Frequency Detection Suggestions

You could divide the frequency down to something that an HC7046 PLL chip can handle, a few MHz maybe. Apply a clock as a reference. It might not actually phase lock, but the frequency detector part will work.

John,

Thanks for your input.

I however gave you the wrong units. The frequency range is 400 to 650 kHz, not MHz.

Regards,

Brad Kimbrough

Then you needn't divide.

A v/f converter would work too, and an analog control loop.

If you have a clean signal, a one-shot does v/f cheap.

Hz, not MHz.

Unfortunately the beat signal is a frequency, and needs to be turned into a voltage to drive the heater that controls the length of the tube.

The problem is that slope of the voltage to frequency calibration line depe nds on at least one resistor and at least one capacitor, both of which have tolerances.

If you know what beat frequency you want, and can generate it as accurately as you need (which shouldn't be difficult) you can use 4046-style phase an d frequency detection hardware to compare the beat frequency with the desir ed reference frequency, and use the output to control the heater.

doesn't have the dead spot at small phase offsets which can complicate the use of regular 4046-stye phase and frequency comparison outputs.

Phase-locked loops can work fine with pure "product" - phase-only - detecto rs, but they tend to start up rather slowly.

Floyd M. Gardner's "Phaselock Techniques" ISBN 0-471-04294-3 is old - 1979

- but does cover the basics.

We seem to have two threads going on this topic. It would be worth picking just one.

From my followup to the other one:

I'd amplify this by saying that a barefoot HeNe is a much better frequency standard than the beat frequency. For one thing, the laser line is

c/632.8 nm = 474 THz

so you're effectively trying to frequency-multiply by a factor of a million or so, which will multiply the phase noise by 120 dB.

For another, adjacent modes in a HeNe get pulled around by hundreds of kilohertz just by temperature gradients and stuff like that. You can see this by shining a normal 5 mW cylindrical HeNe into a photodiode and looking at the output on a spectrum analyzer. If you put your hand on one end of the case, you'll break the locking between adjacent modes, and you'll get spurs sweeping back and forth between ~200 kHz and 1 MHz. These are fourth-order mixing products, specifically the beat between one mode and a third-order IM product of two others. This shows that the mode spacing is uncertain at the level of hundreds of ppm.

Multiplying that up by a million times would get you a frequency uncertainty in the hundreds of gigahertz, which is far, far worse than the stability of a free-running HeNe, even if it could oscillate over that wide a range, which it can't.

What are you trying to do that needs better stability than a free-running HeNe?

Cheers

Phil Hobbs

Use the phase / frequency digital detector circuit in any modern PLL chip.

Supply it your variable frequency and a suitable reference frequency and a small analog filter with op amp and it will give you a control voltage with any range you choose.

Go over to Sam's laser FAQ, find the commercial stabilized laser section. T hen take a look at the details for the "Laboratory for Science" Model 220. If you look at the patents and description you should find everything you need except for the loop filter constants.

Steve

Sam's Laser FAQ references US patent 4,468,773 for the main control loop, which gives you a pretty good block diagram and waveforms for the controller..

J.L. Hall of the University of Colorado also did some work with HENEs and F->V converters.

Steve

Phil,

I appreciate your feedback. Here is a link to a good reference paper on the topic of three mode HeNe stabilization.

The goal is increased power from a stabilized HeNe laser. For me, small, slow drift is fine, I'm not as interested in the absolute frequency

Thank you,

Brad

As I'm working on a few stabilized lasers right now, I did a bit more digging.

See:

Frequency stabilization of a multimode high-power He-Ne laser Shuko YokoyamaTsutomu ArakiTakanori OshioNorihito Suzuki

Review of Scientific Instruments 64, 2796 (1993); doi: 10.1063/1.1144365

It has a nice Op-amp based IF processing circuit, although for ultimate stability Hall's F->V scheme would be better..

Good Luck!

Steve

Steve,

Thanks for the reference. I'll take a look.

Brad

The catch is that the phase/frequency detector in most 4046 style PLL chips has a dead spot around zero phase difference. The Nexperia 74HCT9046 doesn't and the data sheet talks about the problem (see Fig. 11) such as it is.