# Heck of a way to run an oscillator?

Started by May 13, 2014
```I was looking at the SAW oscillator circuits at a co. called
Epcos, and they just take an op amp with the SAW filter in the feedback
loop. They explain that the op amp amplifies noise, but preferentially
at the filter allowed frequency. 2 components.

I love the simplicity, but does this approach have any drawbacks?
```
```On Tuesday, May 13, 2014 2:14:54 PM UTC+1, skeptical engineer wrote:

> I was looking at the SAW oscillator circuits at a co. called
> Epcos, and they just take an op amp with the SAW filter in the feedback
> loop. They explain that the op amp amplifies noise, but preferentially
> at the filter allowed frequency. 2 components.
> I love the simplicity, but does this approach have any drawbacks?

The amp just needs to be in its linear region to begin with

NT
```
```"skeptical engineer"

>I was looking at the SAW oscillator circuits at a co. called
> Epcos, and they just take an op amp with the SAW filter in the feedback
> loop. They explain that the op amp amplifies noise, but preferentially
> at the filter allowed frequency. 2 components.

**  I see no "op-amps " in their VHF /UHF oscillator designs.

You need to post a link to stuff like that.

....  Phil

```
```On 05/13/2014 03:14 PM, skeptical engineer wrote:
> I was looking at the SAW oscillator circuits at a co. called
> Epcos, and they just take an op amp with the SAW filter in the feedback
> loop. They explain that the op amp amplifies noise, but preferentially
> at the filter allowed frequency. 2 components.
>
> I love the simplicity, but does this approach have any drawbacks?

That was probably a gain block fed back by the SAW device. And yes, this
is a classical way of building an oscillator: the whole thing will
oscillate at the frequency where the phase shift due to the whole loop
is a multiple of 360 deg, provided the gain there is slightly more than
unity.
Startup can be explained by noise triggering the circuit's free
response, which is an exponentially growing sine wave. Eventually,
nonlinearities will set in, distorting the waveform and changing the
oscillation frequency.
As the SAW device is high-Q, with a steep phase slope around the
intended frequency, this usually turns out to be a pretty good and
simple oscillator. Just watch out for not having excessive gain.

Pere

```
```On Tuesday, May 13, 2014 9:30:08 PM UTC-4, Phil Allison wrote:
> "skeptical engineer"
>
>
>
> >I was looking at the SAW oscillator circuits at a co. called
>
> > Epcos, and they just take an op amp with the SAW filter in the feedback
>
> > loop. They explain that the op amp amplifies noise, but preferentially
>
> > at the filter allowed frequency. 2 components.
>
>
>
> **  I see no "op-amps " in their VHF /UHF oscillator designs.
>
>
>
>     You need to post a link to stuff like that.
>
>
>
>
>
>
>
> ....  Phil

OK - right you are.
http://www.mouser.com/pdfdocs/_Application%20Toolkit_050518_17h.pdf
See page 10.

This is posted at Mouser, but it is an Epcos doc. The question is, would an
op-amp (generic amplifier) work in this circuit?

jb
```
```On Wednesday, May 14, 2014 4:04:15 AM UTC-4, o pere o wrote:
> On 05/13/2014 03:14 PM, skeptical engineer wrote:
>
> > I was looking at the SAW oscillator circuits at a co. called
>
> > Epcos, and they just take an op amp with the SAW filter in the feedback
>
> > loop. They explain that the op amp amplifies noise, but preferentially
>
> > at the filter allowed frequency. 2 components.
>
> >
>
> > I love the simplicity, but does this approach have any drawbacks?
>
>
>
> That was probably a gain block fed back by the SAW device. And yes, this
>
> is a classical way of building an oscillator: the whole thing will
>
> oscillate at the frequency where the phase shift due to the whole loop
>
> is a multiple of 360 deg, provided the gain there is slightly more than
>
> unity.
>
> Startup can be explained by noise triggering the circuit's free
>
> response, which is an exponentially growing sine wave. Eventually,
>
> nonlinearities will set in, distorting the waveform and changing the
>
> oscillation frequency.
>
> As the SAW device is high-Q, with a steep phase slope around the
>
> intended frequency, this usually turns out to be a pretty good and
>
> simple oscillator. Just watch out for not having excessive gain.
>
>
>
> Pere

Interesting. Relating to the SAW design itself, this brings up the issue of
high frequency 'sound' waves in various media. At sound speeds of 3x10exp3 M/s,
a gigahertz f wave will have a wavelength of 3x10exp3/10exp9 = 3 microns.
in quartz.

```
```haiticare2011@gmail.com wrote:
> On Tuesday, May 13, 2014 9:30:08 PM UTC-4, Phil Allison wrote:
>> "skeptical engineer"
>>
>>
>>
>>> I was looking at the SAW oscillator circuits at a co. called
>>> Epcos, and they just take an op amp with the SAW filter in the feedback
>>> loop. They explain that the op amp amplifies noise, but preferentially
>>> at the filter allowed frequency. 2 components.
>>
>>
>> **  I see no "op-amps " in their VHF /UHF oscillator designs.
>>
>>
>>
>>     You need to post a link to stuff like that.
>>
>>
>>
>>
>>
>>
>>
>> ....  Phil
>
> OK - right you are.
> http://www.mouser.com/pdfdocs/_Application%20Toolkit_050518_17h.pdf
> See page 10.
>
> This is posted at Mouser, but it is an Epcos doc. The question is, would an
> op-amp (generic amplifier) work in this circuit?
>

That's not an opamp but a generic amplifier, and only meant as
illustration, as an amp "building block". Yes, a fast enough opamp can
do this but that isn't necessarily the wisest and most frugal approach.
Cheapskates use logic parts for that because you get a six-pack for a
nickel :-)

--
Regards, Joerg

http://www.analogconsultants.com/
```
```On Wednesday, May 14, 2014 10:46:54 AM UTC-4, Joerg wrote:
> haitdicare2011@gmail.com wrote:
>
> > On Tuesday, May 13, 2014 9:30:08 PM UTC-4, Phil Allison wrote:
>
> >> "skeptical engineer"
>
> >>
>
> >>
>
> >>
>
> >>> I was looking at the SAW oscillator circuits at a co. called
>
> >>> Epcos, and they just take an op amp with the SAW filter in the feedback
>
> >>> loop. They explain that the op amp amplifies noise, but preferentially
>
> >>> at the filter allowed frequency. 2 components.
>
> >>
>
> >>
>
> >> **  I see no "op-amps " in their VHF /UHF oscillator designs.
>
> >>
>
> >>
>
> >>
>
> >>     You need to post a link to stuff like that.
>
> >>
>
> >>
>
> >>
>
> >>
>
> >>
>
> >>
>
> >>
>
> >> ....  Phil
>
> >
>
> > OK - right you are.
>
> > http://www.mouser.com/pdfdocs/_Application%20Toolkit_050518_17h.pdf
>
> > See page 10.
>
> >
>
> > This is posted at Mouser, but it is an Epcos doc. The question is, would an
>
> > op-amp (generic amplifier) work in this circuit?
>
> >
>
>
>
> That's not an opamp but a generic amplifier, and only meant as
>
> illustration, as an amp "building block". Yes, a fast enough opamp can
>
> do this but that isn't necessarily the wisest and most frugal approach.
>
> Cheapskates use logic parts for that because you get a six-pack for a
>
> nickel :-)
>
>
>
> --
>
> Regards, Joerg
>
>
>
> http://www.analogconsultants.com/

OK thanks. - What logic part would produce a sine wave output?
jb
```
```On 15/05/14 04:43, haiticare2011@gmail.com wrote:
> On Wednesday, May 14, 2014 10:46:54 AM UTC-4, Joerg wrote:
>> Cheapskates use logic parts for that because you get a six-pack for a
> OK thanks. - What logic part would produce a sine wave output?

One that has no extra gain than what is required to start, so it's not
constantly slamming against the amplitude limit. One that limits its
amplitude into a resistive load. Or one fitted with an AGC circuit. It
helps if the clipping is somewhat soft. Look at the 74HCU04 inverters
for a start.

```
```On Thursday, 15 May 2014 00:37:03 UTC+10, haitic...@gmail.com  wrote:
> On Wednesday, May 14, 2014 4:04:15 AM UTC-4, o pere o wrote:=20
> > On 05/13/2014 03:14 PM, skeptical engineer wrote:=20
> >=20
> > > I was looking at the SAW oscillator circuits at a co. called Epcos, a=
nd they just take an op amp with the SAW filter in the feedback loop. They =
explain that the op amp amplifies noise, but preferentially at the filter a=
llowed frequency. 2 components.
> > >=20
> > > I love the simplicity, but does this approach have any drawbacks?
> > =20
> > That was probably a gain block fed back by the SAW device. And yes, thi=
s  is a classical way of building an oscillator: the whole thing will  osci=
llate at the frequency where the phase shift due to the whole loop   is a m=
ultiple of 360 deg, provided the gain there is slightly more than  unity.=
=20
> > =20
> > Startup can be explained by noise triggering the circuit's free  respon=
se, which is an exponentially growing sine wave. Eventually,  nonlinearitie=
s will set in, distorting the waveform and changing the  oscillation freque=
ncy.=20
> > =20
> > As the SAW device is high-Q, with a steep phase slope around the   inte=
nded frequency, this usually turns out to be a pretty good and  simple osci=
llator. Just watch out for not having excessive gain.
>=20
> Interesting. Relating to the SAW design itself, this brings up the issue =
of   high frequency 'sound' waves in various media. At sound speeds of 3x10=
exp3 M/s,  a gigahertz f wave will have a wavelength of 3x10exp3/10exp9 =3D=
3 microns.  in quartz.

http://en.wikipedia.org/wiki/Acoustic_microscopy

As usual, Wikipedia hasn't got it entirely right. There were acoustic micro=
scopes that went up into the GHz region, though thye may not have been comm=
ercially successful.

--=20
Bill Sloman, Sydney

```