Oscillator ALC Rectifier

[Roger Lascelles]

The HP 8654 RF generator has an ALC detector which passes current through a
pair of diodes even when no signal is present. This seems to make the
detector follow the waveform envelope without holding the peaks too long.

The Linear Technology App Note : AN43-32 "Bridge Circuits" shows Wien Bridge
oscillators with two diodes in the ALC.

ALC rectifier attack time and decay time combine with loop filter response
to make a circuit which I find hard to tweak experimentally.


Please share your insights, experience, references.

thanks,
Roger

 

[Joerg]

Hello Jim, Hello Roger,

The two-diode arrangement is to eliminate the TC and forward drop of
the diode from the amplitude set-point.

The fwd drop issue could be tackled nicely with the inverted transistor,
as suggested in EDN Magazine Nov 9, 2000, page 172ff:

http://www.edn.com/contents/images/110900di.pdf

Regards, Joerg

http://www.analogconsultants.com

 

[Ken Smith]

In article <4qlnu0tig8ook4vsfs54d5u11g5f1j4vcb@4ax.com>,
Jim Thompson <thegreatone@example.com> wrote:

On Tue, 18 Jan 2005 01:49:43 +1100, "Roger Lascelles"
invalidl@invalid.invalid> wrote:
[...]
ALC rectifier attack time and decay time combine with loop filter response
to make a circuit which I find hard to tweak experimentally.
[...]
The two-diode arrangement is to eliminate the TC and forward drop of
the diode from the amplitude set-point.

When I do this sort of thing I usually use a comparator to find the
peaks.

Peak detection is generally noiser than 1/2 wave averaging. The noise
will increases the near carrier noise in the oscillator output. If you
are going with an ALC oscillator to minimize the near carrier noise, it is
best not to peak detect in the rectifier.

--
--
kensmith@rahul.net forging knowledge

 

[Roger Lascelles]

"Ken Smith" <kensmith@green.rahul.net> wrote in message
news:cshauo$d1t$1@blue.rahul.net...

Peak detection is generally noiser than 1/2 wave averaging. The noise
will increases the near carrier noise in the oscillator output. If you
are going with an ALC oscillator to minimize the near carrier noise, it is
best not to peak detect in the rectifier.

It is so easy to get a sawtooth amplitude jitter when using peak detection.
The peak detector grabs even short noise pulses and then the amplitude ramps
back down again. Winding up the CRO gain and looking at the envelope can
show this. This level of jitter is obscene for low noise RF oscillators,
but bench lash ups often show this.

- Jim tells us that the second diode is for temp compensation.

- Ken tells us half wave rectification is less noisy.

Looking at equipment manuals, I see both of these features. A couple of
basic principles for me.

thanks
Roger

 

[Ben Bradley]

On Tue, 18 Jan 2005 01:49:43 +1100, "Roger Lascelles"
<invalidl@invalid.invalid> wrote:

The HP 8654 RF generator has an ALC detector which passes current through a
pair of diodes even when no signal is present. This seems to make the
detector follow the waveform envelope without holding the peaks too long.

The Linear Technology App Note : AN43-32 "Bridge Circuits" shows Wien Bridge
oscillators with two diodes in the ALC.

ALC rectifier attack time and decay time combine with loop filter response
to make a circuit which I find hard to tweak experimentally.


Please share your insights, experience, references.

[you can ignore me, call me a troll, whatever, but...]

I was looking through info on Wein bridge oscillators, including
AN43, a year ago when I wanted a stable amplitude, fixed-frequency
oscillator for the exitation voltage for an LVDT for position
measurement, as I've seen LVDT schematics that used such an
oscillator.
I didn't like all the complications and tweakings of those types of
oscillators, so from parts I had laying around, I got a 1.6MHz crystal
ocillator driving a CMOS 4024 7-stage ripple counter for frequency
divide by 128 (resulting in 12.5kHz - I considered 1MHz / 64, but
15.625kHz rings a bell, it's the US TV horizontal sweep rate, and if
there's a TV on in the house it's sure to cause interference), then
put that square wave output through three or four poles of low-pass
filtering [thanks once again to Don's "Active Filter Cookbook") to get
a reasonably good sine wave. With the whole thing powered by voltage
regulators, it's as steady (both amplitude and frequency) as I need it
to be.

While needing the complication of a microcontroller to control it
or otherwise set up the registers, a DDS chip would be a VERY stable
alternative that needs no tweaking.

thanks,
Roger

-----
http://mindspring.com/~benbradley

 

[Tim Shoppa]

Roger Lascelles wrote:

This seems to make the
detector follow the waveform envelope without holding the peaks too
long.

There's a name for diode-detector-following distortion, but I cannot
recall it. But I agree with Jim, the circuit you see is probably to
remove tempcos in diode forward voltages.

ALC rectifier attack time and decay time combine with loop filter
response
to make a circuit which I find hard to tweak experimentally.

Poor choice of time constants results in a phenomonon known as
"squegging". The control voltage going through undesired and sometimes
chaotic oscillations does this. I know at least one oldish (60's?
70's?) RF textbook descusses this in terms of poles and zeroes. Is
this the problem you're having?

Tim.

 

[Ken Smith]

In article <IQ73tCBF3g9BFw3j@ddwyer.demon.co.uk>,
dd <dd@ddwyer.demon.co.uk> wrote:

I succeeded in modelling a level loop on (LT) spice.
The build up of oscillation takes many cycles so simulating can take
minutes

Mere minutes! My DC-DC converter circuit took all weekend to simulate up
to steady state. Lucky for me, the sim matched what I expected (well
close enough) so I only had to run it twice as the full up test.

I was brave enough to test it at 2 voltages and assume it didn't do
anything surprising between them.

--
--
kensmith@rahul.net forging knowledge

 

[Tim Shoppa]

I wrote:

... name for diode-detector-following distortion ...

and

I know at least one oldish RF textbook discusses squegging

Almost certainly I'm thinking of Clarke and Hess. (Not all that old,
either...) They discuss AM detectors other than diode detectors IIRC.
Tim.

 

[John Woodgate]

I read in sci.electronics.design that Tim Shoppa <shoppa@trailing-
edge.com> wrote (in <1106662154.385784.307310@c13g2000cwb.googlegroups.c
om&gt about 'Oscillator ALC Rectifier', on Tue, 25 Jan 2005:

There's a name for diode-detector-following distortion, but I cannot
recall it.

Diagonal clipping.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk