50 kHz VCO w/sine output

On Sep 28, 10:40=A0am, 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 wrot=
e:
> >>>>>> 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 t=
he
> >>>>> 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 don=
e 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 LTspic=
e.
> >>>>> Good sinewave LC VCOs in narrow ranges are easier, and a 4.0 MHz si=
ne
> >>>>> 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 wan=
ted.
> >>>> 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 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? =A0:^)
>
> Your wife might object :-)

Well looking is OK, just no touching!

>
> > 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 some
> > 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.

OK I've ordered some.  These even come in DIP's!

George H.
>
> 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 ...

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

On Sep 28, 1:38=A0pm, Tim Wescott <t...@seemywebsite.com> wrote:
> On Wed, 28 Sep 2011 06:21:38 -0700, George Herold wrote:
> > On Sep 27, 5:00=A0pm, 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. =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 =A0wit=
h
> >> >>>> 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 do=
ne
> >> >>> with a few trimmers (probably easiest to trim the offset + gain of
> >> >>> the control voltages). =A0Then, there's the level translation prob=
lem
> >> >>> (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 LTspi=
ce.
> >> >>> 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. =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 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? =A0:^)
>
> > 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 some
> > varactor diodes and build some LC tank ciruit. =A0Got 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. =A0If 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? =A0I spent years working at a compan=
y
> 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. =
=A0
> One learned to do what was going to get through the design review in the
> end...
>
> --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.

George H.

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 outside
> > 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 Devices
> website. =A0There is also VB source code & 80*51 source code availible fo=
r
> 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,  there's so many DDS chips.  Which ones will still be
around in ten years?

George H.

On Sep 28, 1:46=A0pm, "Michael A. Terrell" <mike.terr...@earthlink.net>
wrote:
> George Herold wrote:
>
> > Can I get some scantily clad drum majorettes too? =A0:^)
>
> =A0 =A0I 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)

George H.

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

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

-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

On Sep 28, 12:34=A0pm, George Herold <gher...@teachspin.com> wrote:
> On Sep 28, 1:38=A0pm, Tim Wescott <t...@seemywebsite.com> wrote:
>
>
>
>
>
> > On Wed, 28 Sep 2011 06:21:38 -0700, George Herold wrote:
> > > On Sep 27, 5:00=A0pm, 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 variab=
le
> > >> >>>> clock into a DSG chip. =A0This looks easier to my unsophisticat=
ed
> > >> >>>> 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 =A0w=
ith
> > >> >>>> 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 pr=
oblem
> > >> >>> (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 LTs=
pice.
> > >> >>> 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. =A0This was my first suggestion on how to get what was
> > >> >> wanted. It was rejected. =A0But perhaps I should push it a bit mo=
re!
>
> > >> > 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 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:^)
>
> > > 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?
>
> > > Now I just have to convince the 'powers that be'.
>
> > If this is a low production volume system then you'll probably spend le=
ss
> > $$ in the end to just buy a VCO from MiniCircuits. =A0If it's high volu=
me
> > then you can spend more time engineering the VCO yourself and the parts
> > will cost less.
>
> > What do the 'powers that be' expect? =A0I spent years working at a comp=
any
> > that was very "transistor averse" -- they were much happier with a squa=
re
> > 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.=
 =A0
> > One learned to do what was going to get through the design review in th=
e
> > end...
>
> > --www.wescottdesign.com-Hide quoted text -
>
> > - Show quoted text -
>
> Yeah, low low volume. (say 20 per year) =A0So it's cheaper to just buy
> something. =A0But then I don't learn anything....
>
> The lowest frequency VCO that mini circuits makes is 12-25 MHz. =A0And
> higher frequencies for the peices with a smaller tuning range. =A0It'd
> be nice to have something a bit lower. =A0So maybe I can 'make my own'
> out of Joerg's SA612A part.
>
> George H.- Hide quoted text -
>
> - Show quoted text -

Hey George,

Here=92s some more food for thought re the downconverter approach:

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

If you=92ve never done this before I would discourage you:

1)	 Unless you=92ve got the time to put into the learning curve, not
only RF but the Freq Synth part
2)	You have access to a decent SA or FFT analyzer that can cover your
frequency of interest
3)	A low frequency network analyzer would also make the job a lot
easier

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

So you have to make sure your system has enough isolation between the
two oscillators.  You need to take into account things like LO to RF
isolation when looking at the mixer.  In =93S=94 parameter terms it will
be the S12 term of the VCO chain.

Again I will state, I really believe your VCO will have to be phased
locked.  Otherwise it=92ll be hit and miss when trying to set it=92s
freq.  Each one of those gain curves will be slightly different from
part to part.

I don=92t mean to discourage you.  In fact, for somebody that does synth
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.  The digital approach is just a matter of
ramping up and not equipment intensive.  And software is comparatively
easier to change then hardware.

I=92m just trying to be helpful and I wish you the best of luck.

Btw, you do want to go with frequencies up over 10MHz.  Get far away
from your baseband, just makes the IF filtering all that much
easier.

John Larkin 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? I can imagine all sorts
> of silly or chaotic behavior. This seems like an interesting problem.
>
> 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.
>
> For shaping a triangle into a sine, what should be optimized is the
> harmonic distortion, not the RMS error, I guess.

Same thing, isn't it?

> Fiddling in Spice is
> probably as good a way as any.
>
> John
>
>
--
Les Cargill

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 outsid=
e
> > > 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 Devices
> > website. =A0There is also VB source code & 80*51 source code availible =
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

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 wrot=
e:
> >>>>>> 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 t=
he
> >>>>> 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 don=
e 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 LTspic=
e.
> >>>>> Good sinewave LC VCOs in narrow ranges are easier, and a 4.0 MHz si=
ne
> >>>>> 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 wan=
ted.
> >>>> 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 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? =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 some
> > 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

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.
> 
> 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
Back in the hay day, we used double balanced mixers. You could generate
a vary wide dynamic signal that was clean. Just use a low pass filter to
remove the upper end.

  They made great audio scramblers too! :)

http://www.mit.edu/~6.301/LM1496.pdf

Jamie