50 kHz VCO w/sine output

30kHz to 100kHz VCO with sine wave output.

I know this has been covered before in previous threads.

The obvious way to do this is a digital signal generator (DSG) IC with
maybe an A-D to measure the voltage.  I=92ve not done much digital stuff
in a while, so this has a bit of a learning curve.  But if anyone has
some good chips to look at or app notes then please do share.  I've
been looking at IC's on Analogs web site, but there are a lot to
choose from!

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.

Finally I offer the following,  a Wien bridge oscillator  with
varactor diodes as the capacitors.    I was also thinking I could use
crappy Z5U ceramic caps instead of the varactors.

http://imageshack.us/photo/my-images/88/wien1.jpg/

(Excuse my chicken scratching.)
Comments welcome.

George H.

On Tue, 27 Sep 2011 09:54:36 -0700, George Herold wrote:

> 30kHz to 100kHz VCO with sine wave output.
> 
> I know this has been covered before in previous threads.
> 
> The obvious way to do this is a digital signal generator (DSG) IC with
> maybe an A-D to measure the voltage.  I’ve not done much digital stuff
> in a while, so this has a bit of a learning curve.  But if anyone has
> some good chips to look at or app notes then please do share.  I've been
> looking at IC's on Analogs web site, but there are a lot to choose from!
> 
> 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.
> 
> Finally I offer the following,  a Wien bridge oscillator  with varactor
> diodes as the capacitors.    I was also thinking I could use crappy Z5U
> ceramic caps instead of the varactors.

It'll be rough to get the high impedances and low voltage swings you need 
for the Wien bridge oscillator to work either with varactors or with a 
suggestion that I've made previously of using JFETs as variable resistors.

So: a high frequency VCO to a DDS chip, or a fixed-frequency reference to 
a DDS that you write a frequency to with a micro, or (if you are for some 
reason insisting on "analog only") a VCO feeding a mixer, beating against 
a crystal oscillator.

I'd look to the DDS solution, or see if you can make a suitable block 
with a PIC or similar, a good DAC, and some code.

-- 
www.wescottdesign.com

George Herold wrote:
> 30kHz to 100kHz VCO with sine wave output.
> 
> I know this has been covered before in previous threads.
> 
> The obvious way to do this is a digital signal generator (DSG) IC with
> maybe an A-D to measure the voltage.  I�ve not done much digital stuff
> in a while, so this has a bit of a learning curve.  But if anyone has
> some good chips to look at or app notes then please do share.  I've
> been looking at IC's on Analogs web site, but there are a lot to
> choose from!
> 
> 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.
> 
> Finally I offer the following,  a Wien bridge oscillator  with
> varactor diodes as the capacitors.    I was also thinking I could use
> crappy Z5U ceramic caps instead of the varactors.
> 
> http://imageshack.us/photo/my-images/88/wien1.jpg/
> 
> (Excuse my chicken scratching.)
> Comments welcome.
> 

This is the TV dinner version, costs a bit more but you just rip open,
microwave, and eat:

http://www.analog.com/static/imported-files/data_sheets/AD9837.PDF

Z5U as a VCO works but it's tough to get the distortion low enough,
since you want a nice sine wave and not a crummy one I assume.

If it has to be cheap there's this secret trick:

Let the VCO run on a much higher and more practical frequency. Let's say
 2030kHz to 2100kHz, for example. That can easily be done with a
varicap, a transistor, some R, C, and an inductor. Very cheap. Then mix
that with a 2MHz clock. The result will be these three spectra:

30kHz-100kHZ
2030kHz-2100kHz (40dB down or better if using a DBM)
4030kHz-4100kHz
plus some harmonics.

A very simple lowpass that cuts off at a few hundred kHZ can fish out
30kHz-100kHz and ... bingo.

If you search around you might find a radio chip that does most of
nearly all of this. Depends a bit on what supply voltages you have
available. Something like this maybe:

http://www.nxp.com/documents/data_sheet/SA612A.pdf

-- 
Regards, Joerg

http://www.analogconsultants.com/

On Tue, 27 Sep 2011 09:54:36 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

>30kHz to 100kHz VCO with sine wave output.
>
>I know this has been covered before in previous threads.
>
>The obvious way to do this is a digital signal generator (DSG) IC with
>maybe an A-D to measure the voltage.  I&#4294967295;ve not done much digital stuff
>in a while, so this has a bit of a learning curve.  But if anyone has
>some good chips to look at or app notes then please do share.  I've
>been looking at IC's on Analogs web site, but there are a lot to
>choose from!
>
>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.
>
>Finally I offer the following,  a Wien bridge oscillator  with
>varactor diodes as the capacitors.    I was also thinking I could use
>crappy Z5U ceramic caps instead of the varactors.
>
>http://imageshack.us/photo/my-images/88/wien1.jpg/
>
>(Excuse my chicken scratching.)
>Comments welcome.
>
>George H.

You could do a varactor-tuned LC oscillator, but a 3:1 range is
pushing things. 

Some classic integrator-based function generator thing wouldn't be
hard, with a sine shaper downstream. There are chips to do this, and
some probably run up to 100K.

John

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).

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.

On Sep 27, 2:17=A0pm, Tim Wescott <t...@seemywebsite.com> wrote:
> On Tue, 27 Sep 2011 09:54:36 -0700, George Herold wrote:
> > 30kHz to 100kHz VCO with sine wave output.
>
> > I know this has been covered before in previous threads.
>
> > The obvious way to do this is a digital signal generator (DSG) IC with
> > maybe an A-D to measure the voltage. =A0I=92ve not done much digital st=
uff
> > in a while, so this has a bit of a learning curve. =A0But if anyone has
> > some good chips to look at or app notes then please do share. =A0I've b=
een
> > looking at IC's on Analogs web site, but there are a lot to choose from=
!
>
> > I was also thinking I could do this with a ~20MHz varactor controlled
> > VCO as a variable clock into a DSG chip. =A0This looks easier to my
> > unsophisticated digital mind.
>
> > Finally I offer the following, =A0a Wien bridge oscillator =A0with vara=
ctor
> > diodes as the capacitors. =A0 =A0I was also thinking I could use crappy=
 Z5U
> > ceramic caps instead of the varactors.
>
> It'll be rough to get the high impedances and low voltage swings you need
> for the Wien bridge oscillator to work either with varactors or with a
> suggestion that I've made previously of using JFETs as variable resistors=
.

Hi Tim,  I've got a spice simulation with just diode limiting for the
varible gain.  ~1.2 Vp-p on the output.  I couldn't find any varactors
in LTspice so I just used a diode and added some more C in parallel.
The AC voltage across the 'varactor' is less than a volt.  By playing
around with the gain control I can probably get reduce that by a
factor of two.  Is that too much?


>
> So: a high frequency VCO to a DDS chip, or a fixed-frequency reference to
> a DDS that you write a frequency to with a micro, or (if you are for some
> reason insisting on "analog only") a VCO feeding a mixer, beating against
> a crystal oscillator.

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.

George H.

>
> I'd look to the DDS solution, or see if you can make a suitable block
> with a PIC or similar, a good DAC, and some code.


>
> --www.wescottdesign.com- Hide quoted text -
>
> - Show quoted text -

On Sep 27, 2:49=A0pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> On Tue, 27 Sep 2011 09:54:36 -0700 (PDT), George Herold
>
>
>
>
>
> <gher...@teachspin.com> wrote:
> >30kHz to 100kHz VCO with sine wave output.
>
> >I know this has been covered before in previous threads.
>
> >The obvious way to do this is a digital signal generator (DSG) IC with
> >maybe an A-D to measure the voltage. =A0I ve not done much digital stuff
> >in a while, so this has a bit of a learning curve. =A0But if anyone has
> >some good chips to look at or app notes then please do share. =A0I've
> >been looking at IC's on Analogs web site, but there are a lot to
> >choose from!
>
> >I was also thinking I could do this with a ~20MHz varactor controlled
> >VCO as a variable clock into a DSG chip. =A0This looks easier to my
> >unsophisticated digital mind.
>
> >Finally I offer the following, =A0a Wien bridge oscillator =A0with
> >varactor diodes as the capacitors. =A0 =A0I was also thinking I could us=
e
> >crappy Z5U ceramic caps instead of the varactors.
>
> >http://imageshack.us/photo/my-images/88/wien1.jpg/
>
> >(Excuse my chicken scratching.)
> >Comments welcome.
>
> >George H.
>
> You could do a varactor-tuned LC oscillator, but a 3:1 range is
> pushing things.
Yeah the dang square root of C.... well maybe only a 2:1 range would
be enough.

>
> Some classic integrator-based function generator thing wouldn't be
> hard, with a sine shaper downstream. There are chips to do this, and
> some probably run up to 100K.

Yeah the triangle wave/ integrator is nice, but the sine shaper is a
bitch... and ugly!   We got some of the old ICL8038 chips, I'll have
to fire one of those up too.

George H.


>
> John- Hide quoted text -
>
> - Show quoted text -

On Sep 27, 2:35=A0pm, Joerg <inva...@invalid.invalid> wrote:
> George Herold wrote:
> > 30kHz to 100kHz VCO with sine wave output.
>
> > I know this has been covered before in previous threads.
>
> > The obvious way to do this is a digital signal generator (DSG) IC with
> > maybe an A-D to measure the voltage. =A0I ve not done much digital stuf=
f
> > in a while, so this has a bit of a learning curve. =A0But if anyone has
> > some good chips to look at or app notes then please do share. =A0I've
> > been looking at IC's on Analogs web site, but there are a lot to
> > choose from!
>
> > I was also thinking I could do this with a ~20MHz varactor controlled
> > VCO as a variable clock into a DSG chip. =A0This looks easier to my
> > unsophisticated digital mind.
>
> > Finally I offer the following, =A0a Wien bridge oscillator =A0with
> > varactor diodes as the capacitors. =A0 =A0I was also thinking I could u=
se
> > crappy Z5U ceramic caps instead of the varactors.
>
> >http://imageshack.us/photo/my-images/88/wien1.jpg/
>
> > (Excuse my chicken scratching.)
> > Comments welcome.
>
> This is the TV dinner version, costs a bit more but you just rip open,
> microwave, and eat:
>
> http://www.analog.com/static/imported-files/data_sheets/AD9837.PDF

Thanks,  I only did a quick scan... To use it as a VCO I'd have to
write new frequencies via the SPI interface.
>
> Z5U as a VCO works but it's tough to get the distortion low enough,
> since you want a nice sine wave and not a crummy one I assume.

Hmm, I'm not so sure.  What we really want to do.  (I think.....)  is
drive the piss out of a VCO so we can see all the various side bands.
Crank it up to where you see the fundamental dissapear...(The first
zero of the Bessel function)  And even higher... watch the first
sideband go away... Anyway I think this is what we want, I'll have to
check.  So I'm not sure I care too much about a crummy sine wave.
Maybe the first few harmonics down by 40dB is OK.

>
> If it has to be cheap there's this secret trick:
>
> Let the VCO run on a much higher and more practical frequency. Let's say
> =A02030kHz to 2100kHz, for example. That can easily be done with a
> varicap, a transistor, some R, C, and an inductor. Very cheap. Then mix
> that with a 2MHz clock. The result will be these three spectra:
>
> 30kHz-100kHZ
> 2030kHz-2100kHz (40dB down or better if using a DBM)
> 4030kHz-4100kHz
> plus some harmonics.
>
> A very simple lowpass that cuts off at a few hundred kHZ can fish out
> 30kHz-100kHz and ... bingo.

Yeah I thought of that too.  Cheap is not really important.  But I'd
really like parts that are still going to be around in ten years.
>
> If you search around you might find a radio chip that does most of
> nearly all of this. Depends a bit on what supply voltages you have
> available. Something like this maybe:
>
> http://www.nxp.com/documents/data_sheet/SA612A.pdf
Nice, thanks.  I usually just go to minicircuits for this type of
stuff.

George H.
>
> --
> Regards, Joerg
>
> http://www.analogconsultants.com/- Hide quoted text -
>
> - Show quoted text -

George Herold wrote:
> On Sep 27, 2:17 pm, Tim Wescott <t...@seemywebsite.com> wrote:
>> On Tue, 27 Sep 2011 09:54:36 -0700, George Herold wrote:
>>> 30kHz to 100kHz VCO with sine wave output.
>>> I know this has been covered before in previous threads.
>>> The obvious way to do this is a digital signal generator (DSG) IC with
>>> maybe an A-D to measure the voltage.  I&#4294967295;ve not done much digital stuff
>>> in a while, so this has a bit of a learning curve.  But if anyone has
>>> some good chips to look at or app notes then please do share.  I've been
>>> looking at IC's on Analogs web site, but there are a lot to choose from!
>>> 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.
>>> Finally I offer the following,  a Wien bridge oscillator  with varactor
>>> diodes as the capacitors.    I was also thinking I could use crappy Z5U
>>> ceramic caps instead of the varactors.
>> It'll be rough to get the high impedances and low voltage swings you need
>> for the Wien bridge oscillator to work either with varactors or with a
>> suggestion that I've made previously of using JFETs as variable resistors.
> 
> Hi Tim,  I've got a spice simulation with just diode limiting for the
> varible gain.  ~1.2 Vp-p on the output.  I couldn't find any varactors
> in LTspice so I just used a diode and added some more C in parallel.
> The AC voltage across the 'varactor' is less than a volt.  By playing
> around with the gain control I can probably get reduce that by a
> factor of two.  Is that too much?
> 

Not sure what granularity you need but there are also digital
potentiometers to vary the "R" part.

Switching caps in and our can be done via 75HC4051 or CD4051 if you need
more volts. One of those plus 8 caps would give you 256 frequencies, two
plus 16 caps would (well, in theory) give you 65536 frequencies.

> 
>> So: a high frequency VCO to a DDS chip, or a fixed-frequency reference to
>> a DDS that you write a frequency to with a micro, or (if you are for some
>> reason insisting on "analog only") a VCO feeding a mixer, beating against
>> a crystal oscillator.
> 
> 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.
> 

Search for this:

"<your favorite uC goes here>" plus "sine wave generator"

and

"<your favorite uC goes here>" plus "wave digital filters" (or "WDF")

That can point you to resources where most of the coding has already
been done and you may only have to do some adaptations. WDFs are method
to do this with really cheap uC that don't come with a HW multiplier on
board. But you do need a DAC. In a pinch maybe a "poor man's DAC" using
a port register plus eight resistors R2R-style.

[...]

-- 
Regards, Joerg

http://www.analogconsultants.com/

On Sep 27, 3:40=A0pm, 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. =A0This 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, =A0a Wien bridge oscillator =A0with
> > 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 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?  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!

George H.