > > I made aBandaxallClass D Oscillator with a 11mm x 7mm pot core
> > > > > transformer and it works quite well. The input is 8VDC and the
> > > > > secondary output is 800V peak to peak.
>
> > > > > The transformer is very difficult (for me) to wind because the
> > > > > secondary uses #46 awg wire and it breaks easily.
>
> > > > I could never do better than 0.1mm OD wire - 42 awg. Professional c=
oil
> > > > winders can do a lot better - down to 0.2mm wire, around 52 awg.
>
> > > > > Also, I would like
> > > > > to use a shorter transformer, around 5mm tall. There are smaller =
pot
> > > > > cores but there is no way I could wind them.
>
> > > > Again, sub-contract the job to a professional. It's not a business
> > > > that lends itself to automation, so get one-off jobs down isn't too
> > > > difficult.
>
> > > > > I'm looking for a SMT off the shelf transformer to use in this
> > > > > circuit.
> > > > > A tried a CCFL transformer made by Coilcraft (FL2810-1L)
>
> > > > >http://www.coilcraft.com/misc/ccflp.html
>
> > > > There do seem to offer quite a few variants on that theme.
>
> > > >http://www.coilcraft.com/search/us_search_results.cfm
>
> > > > > but it didn't work. I believe the winding capacitance is too low.=
 The
> > > > > secondary is wound on a segmented bobbin as opposed to having ent=
ire
> > > > > layers wound on top of each other.
>
> > > > That's not likely to be a correct explanation of why it didn't work=
.
>
> > > Here's a simulation of the circuit I've been working with.
>
> > >http://www.flickr.com/photos/58722598@N07/7083763563/in/photostream
>
> > >http://www.flickr.com/photos/58722598@N07/6937689758/in/photostream/
>
> > > So far, I haven't had to add a capacitor to the primary to get it to
> > > work because there is enough reflected capacitance (15pF from 1000
> > > turns on a pot core transformer).
>
> > > In the simulation I removed the 15pF on the secondary and added a
> > > 4.2nF across the primary and got the same results.
>
> > > I don't know why the circuit didn't work in the lab using the FL2180
> > > (low winding capacitance) with a capacitance added to the primary as
> > > shown onBill'spage :
>
> > >http://home.planet.nl/~sloma000/Baxandall%20parallel-resonant%20Class.=
..
>
> > > but I will keep trying.....
>
> > > > > I tried adding a small cap across the secondary, but it wouldn't
> > > > > oscillate, I'm guessing the winding resistance on the secondary (=
300
> > > > > ohms) is too high which prevents the circuit from oscillating.
>
> > > > It would have needed to be a very small capacitor with a very high
> > > > voltage rating
>
> > > > > I tried adding a cap across the primary, the circuit did oscillat=
e but
> > > > > the input current seemed too high and the current waveform didn't=
 look
> > > > > right.
>
> > > > A high winding capacitance is unlikely to be all that helpful.
>
> > > > If you haven't already read J Williams application notes on the
> > > > subject (AN45, AN49, AN51, AN55, AN61, AN65 on the Linear Technolog=
y
> > > > web-site) you should do so. Jim Williams never called the circuit a
> > > > Baxandall class -D oscillator but that's what he's describing.
>
> > >Bill, I looked through those notes....there is an extra winding on the
> > > transformer to drive the bipolar transistors, is this circuit the sam=
e
> > > principle as theBandaxallosc?
>
> > Read the original Baxandall paper from 1959, back before MOSFETs had
> > been invented and some 15 years before they became commercially
> > available.
>
> >http://home.planet.nl/~sloma000/0344_001_Baxandal.pdf
>
> > > =A0Also, I've been using mosfets because
> > > I thought this was necessary since they need to conduct both positive
> > > and negative current?
>
> > Obviously not true, since the classic Baxandall uses bipolar
> > transistors. MOSFETs are better than bipolar transistors, in that they
> > don't squeg if the feed inductor (L1 in your circuit) gets too big.
>
> > --
> >BillSloman, Nijmegen
>
> Bill...sorry for the late response. =A0In Baxandall's paper, fig 9e the
> collector current is shown as positive (half sinewave). However, in my
> simulations (see above) =A0and in the lab (where I use mosfets) the
> drain current is a full sine wave (positive and negative current). In
> the simulation you have on your site I don't see the drain current. Is
> it a half sinewave or full sinewave?

The current you are seeing is segments a full sine-wave at twice the
operating frequency - if you loaded the oscillator so that you were
dissipating an appreciable amount of power in the load, the current
through the drain would look a lot more like a positive half-sine
wave.

The component at twice the operating frequency is the reason that the
Baxandall class-D oscillator doesn't produce a particularly pure sine
wave - there's pretty much always an irreducible minimum third
harmonic content in the output at around 1% of the fundamental (plus
progressively smaller proportions of the higher odd harmonics).

I've got a way of reducing that by a couple of orders of magnitude,
but it drops the efficiency of the device  (as an inverter) from
around 95% to closer to 50%. E-mail me for details -
bill.sloman@ieee.org is a real address.

--
Bill Sloman, Nijmegen