50 kHz VCO w/sine output

On Sep 28, 3:27=A0pm, Klaus Kragelund <klausk...@hotmail.com> wrote:
> On 28 Sep., 16:40, Joerg <inva...@invalid.invalid> wrote:
>
>
>
>
>
> > George Herold wrote:
> > > On Sep 27, 5:00 pm,Joerg<inva...@invalid.invalid> wrote:
> > >> Tim Wescott wrote:
> > >>> On Tue, 27 Sep 2011 13:18:06 -0700, George Herold wrote:
> > >>>> On Sep 27, 3:40 pm, whit3rd <whit...@gmail.com> wrote:
> > >>>>> On Tuesday, September 27, 2011 9:54:36 AM UTC-7, George Herold wr=
ote:
> > >>>>>> 30kHz to 100kHz VCO with sine wave output. I was also thinking I
> > >>>>>> could do this with a ~20MHz varactor controlled VCO as a variabl=
e
> > >>>>>> clock into a DSG chip. =A0This looks easier to my unsophisticate=
d
> > >>>>>> digital mind.
> > >>>>> Workable, of course, but not exactly a VCO anymore... and there's=
 the
> > >>>>> Nyquist filtering to do afterward.
> > >>>>>> Finally I offer the following, =A0a Wien bridge oscillator =A0wi=
th
> > >>>>>> varactor diodes as the capacitors.
> > >>>>> Not a great choice, because the Wein bridge requires MATCHED
> > >>>>> capacitors, tuned together, for best performance. =A0 It can be d=
one with
> > >>>>> a few trimmers (probably easiest to trim the offset + gain of the
> > >>>>> control voltages). =A0Then, there's the level translation problem=
 (the
> > >>>>> capacitors in a Wien bridge don't have one end grounded).
> > >>>> Hi Whit3rd,
> > >>>> Did you look at my schematic? =A0I seem to have it running in LTsp=
ice.
> > >>>>> Good sinewave LC VCOs in narrow ranges are easier, and a 4.0 MHz =
sine
> > >>>>> source and 4.030 to 4.100 MHz sine VCO can be mixed down to get y=
ou
> > >>>>> what you want.
> > >>>> Ah, good. =A0This was my first suggestion on how to get what was w=
anted.
> > >>>> It was rejected. =A0But perhaps I should push it a bit more!
> > >>> It's what I'd push, if I couldn't convince people to use a DDS or i=
f
> > >>> there were some overriding systems reason why a DDS wouldn't do. ..=
.
> > >> I'll second that. Maybe we should make some signs and start a virtua=
l
> > >> picket line. Need a nice chant though. "Rah-rah-rah, os-ci-llate,
> > >> down-con-vert, rah-rah-rah". Oh, and drums, of course :-)
>
> > > Can I get some scantily clad drum majorettes too? =A0:^)
>
> > Your wife might object :-)
>
> > > Seriously walking about last evening, I think this is the way to go!
> > > We'll do heterodyning and VCO side band generation. =A0I'll order som=
e
> > > varactor diodes and build some LC tank ciruit. =A0Got a favorite
> > > oscillator circuit?
> > > Or should I get something ready to run?
>
> > You could just use the SA612, figure 7 shows how to hook up LC to pins =
6
> > and 9. You can do Colpitts and Hartley, the transistor would be behind
> > those two pins with its base at pin 6.
>
> > As the next poster (me0223) wrote, this is a fairly wimpy chip. So don'=
t
> > push it. If you need more amplitude hang an opamp behind it. Or a small
> > audio amplifier chip if you also need a low impedance output, many of
> > those easily go up to 100kHz. Then you could generate several watts and
> > scare the bats out of the rafters :-)
>
> > > Now I just have to convince the 'powers that be'.
>
> > Been there ...
>
> > --
>
> I was thinking about the mixer approach for a simple network analyser
> gismo. In essense I need a amplitude/phase plot developed from two
> signals, operating from say 1kHz to 10MHz.
>
> Use a DDS to generate a local frequency (the one that is
> investigated), feed that to the input of the back box. Feed a mixer
> with the output of the black box and another DDS with a slightly lower
> frequency (50kHz). Feed the output of the mixer to a high resolution
> low frequency aquisition device (low sample rate ADC). For each local
> frequency, derive the amplitude and phase difference.
>
> Never done mixer designs, is mixers available which has predictable
> and stable gain? (from inputs to output). Or would a pre-calibration
> procedure be better?
>
> Regards
>
> Klaus- Hide quoted text -
>
> - Show quoted text -


Hi Klaus,

Re Mixers:
Typically mixers come spec=92d for conversion gain, IIP3, drive levels,
impedance data and isolation from port to port.  All as a function of
frequency.  When used as a component in an acquisition system such as
a SA typically the system has some mechanism for completing  self
cal.  That requirement is typically determined by your product
accuracy specification.

Having said that, I=92m confused by your application.

You have a DUT, the black box.  Driving it with a source.  Take the
output of the DUT and drive a mixer that uses another source that is a
few KHz below the original source?

Are the souces phased locked?

Why are using the mixer?  Do you have a low frequency A/D that you're
doing frequency translation to?

What about just using characterized switched paths to drive a higher
frequency A/D.  Terminate the black box into a load, stimulate it with
the DDS and look at the input and output.

I=92m probably just missing something.

Regards

Klaus Kragelund wrote:
> On 28 Sep., 16:40, Joerg <inva...@invalid.invalid> wrote:
>> George Herold wrote:
>>> On Sep 27, 5:00 pm,Joerg<inva...@invalid.invalid> wrote:
>>>> Tim Wescott wrote:
>>>>> On Tue, 27 Sep 2011 13:18:06 -0700, George Herold wrote:
>>>>>> On Sep 27, 3:40 pm, whit3rd <whit...@gmail.com> wrote:
>>>>>>> On Tuesday, September 27, 2011 9:54:36 AM UTC-7, George Herold wrote:
>>>>>>>> 30kHz to 100kHz VCO with sine wave output. I was also thinking I
>>>>>>>> could do this with a ~20MHz varactor controlled VCO as a variable
>>>>>>>> clock into a DSG chip.  This looks easier to my unsophisticated
>>>>>>>> digital mind.
>>>>>>> Workable, of course, but not exactly a VCO anymore... and there's the
>>>>>>> Nyquist filtering to do afterward.
>>>>>>>> Finally I offer the following,  a Wien bridge oscillator  with
>>>>>>>> varactor diodes as the capacitors.
>>>>>>> Not a great choice, because the Wein bridge requires MATCHED
>>>>>>> capacitors, tuned together, for best performance.   It can be done with
>>>>>>> a few trimmers (probably easiest to trim the offset + gain of the
>>>>>>> control voltages).  Then, there's the level translation problem (the
>>>>>>> capacitors in a Wien bridge don't have one end grounded).
>>>>>> Hi Whit3rd,
>>>>>> Did you look at my schematic?  I seem to have it running in LTspice.
>>>>>>> Good sinewave LC VCOs in narrow ranges are easier, and a 4.0 MHz sine
>>>>>>> source and 4.030 to 4.100 MHz sine VCO can be mixed down to get you
>>>>>>> what you want.
>>>>>> Ah, good.  This was my first suggestion on how to get what was wanted.
>>>>>> It was rejected.  But perhaps I should push it a bit more!
>>>>> It's what I'd push, if I couldn't convince people to use a DDS or if
>>>>> there were some overriding systems reason why a DDS wouldn't do. ...
>>>> I'll second that. Maybe we should make some signs and start a virtual
>>>> picket line. Need a nice chant though. "Rah-rah-rah, os-ci-llate,
>>>> down-con-vert, rah-rah-rah". Oh, and drums, of course :-)
>>> Can I get some scantily clad drum majorettes too?  :^)
>> Your wife might object :-)
>>
>>> Seriously walking about last evening, I think this is the way to go!
>>> We'll do heterodyning and VCO side band generation.  I'll order some
>>> varactor diodes and build some LC tank ciruit.  Got a favorite
>>> oscillator circuit?
>>> Or should I get something ready to run?
>> You could just use the SA612, figure 7 shows how to hook up LC to pins 6
>> and 9. You can do Colpitts and Hartley, the transistor would be behind
>> those two pins with its base at pin 6.
>>
>> As the next poster (me0223) wrote, this is a fairly wimpy chip. So don't
>> push it. If you need more amplitude hang an opamp behind it. Or a small
>> audio amplifier chip if you also need a low impedance output, many of
>> those easily go up to 100kHz. Then you could generate several watts and
>> scare the bats out of the rafters :-)
>>
>>> Now I just have to convince the 'powers that be'.
>> Been there ...
>>
>> --
> 
> I was thinking about the mixer approach for a simple network analyser
> gismo. In essense I need a amplitude/phase plot developed from two
> signals, operating from say 1kHz to 10MHz.
> 
> Use a DDS to generate a local frequency (the one that is
> investigated), feed that to the input of the back box. Feed a mixer
> with the output of the black box and another DDS with a slightly lower
> frequency (50kHz). Feed the output of the mixer to a high resolution
> low frequency aquisition device (low sample rate ADC). For each local
> frequency, derive the amplitude and phase difference.
> 

Or if you can, mix down a little lower so the sound card of a computer
can pick it up. Some go up to 25-30kHz. But all this depends on your
signal source. If it is by itself and no other noise is present, ok. But
if there are other spectra then keep in mind that if, for example, you
mix 1.45MHz with 1.4MHz you get a 50kHz output signal as a result. Plus
2.85MHz but that's easy to filter away. However, you also get a 50kHz
output signal if the input sees a strong enough signal at 1.35MHz. In
the AM radio band this can easily happen.


> Never done mixer designs, is mixers available which has predictable
> and stable gain? (from inputs to output). Or would a pre-calibration
> procedure be better?
> 

All the commercial and also the discrete mixers I have ever used are
very true to their specs and stable. Ok, cheap single-transistor
versions can be iffy. If you want top performance use a DBM from
Mini-Circuits or similar. We have used them in ultrasound machines and
there the whole algorithm would fall off the cliff if the amplitude
error was larger than 0.5dB or the phase error was larger than 1% (often
much less).

-- 
Regards, Joerg

http://www.analogconsultants.com/

George Herold wrote:
> On Sep 28, 1:38 pm, Tim Wescott <t...@seemywebsite.com> wrote:
>> On Wed, 28 Sep 2011 06:21:38 -0700, George Herold wrote:
>>> On Sep 27, 5:00 pm, Joerg <inva...@invalid.invalid> wrote:
>>>> Tim Wescott wrote:
>>>>> On Tue, 27 Sep 2011 13:18:06 -0700, George Herold wrote:
>>>>>> On Sep 27, 3:40 pm, whit3rd <whit...@gmail.com> wrote:
>>>>>>> On Tuesday, September 27, 2011 9:54:36 AM UTC-7, George Herold
>>>>>>> wrote:
>>>>>>>> 30kHz to 100kHz VCO with sine wave output. I was also thinking I
>>>>>>>> could do this with a ~20MHz varactor controlled VCO as a variable
>>>>>>>> clock into a DSG chip.  This looks easier to my unsophisticated
>>>>>>>> digital mind.
>>>>>>> Workable, of course, but not exactly a VCO anymore... and there's
>>>>>>> the Nyquist filtering to do afterward.
>>>>>>>> Finally I offer the following,  a Wien bridge oscillator  with
>>>>>>>> varactor diodes as the capacitors.
>>>>>>> Not a great choice, because the Wein bridge requires MATCHED
>>>>>>> capacitors, tuned together, for best performance.   It can be done
>>>>>>> with a few trimmers (probably easiest to trim the offset + gain of
>>>>>>> the control voltages).  Then, there's the level translation problem
>>>>>>> (the capacitors in a Wien bridge don't have one end grounded).
>>>>>> Hi Whit3rd,
>>>>>> Did you look at my schematic?  I seem to have it running in LTspice.
>>>>>>> Good sinewave LC VCOs in narrow ranges are easier, and a 4.0 MHz
>>>>>>> sine source and 4.030 to 4.100 MHz sine VCO can be mixed down to
>>>>>>> get you what you want.
>>>>>> Ah, good.  This was my first suggestion on how to get what was
>>>>>> wanted. It was rejected.  But perhaps I should push it a bit more!
>>>>> It's what I'd push, if I couldn't convince people to use a DDS or if
>>>>> there were some overriding systems reason why a DDS wouldn't do. ...
>>>> I'll second that. Maybe we should make some signs and start a virtual
>>>> picket line. Need a nice chant though. "Rah-rah-rah, os-ci-llate,
>>>> down-con-vert, rah-rah-rah". Oh, and drums, of course :-)
>>> Can I get some scantily clad drum majorettes too?  :^)
>>> Seriously walking about last evening, I think this is the way to go!
>>> We'll do heterodyning and VCO side band generation.  I'll order some
>>> varactor diodes and build some LC tank ciruit.  Got a favorite
>>> oscillator circuit?
>>> Or should I get something ready to run?
>>> Now I just have to convince the 'powers that be'.
>> If this is a low production volume system then you'll probably spend less
>> $$ in the end to just buy a VCO from MiniCircuits.  If it's high volume
>> then you can spend more time engineering the VCO yourself and the parts
>> will cost less.
>>
>> What do the 'powers that be' expect?  I spent years working at a company
>> that was very "transistor averse" -- they were much happier with a square
>> inch of board space and a $5.00 super-zoot op amp than they were with a
>> quarter square inch occupied by a $0.25 transistor and a few resistors.  
>> One learned to do what was going to get through the design review in the
>> end...
>>

Best comment I ever received when I "transistorized" a contemplated
massive army of ICs: "You mean that's IT?" ... "Yeah, that's it" ...
"Amazing. So what's it gonna cost?" ... "About three fifty" ...
"Threehundredfifty?" ... "No, three Dollars fifty" ... "WHAT?"


>> --www.wescottdesign.com- Hide quoted text -
>>
>> - Show quoted text -
> 
> Yeah, low low volume. (say 20 per year)  So it's cheaper to just buy
> something.  But then I don't learn anything....
> 
> The lowest frequency VCO that mini circuits makes is 12-25 MHz.  And
> higher frequencies for the peices with a smaller tuning range.  It'd
> be nice to have something a bit lower.  So maybe I can 'make my own'
> out of Joerg's SA612A part.
> 

You can use that VCO and then mix it with 15MHz or whatever. But I don't
know how you control the VCO. If via PLL, ok. If via hand and potmeter
then definitely not because it'll be way too sensitive.

-- 
Regards, Joerg

http://www.analogconsultants.com/

On Sep 28, 11:57=A0am, John Larkin wrote:
> On Wed, 28 Sep 2011 11:32:24 -0400, Phil Hobbs wrote:
> >On 09/28/2011 09:49 AM, George Herold wrote:
> >> On Sep 27, 10:06 pm, Phil Hobbs =A0wrote:

> >>> I'd probably be wanting to use a tri-wave oscillator with a sine shap=
er.
> >>> =A0 =A0(You don't have to use the crappy diode ones--you can use a ta=
nh to
> >>> roll over the peaks and then subtract a bit of the original sine, as =
we
> >>> discussed a year or so back. =A0I posted a suggestion athttp://electr=
ooptical.net/www/sed/sed.html, but it's a much older
> >>> technique.
>
> >> Hmmm, Thanks Phil, I'd forgotten about that. =A0(I had no need for it =
at
> >> the time.) =A0How do I get a tanh function out of a diff pair. =A0(Yea=
h,
> >> I'll try google too.)
>
> >> George H.
>
> >Diff pairs do tanh automatically--it's getting them to do anything else
> >that takes work.
>
> >You just shove the tri wave between the bases, and subtract the
> >collector currents, either with a current mirror or an op amp. =A0If you
> >don't mind an offset current of I_e/2, just connect one collector to the
> >supply and come out the other one.
>
>
> Another way to smooth a triangle onto a sine is with linear segment
> breakpoints. One opamp and a mess of resistors and diodes can do that,
> but there are several ways.
>
> Two breaks works pretty well. It can be done at high level, which is
> convenient.
>
> I wonder how you'd find the optimum break formula. Or, in general, how
> one best approximates a function, over some range, with N linear
> segments.

A VCO feeding a counter =3D=3D> simple decoder =3D=3D> a few resistors maki=
ng
a sine-weighted DAC might be a convenient way of making a sweepable
break-pointed sine.  A homemade DDS, as it were.

You could clock it at 256x clock rate or whatever, and get the
harmonics super low with just a few breakpoints.

It has the virtues of purity, simplicity, stability, and
repeatability.  Easy to design.  Oh, and it's cheap too.  Joerg likes
that.

--
Cheers,
James Arthur

George Herold wrote:
> 
> On Sep 28, 1:41 pm, "Michael A. Terrell" <mike.terr...@earthlink.net>
> wrote:
> > George Herold wrote:
> >
> > > Yeah DDS from a micro looks like the 'best' path.  It's just outside
> > > my present comfort zone and so hard to predict how long it will
> > > take.
> >
> >    You can get a "AD9850 Module DDS Signal Generator with Circuit
> > Diagram" in ebay for about $13 and demo software on the Analog Devices
> > website.  There is also VB source code & 80*51 source code availible for
> > free.
> >
> >    I just bought a couple for a project I'm working on.
> 
> Thanks Michael,  there's so many DDS chips.  Which ones will still be
> around in ten years?


   That chip has been around for 10 years.  If you are designing
something for long product life you can always make the DDS a module,
and the software updatable to use any new part.  Once you know how they
work, they are fairly simple. 


-- 
You can't have a sense of humor, if you have no sense.

George Herold wrote:
> 
> On Sep 28, 1:46 pm, "Michael A. Terrell" <mike.terr...@earthlink.net>
> wrote:
> > George Herold wrote:
> >
> > > Can I get some scantily clad drum majorettes too?  :^)
> >
> >    I don't know. CAN you? :)
> 
> Nah, too old.  My wife taught high school science for a few years
> while we were in Nashville.  The school was 'on the wrong side of the
> tracks' or river in this case.  (We lived on the wrong side of the
> river too, houses were cheaper.)  On Friday nights teachers were
> encouraged to attend the football game.  The best part was the half
> time show, lot's of drums, dancing and scantily clad drum majorettes
> shaking their bootie. (sp)


  I used to rent the sound system to a high school for their football
games.  I would sit in my service truck & watch Wonder Woman on a 12"
B&W TV I had modified to run on 12 volts.  I had a member of their band
walk out to my truck and start to tell me off because I wasn't watching
the game.  Then he saw what was on TV.  A couple minutes later, most of
the band was crowded around my truck to watch TV. :)

   The contract specified that I had to be at the games, it didn't say
that I had to watch them. :)


-- 
You can't have a sense of humor, if you have no sense.

On 29 Sep., 01:23, me0...@yahoo.com wrote:
> On Sep 28, 3:27=A0pm, Klaus Kragelund <klausk...@hotmail.com> wrote:
>
>
>
>
>
> > On 28 Sep., 16:40, Joerg <inva...@invalid.invalid> wrote:
>
> > > George Herold wrote:
> > > > On Sep 27, 5:00 pm,Joerg<inva...@invalid.invalid> wrote:
> > > >> Tim Wescott wrote:
> > > >>> On Tue, 27 Sep 2011 13:18:06 -0700, George Herold wrote:
> > > >>>> On Sep 27, 3:40 pm, whit3rd <whit...@gmail.com> wrote:
> > > >>>>> On Tuesday, September 27, 2011 9:54:36 AM UTC-7, George Herold =
wrote:
> > > >>>>>> 30kHz to 100kHz VCO with sine wave output. I was also thinking=
 I
> > > >>>>>> could do this with a ~20MHz varactor controlled VCO as a varia=
ble
> > > >>>>>> clock into a DSG chip. =A0This looks easier to my unsophistica=
ted
> > > >>>>>> digital mind.
> > > >>>>> Workable, of course, but not exactly a VCO anymore... and there=
's the
> > > >>>>> Nyquist filtering to do afterward.
> > > >>>>>> Finally I offer the following, =A0a Wien bridge oscillator =A0=
with
> > > >>>>>> varactor diodes as the capacitors.
> > > >>>>> Not a great choice, because the Wein bridge requires MATCHED
> > > >>>>> capacitors, tuned together, for best performance. =A0 It can be=
 done with
> > > >>>>> a few trimmers (probably easiest to trim the offset + gain of t=
he
> > > >>>>> control voltages). =A0Then, there's the level translation probl=
em (the
> > > >>>>> capacitors in a Wien bridge don't have one end grounded).
> > > >>>> Hi Whit3rd,
> > > >>>> Did you look at my schematic? =A0I seem to have it running in LT=
spice.
> > > >>>>> Good sinewave LC VCOs in narrow ranges are easier, and a 4.0 MH=
z sine
> > > >>>>> source and 4.030 to 4.100 MHz sine VCO can be mixed down to get=
 you
> > > >>>>> what you want.
> > > >>>> Ah, good. =A0This was my first suggestion on how to get what was=
 wanted.
> > > >>>> It was rejected. =A0But perhaps I should push it a bit more!
> > > >>> It's what I'd push, if I couldn't convince people to use a DDS or=
 if
> > > >>> there were some overriding systems reason why a DDS wouldn't do. =
...
> > > >> I'll second that. Maybe we should make some signs and start a virt=
ual
> > > >> picket line. Need a nice chant though. "Rah-rah-rah, os-ci-llate,
> > > >> down-con-vert, rah-rah-rah". Oh, and drums, of course :-)
>
> > > > Can I get some scantily clad drum majorettes too? =A0:^)
>
> > > Your wife might object :-)
>
> > > > Seriously walking about last evening, I think this is the way to go=
!
> > > > We'll do heterodyning and VCO side band generation. =A0I'll order s=
ome
> > > > varactor diodes and build some LC tank ciruit. =A0Got a favorite
> > > > oscillator circuit?
> > > > Or should I get something ready to run?
>
> > > You could just use the SA612, figure 7 shows how to hook up LC to pin=
s 6
> > > and 9. You can do Colpitts and Hartley, the transistor would be behin=
d
> > > those two pins with its base at pin 6.
>
> > > As the next poster (me0223) wrote, this is a fairly wimpy chip. So do=
n't
> > > push it. If you need more amplitude hang an opamp behind it. Or a sma=
ll
> > > audio amplifier chip if you also need a low impedance output, many of
> > > those easily go up to 100kHz. Then you could generate several watts a=
nd
> > > scare the bats out of the rafters :-)
>
> > > > Now I just have to convince the 'powers that be'.
>
> > > Been there ...
>
> > > --
>
> > I was thinking about the mixer approach for a simple network analyser
> > gismo. In essense I need a amplitude/phase plot developed from two
> > signals, operating from say 1kHz to 10MHz.
>
> > Use a DDS to generate a local frequency (the one that is
> > investigated), feed that to the input of the back box. Feed a mixer
> > with the output of the black box and another DDS with a slightly lower
> > frequency (50kHz). Feed the output of the mixer to a high resolution
> > low frequency aquisition device (low sample rate ADC). For each local
> > frequency, derive the amplitude and phase difference.
>
> > Never done mixer designs, is mixers available which has predictable
> > and stable gain? (from inputs to output). Or would a pre-calibration
> > procedure be better?
>
> > Regards
>
> > Klaus- Hide quoted text -
>
> > - Show quoted text -
>
> Hi Klaus,
>
> Re Mixers:
> Typically mixers come spec=92d for conversion gain, IIP3, drive levels,
> impedance data and isolation from port to port. =A0All as a function of
> frequency. =A0When used as a component in an acquisition system such as
> a SA typically the system has some mechanism for completing =A0self
> cal. =A0That requirement is typically determined by your product
> accuracy specification.
>
> Having said that, I=92m confused by your application.
>
> You have a DUT, the black box. =A0Driving it with a source. =A0Take the
> output of the DUT and drive a mixer that uses another source that is a
> few KHz below the original source?
>
> Are the souces phased locked?
>
> Why are using the mixer? =A0Do you have a low frequency A/D that you're
> doing frequency translation to?
>
> What about just using characterized switched paths to drive a higher
> frequency A/D. =A0Terminate the black box into a load, stimulate it with
> the DDS and look at the input and output.
>
> I=92m probably just missing something.
>

In essense I want to measure the gain/phase of a black box. It could
be a filter, or could be a amplier, PSU etc

To measure the gain/phase plot, I will measure at a lot of discrete
frequencies, up to about 10MHz (what makes sense for my application)

But, to measure phase in a simple way, I need to have the ADC capture
the signal with a sample rate of at least 20-50 times higher than my
signal, so that would be a very high speed ADC.

Instead, I purpose to shift the signal down instead, so I can use a
cheap and LF ADC. The local oscillator is the exersitation for the
filter, the second DDS is just used to shift the signal down. I am not
sure if the need to be phase aligned.

Regards

Klaus

On Wed, 28 Sep 2011 16:04:49 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>On 09/28/2011 03:11 PM, Spehro Pefhany wrote:
>> On Wed, 28 Sep 2011 11:57:41 -0700, John Larkin
>> <jjlarkin@highNOTlandTHIStechnologyPART.com>  wrote:
>>
>>> On Wed, 28 Sep 2011 14:40:01 -0400, Spehro Pefhany
>>> <speffSNIP@interlogDOTyou.knowwhat>  wrote:
>>>
>>>> On Wed, 28 Sep 2011 08:57:52 -0700, John Larkin
>>>> <jjlarkin@highNOTlandTHIStechnologyPART.com>  wrote:
>>>>
>>>>>
>>>>>
>>>>> Another way to smooth a triangle onto a sine is with linear segment
>>>>> breakpoints. One opamp and a mess of resistors and diodes can do that,
>>>>> but there are several ways.
>>>>>
>>>>> Two breaks works pretty well. It can be done at high level, which is
>>>>> convenient.
>>>>>
>>>>> I wonder how you'd find the optimum break formula. Or, in general, how
>>>>> one best approximates a function, over some range, with N linear
>>>>> segments.
>>>>>
>>>>> John
>>>>
>>>> Many years ago I wrote an optimization program for doing this for a
>>>> nonlinear function with 'n' breakpoints. It would slide around the
>>>> points from reasonable initial positions and optimize whatever error
>>>> function you wanted (absolute error, error squared or whatever). These
>>>> days you could probably do it with Excel's solver. I used it for
>>>> fitting thermocouple curves.
>>>
>>> Did it always converge to a sensible result?
>>
>> It was a long time ago, but provided you started with a sensible start
>> position I think it did.
>>
>>> I can imagine all sorts
>>> of silly or chaotic behavior. This seems like an interesting problem.
>>
>> I think it would be rather difficult to do in general, and I think I
>> spent a bit of time looking at the general problem, but really I just
>> wanted to calculate optimal precision resistor values for each input
>> type so I could send the design to production and go have a beer.
>> Plotting the nonlinearity and picking points manually was good enough
>> to meet the spec.
>>
>>> I usually just eyeball it, for soft curves at least. I guess you want
>>> shorter segments on the curvier parts. For software fits, we just do
>>> some insane number of equally-spaced segments, like 64 or so.
>>
>> The BFI method works fine.
>>
>>> For shaping a triangle into a sine, what should be optimized is the
>>> harmonic distortion, not the RMS error, I guess. Fiddling in Spice is
>>> probably as good a way as any.
>>>
>>> John
>>>
>>
>
>For approximating a sine wave, the harmonic distortion is identical to 
>the RMS error, assuming there's no DC offset.
>
>Cheers
>
>Phil

Oh, right, the conservation of energy thing.

John

On Sep 28, 5:38=A0pm, Bill Sloman <bill.slo...@ieee.org> wrote:
> On Sep 28, 9:41=A0pm, George Herold <gher...@teachspin.com> wrote:
>
>
>
>
>
> > On Sep 28, 1:41=A0pm, "Michael A. Terrell" <mike.terr...@earthlink.net>
> > wrote:
>
> > > George Herold wrote:
>
> > > > Yeah DDS from a micro looks like the 'best' path. =A0It's just outs=
ide
> > > > my present comfort zone and so hard to predict how long it will
> > > > take.
>
> > > =A0 =A0You can get a "AD9850 Module DDS Signal Generator with Circuit
> > > Diagram" in ebay for about $13 and demo software on the Analog Device=
s
> > > website. =A0There is also VB source code & 80*51 source code availibl=
e for
> > > free.
>
> > > =A0 =A0I just bought a couple for a project I'm working on.
>
> > > --
> > > You can't have a sense of humor, if you have no sense.
>
> > Thanks Michael, =A0there's so many DDS chips. =A0Which ones will still =
be
> > around in ten years?
>
> The first mention of the AD9850 I can find here dates back to 2000, so
> it has been around for more than ten years already - always a good
> sign - and I think that it had been around for a while then. IIRR it
> was the first of Analog Devices DDS chips, and it seems to have been
> designed into a lot of applications, so it may be around for a while
> yet.
>
> --
> Bill Sloman, Nijmegen- Hide quoted text -
>
> - Show quoted text -

Thanks Bill, that's good to know.

George H.

On Sep 28, 4:33=A0pm, me0...@yahoo.com wrote:
> On Sep 28, 12:34=A0pm, George Herold <gher...@teachspin.com> wrote:
>
>
>
>
>
> > On Sep 28, 1:38=A0pm, Tim Wescott <t...@seemywebsite.com> wrote:
>
<big snip>
> > - Show quoted text -
>
> Hey George,
>
> Here=92s some more food for thought re the downconverter approach:

Hi MeO
>
> (you=92ve never said what you=92re looking for in Frequency stability, bu=
t
> I=92m assuming you want accuracy set by your XOSC and phase noise is
> really not an issue)

Hmm, this is a small part of a new project.  I'm still trying to get a
handle on the specs I need.
>
> If you=92ve never done this before I would discourage you:
>
> 1) =A0 =A0 =A0 Unless you=92ve got the time to put into the learning curv=
e, not
> only RF but the Freq Synth part
> 2) =A0 =A0 =A0You have access to a decent SA or FFT analyzer that can cov=
er your
> frequency of interest
> 3) =A0 =A0 =A0A low frequency network analyzer would also make the job a =
lot
> easier

Well, an RF amplifier design ~20 years ago.  (would have been nice to
have a spectrum analyzer at the time, I had some ring down issues in
one of the amp stages.)  The only RF recently is a few watt oscillator
for a Rubidium discharge lamp.  (Where frequency stability is not much
of an issue.)  I've got a 'new' SRS770 spectrum analyzer, (DC-100kHz)
which should tell me all I need to know about what's happening after
the mixer.

>
> You=92ve got another potential problem when you attempt this kind of
> frequency translation. =A0The low =93Q=94 VCO is going to have a tendency=
 to
> be pulled around by the high =93Q=94 XOSC. =A0They =93sniff=94 each other=
 when
> they get close frequency wise.

Hmm a bit of padding should take care of that... I'll just burn
power!
>
> So you have to make sure your system has enough isolation between the
> two oscillators. =A0You need to take into account things like LO to RF
> isolation when looking at the mixer. =A0In =93S=94 parameter terms it wil=
l
> be the S12 term of the VCO chain.
>
> Again I will state, I really believe your VCO will have to be phased
> locked. =A0Otherwise it=92ll be hit and miss when trying to set it=92s
> freq. =A0Each one of those gain curves will be slightly different from
> part to part.
>
> I don=92t mean to discourage you. =A0In fact, for somebody that does synt=
h
> design this is pretty trivial.
>
> But for a first timer it will be daunting, and could really end up
> being a big waste of time. =A0The digital approach is just a matter of
> ramping up and not equipment intensive. =A0And software is comparatively
> easier to change then hardware.

Thanks,  I'd like to do a DDS design sometime, but right now that
looks like a longer learning curve/ time sink.  (strange as that may
sound.)
>
> I=92m just trying to be helpful and I wish you the best of luck.

Thanks again.  At the moment I'm just going to order some VCO's from
minicirciuts... (throw some money at the problem.) and see what the
output looks like.  I'll also push forward on the triangle wave/ sine
shaper idea.

This is a nice app note,
http://www.national.com/an/AN/AN-263.pdf


George H.

>
> Btw, you do want to go with frequencies up over 10MHz. =A0Get far away
> from your baseband, just makes the IF filtering all that much
> easier.- Hide quoted text -
>
> - Show quoted text -