Passive f to 2f converter

[George Herold]

I got a request for a f to 2f converter for our lockin.
http://www.teachspin.com/instruments/signal_processor/
(Freq = 3 - 3k Hz. )
(Well a crappy pic, but I wanted to show that it has a filter module.
(It stinks signal wise too, the photographer didn't like the second
bnc cable from the reference output.))

Passive would be the simplest. I thought of a diode rectifier, and
when my boss mentioned that we could send it through the filter, I was
sold.

I first drew it up with coupling caps, but that wouldn't float.
So a transformer was needed.
I've got some of these,
http://www.digikey.com/product-detail/en/TY-141P/237-1118-ND/242640
Triad transformers. (in both 10k and 600 ohm flavors)
And it worked great.
I hung a 1 k ohm load on the rectifier output of the 10k transformer.
(hmm, that might be my problem.)
And at some low frequency ~30Hz, all hell broke out,
with some jaggedie's during the sine wave.
Did I mention I was driving it from an opamp?

I was wondering if I should add some source resistance?

George H.

 

[George Herold]

Transformer saturating? Try using a higher impedance transformer.

Hmm I don't really know. Here's a 'scope shot of the input to the
transformer.

http://bayimg.com/cAhhdaaeB

At a slightly higher frequency the step and the jaggedies go away.
(Jaggedies look like slew rate limitied oscillations ~2MHz.)

I put 100 ohms between the opamp and transformer and that made
everyone happy.

Of course this is much below the spec for the transformer.
(200 to 15 kHz.)

Another cute way: drive an optoisolator that has back-to-back LEDs, and use a
pullup on the phototransistor. You'll get two sinks per cycle.

Oh that's nice, then I wouldn't need the transformer.
Any hints for an opto-isolater with back to back leds? I see nothing
useful in digikeys search function.

George H.

 

[George Herold]

Hmm I don't really know.  Here's a 'scope shot of the input to the
transformer.

At a slightly higher frequency the step and the jaggedies go away.
(Jaggedies look like slew rate limitied oscillations ~2MHz.)

I put 100 ohms between the opamp and transformer and that made
everyone happy.

Of course this is much below the spec for the transformer.
(200 to 15 kHz.)

Oh that's nice, then I wouldn't need the transformer.
Any hints for an opto-isolater with back to back leds?  I see nothing
useful in digikeys search function.

---
H11AA1, AA2, AA3, AND AA4, but why bother when you can do the same
thing by feeding a single emitter device from a full-wave bridge?

.       +-----+    +-------------+
.AC>----|~   +|----|A---+        |
.       |     |    |    |       C|
.       |     |    |  [LED]-> B  |
.       |     |    |    |       E|
.AC>----|~   -|----|K---+        |
.       +-----+    +-------------+

Thanks John, If I can do it without the brdige and transformer that
will be a win.

I've got a single ended (is that the right term?) input and output.
(Both are referenced to the same ground point.) So I like the double
LED idea. I don't have that much voltage to play with (~ 6 V p-p). I
guess I'll loose about the same on the LED's or bridge.

Oh, but I'll need some voltage to drive the transistor.... but maybe I
can use that idea sometime in the future.

George H.

 

[George Herold]

.       +-----+    +-------------+
.AC>----|~   +|----|A---+        |
.       |     |    |    |       C|
.       |     |    |  [LED]-> B  |
.       |     |    |    |       E|
.AC>----|~   -|----|K---+        |
.       +-----+    +-------------+

- Show quoted text -

Thanks John,  If I can do it without the brdige and transformer that
will be a win.

I've got a single ended (is that the right term?) input and output.
(Both are referenced to the same ground point.)  So I like the double
LED idea.  I don't have that much voltage to play with (~ 6 V p-p).  I
guess I'll loose about the same on the LED's or bridge.

Oh, but I'll need some voltage to drive the transistor.... but maybe I
can use that idea sometime in the future.

George H.- Hide quoted text -

- Show quoted text -

Hey, is there an opto with back to back LEDs and just a photodiode
output?

Or maybe a dual, that I hook up back to back?
I found this on digikey... a bit spendie

http://www.digikey.com/product-detail/en/PVI5013RPBF/PVI5013RPBF-ND/1828126

But a lot less mass than a transformer!

George H.

 

[George Herold]

We use this one:

http://www1.futureelectronics.com/doc/CALIFORNIA EASTERN LAB/PS28...

OK thanks,

What's your signal waveform? What 2f waveform do you need? A true sine-to-sine
doubler is an analog squarer and a DC offset.

The input is a sine wave 3-3kHz, ~6Vp-p. The output can be almost
anything that has some 2f. The user can send the signal through a
filter (with gain = to Q) and then the signal becomes the reference
input for a switched gain lockin amp.

George H.

 

[George Herold]

28 pm, John Larkin
I got a request for a f to 2f converter for our lockin.
http://www.teachspin.com/instruments/signal_processor/
(Freq = 3 - 3k Hz. )
(Well a crappy pic, but I wanted to show that it has a filter module.
(It stinks signal wise too, the photographer didn't like the second
bnc cable from the reference output.))
Passive would be the simplest. I thought of a diode rectifier, and
when my boss mentioned that we could send it through the filter,I was
sold.
I first drew it up with coupling caps, but that wouldn't float.
So a transformer was needed.
I've got some of these,
http://www.digikey.com/product-detail/en/TY-141P/237-1118-ND/242640
Triad transformers. (in both 10k and 600 ohm flavors)
And it worked great.
I hung a 1 k ohm load on the rectifier output of the 10k transformer.
(hmm, that might be my problem.)
And at some low frequency ~30Hz, all hell broke out,
with some jaggedie's during the sine wave.
Did I mention I was driving it from an opamp?
I was wondering if I should add some source resistance?
George H.
Transformer saturating? Try using a higher impedance transformer.
Hmm I don't really know. Here's a 'scope shot of the input to the
transformer.
http://bayimg.com/cAhhdaaeB
At a slightly higher frequency the step and the jaggedies go away.
(Jaggedies look like slew rate limitied oscillations ~2MHz.)
I put 100 ohms between the opamp and transformer and that made
everyone happy.
Of course this is much below the spec for the transformer.
(200 to 15 kHz.)
Another cute way: drive an optoisolator that has back-to-back LEDs, and use a
pullup on the phototransistor. You'll get two sinks per cycle.
Oh that's nice, then I wouldn't need the transformer.
Any hints for an opto-isolater with back to back leds? I see nothing
useful in digikeys search function.
---
H11AA1, AA2, AA3, AND AA4, but why bother when you can do the same
thing by feeding a single emitter device from a full-wave bridge?
. +-----+ +-------------+
.AC>----|~ +|----|A---+ |
. | | | | C|
. | | | [LED]-> B |
. | | | | E|
.AC>----|~ -|----|K---+ |
. +-----+ +-------------+
--
JF- Hide quoted text -
- Show quoted text -
Thanks John, If I can do it without the brdige and transformer that
will be a win.
I've got a single ended (is that the right term?) input and output.
(Both are referenced to the same ground point.) So I like the double
LED idea. I don't have that much voltage to play with (~ 6 V p-p). I
guess I'll loose about the same on the LED's or bridge.
Oh, but I'll need some voltage to drive the transistor.... but maybe I
can use that idea sometime in the future.
George H.- Hide quoted text -
- Show quoted text -

Hey, is there an opto with back to back LEDs and just a photodiode
output?

Or maybe a dual, that I hook up back to back?
I found this on digikey... a bit spendie

But a lot less mass than a transformer!

George H.

---
Here's the data sheet:

http://www.irf.com/product-info/datasheets/data/pvi5013r.pdf

Take a look at the response time specs...

Ughh, that is slow. 1 uA into ~200pF, and with Vf= 8V, that's more
than 1nC so sure more than 1ms response time.... but maybe I can
reduce the load resistance (and forward voltage) and make it a bit
snappier?

OTOH, take a look at this:

http://media.digikey.com/pdf/Data Sheets/Fairchild PDFs/H11F1,2,3...

Wire the LEDs in parallel opposition and the FETs in parallel and
Bob's yer uncle!

Very interesting. I like the circuit apps! But I'll need power to
turn the FET on. (not impossible, but not ideal either.)

Thanks
George H.

 

[Bob Masta]

I got a request for a f to 2f converter for our lockin.
http://www.teachspin.com/instruments/signal_processor/
(Freq = 3 - 3k Hz. )

Hi, George. I don't have an answer to your question, but
I'm just curious if lock-in designs have finally evolved
since the 80's/90's to allow the reference oscillator to
drive the multiplier (or whatever) directly. Back then,
every model I could track down had the reference oscillator
as pretty much a tacked-on free-standing circuit; the
multiplier was *only* driven by the PLL. So if you wanted
to apply a frequency step, you had to apply a step to the
VCO, which was plumbed to the PLL, and wait for the PLL to
settle.

But the PLL is really only needed for cases where you *must*
use an external reference... the classic case being a "fan
blade" optical chopper, where the frequency was determined
by the motor speed.

I imagine *somebody* still uses this arrangement, but in my
case (hearing research) and plenty of others, what people
really needed was simply a reference oscillator to drive
their experiment *and* the lock-in multiplier to analyze the
signal coming back from the experiment. No need for a PLL,
since the reference oscillator can be a quadrature type
right from the get-go (digital, these days), and everything
is always "locked"... no waiting for the PLL.

The solution for me was to go with synchronous waveform
averaging, which has other benefits as well (like preserving
a complex neural response waveform). But lock-ins would
still have an edge in absolute S/N for those who deal with
single response frequencies. So, can you now skip the PLL if
you want to?

Best regards,


Bob Masta

DAQARTA v7.10
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Sound Level Meter
Frequency Counter, Pitch Track, Pitch-to-MIDI
FREE Signal Generator, DaqMusic generator
Science with your sound card!

 

[George Herold]

Hi, George.  I don't have an answer to your question, but
I'm just curious if lock-in designs have finally evolved
since the 80's/90's to allow the reference oscillator to
drive the multiplier (or whatever) directly.  Back then,
every model I could track down had the reference oscillator
as pretty much a tacked-on free-standing circuit; the
multiplier was *only* driven by the PLL.  So if you wanted
to apply a frequency step, you had to apply a step to the
VCO, which was plumbed to the PLL, and wait for the PLL to
settle.

But the PLL is really only needed for cases where you *must*
use an external reference... the classic case being a "fan
blade" optical chopper, where the frequency was determined
by the motor speed.

I imagine *somebody* still uses this arrangement, but in my
case (hearing research) and plenty of others, what people
really needed was simply a reference oscillator to drive
their experiment *and* the lock-in multiplier to analyze the
signal coming back from the experiment.  No need for a PLL,
since the reference oscillator can be a quadrature type
right from the get-go (digital, these days), and everything
is always "locked"... no waiting for the PLL.

The solution for me was to go with synchronous waveform
averaging, which has other benefits as well (like preserving
a complex neural response waveform).  But lock-ins would
still have an edge in absolute S/N for those who deal with
single response frequencies. So, can you now skip the PLL if
you want to?

Best regards,

Bob Masta

              DAQARTA  v7.10
   Data AcQuisition And Real-Time Analysis
             www.daqarta.com
Scope, Spectrum, Spectrogram, Sound Level Meter
 Frequency Counter, Pitch Track, Pitch-to-MIDI
   FREE Signal Generator, DaqMusic generator
          Science with your sound card!

Hi Bob, I have very little idea of what 'modern' lockin design looks
like.
(I'd be tempted to guess that lots is now done in software.)

The lockin we sell is a throw-back. It's basically for students to
follow the signal chain through the insturment. It's has no PLL
inside it. (Gasp)

There are two classes of lockin's that I know of. One uses the
multiplier function to mix the signal with the reference. And the
other uses a switched gain stage. The lockin we sell uses the
switched gain stage topology. It's built around the AD630 chip. So
sure it's possible to do a lockin w/o a PLL... the switched gain stage
approach just needs a comparator to sense the zero crossing. With a
quadrature oscillator (and two swithced gain stages) you wouldn't even
need a phase shifter. Signal changes could be as fast as the final
low pass filter.
(The down side of the switched gain is that it also sees signals (or
noise) at odd harmonics of the reference frequency.)

My favorite lockin from grad school was the 124A from EG&G. I was
talking with the design engineer from SRS at a trade show a year or
two ago and he has redone the 124A for SRS. I think it's an 'all-
analog' signal chain. I hear that the low temperature guys like that,
because the digital based lockin's would send 'crud' back down into
their cryostat's and screw things up.

George H.

 

[whit3rd]

The use of an analog multiplier (the six-transistor kind, like in MC1496) will get
it done without the (circa 4mA) current requirement of an LED solution. It's
easy to drive a center-tapped transformer with the output.
Of course, this is NOT a passive solution, it takes a power supply; the upside, is
that you can bias it for 2 uA current draw and toss two in more AA cells each year.
It's sensitive to level, you'd want an input attenuator, too.