Can I "reset" an AD9901?

It's a method of locking a laser to a moderate-finesse etalon (F = 30 or thereabouts), with accuracy and stability potentially limited only by shot noise.

The trick is superficially similar to slope detection of FM using the skirts of an AM-only receiver. You make a very stable Fabry-Perot etalon, put the beam of a single-frequency diode laser through it, and detect the transmitted (T) and reflected (R) beams separately.

Once you have those, you subtract them to form the tuning signal R-T. Ideally you do it by simply wiring them anode-to-cathode, so that the photocurrents subtract directly, probably with a bit of bootstrap magic to reduce the effects of their capacitance.

The locking loop servos around the point R -T = 0, which is notionally halfway down one side of the transmission peak. Servoing around zero prevents the laser's residual intensity noise (RIN) from coupling into the tuning signal and degrading the FM moise.

(See

With another turn of the crank, you can use this idea to make ultrastable intracavity measurements. If the etalon were lossless, the R and T beams would sum to a constant optical power, so that the sum of the two photocurrents R + T would be constant as well. In reality this isn't so. That means that although the laser's AM noise doesn't get turned into FM sidebands, FM laser noise does become AM noise on the R + T signal.

The cute part is that by attenuating the (stronger) R beam so that

d R / d nu + d T / d nu = 0,

you make the R + T signal decouple from the tuning, so that you can in principle do intracavity amplitude measurements down at the shot noise as well. (You need to use laser noise cancellation to get rid of the RIN on the R+T signal.)

The field strength inside the etalon cavity is about F times that of the incident beam, which greatly enhances the sensitivity of measurements peformed inside the cavity. (Doing measurements inside a passive cavity is pretty straightforward, unlike laser intracavity measurements, which are squirrelly as hell.)

Cheers

Phil Hobbs

Like I said, I've been meaning to try that out. Too cool to ignore.

Cheers

Phil Hobbs

Yeah, but for 4046 compatibility, those resistors are the program current path, and with zero current, the timing capacitor won't charge. If they also bias part of the phase detector, though... the other approach, grounding the timing capacitor, will also stop the oscillation, by wasting the applied current.

The old RCA CD4046 data sheet was much more enlightening than the Nexperia data sheet.

The best time-sweep test generator could be a crazy fast edge and a trombone-type micrometer driven mechanical delay line.

Or maybe stretching or heating a chunk of coax or pcb to make picosecond delay sweeps. Any other ideas? Varicap delay line?

I wonder if the DC bias on a pcb trace affects prop delay. Worth trying.

We used comparators, which probably had more jitter than the flop under test.

One approach would be to mechanically stretch a piece of RG-402 or something like that. For temporary impedance matching jobs, I've been known to make shunt stubs by sticking thumbtacks into RG-58 patch cords. They're surprisingly stable, and the coax survives fine--pull out the tack and it's good as new.

A picosecond is only about 8 mils of coax, so one ought to be able to get a reasonable elastic range by stretching a foot or so of hardline.

Probably a little. Temperature certainly does, or one could maybe use a high enough rep rate that the clock and data are exactly half a cycle out of phase--then the delay can be changed just by changing the rep rate a little.

At 300 MHz, that would be several inches of path difference, not too bad to fit on a board.

Cheers

Phil Hobbs

It was a better part too--you could get a good 100:1 range out of the VCO, and it was nice and linear, if slow.

The oscillators of the HC4046 and its fancier brethren are sufficiently nonlinear (3:1 slope variations, some as bad as 5:1) as to badly degrade loop performance--it'll be way overdamped in part of the range and ring like an SOB in another part if you aren't careful. The oscillators also just quit on you if the control voltage goes below a volt or so. A straight loop will cope with that, but fancier things such as offset loops may not.

Cheers

Phil Hobbs

My post is about the MC4044 and deadband. The AD9901 is an XOR phase detector with horrible ripple and drift. It is also very slow. The MC9046 has the same deadband problem as the MC4044.

Jim Thompson's TTL MC4024 and MC4044 were put together in the CMOS 4046.

The NXP 74HC9046 does solve a specific deadband problem of the 4046. I don't think that Motorola ever produced a version of that circuit , and it's successor - ON Semiconductor - reacts to "MC9046" with a link to the MC74HC4046B.

I'd expect that most materials have a dielectric constant that varies with DC bias. Lithium niobate refractive index varies with bias, which is how Mach-Zender things work.

It wouldn't surprise me if teflon Er changes with bias. That would be easy to check.

Quartz maybe.

Am 10.03.22 um 04:01 schrieb snipped-for-privacy@highlandsniptechnology.com:

It changes big time with temperature, just above room temp. I remember a swearing colleage with a clima chamber and reels of teflon coax. One of the sorry moments when you think you remember the numbers instead of writing them down.

The glass fibers in a board reduce the effect, maybe?

Gerhard

Am 09.03.22 um 22:51 schrieb Mike Monett:

No. You claim false properties of the AD9901 based on a chip that is not even remotely similar. 200 MHz is not slow against anything that has *4046* or *9046* in the name.

There is no such chip as a MC9046.

EOD now for me.

@ Jean-Pierre:

You can take that as a starter for your own. It is not really tested, just typed directly from the ds. I got away with a 2FF comparator in the Coolrunner.

----------------------------------------------------------------------------------

-- Company: Hoffmann RF & DSP

-- This is much like an AD9901 phase / frequency discriminator

-- Revision 0.01 - File Created

-- Additional Comments: Free firmware under BSD license

----------------------------------------------------------------------------------

library IEEE; use IEEE.STD_LOGIC_1164.ALL; use IEEE.numeric_std.ALL;

entity phase_frequency_discriminator is Port ( ref: in STD_LOGIC; osc: in STD_LOGIC; rst: in std_logic; -- this is needed for simulation only, may be false in real use. pfd_out: out STD_LOGIC ); end phase_frequency_discriminator;

architecture beehivioral of phase_frequency_discriminator is

signal q_ul, q_ur, q_ll, q_lr: std_logic; -- flipflop outputs from upper left to lower right signal xnor_out: std_logic; -- see ad9901 data sheet signal nand_out: std_logic;

begin

u_ref_left: process(ref) is begin if rising_edge(ref) then q_ul <= (not rst) and (not q_ul); end if; -- rising_edge() end process u_ref;

u_osc_left: process(osc) is begin if rising_edge(osc) then q_ll <= (not rst) and (not q_ll); end if; -- rising_edge() end process u_ref;

xor_out <= q_ul xor q_ll;

u_ref_right: process (ref, q_lr) is begin if q_lr = '0' then q_ur <= '0' else if rising_edge(ref) then q_ur <= (not rst) and xnor_out; end; -- rising_edge() end if; end process u_ref_right;

u_osc_right: process (osc, q_tr) is begin if q_tr = '1' then q_ur <= '1' else if rising_edge(osc) then q_ur <= (not rst) and xnor_out; end; -- rising_edge() end if; end process u_ref_right;

nand_out <= not (q_lr and xnor_out); pfd_out <= not (nand_out and not q_ur);

end beehivioral;

The glass transition in Teflon mainly shows up in the mechanical properties--the CTE goes up to over 2000 ppm/K. The effect on transmission lines is much less, but still important.

If it were my measurement, I'd certainly use the unbalanced path length trick--that puts everything into time and frequency, provided that the measurement is sufficiently fast compared with the thermal timescales.

Cheers

Phil Hobbs

Here's our flipflop tester:

In phased-array radar applications, the "Teflon knee" can be very important, to the point that ordinary Teflon cables cannot be used. The phase change per degree Kelvin (=centigrade) becomes very large around 20 C, where there is a phase change between crystal types. I thing that glass transition is a different beast.

Joe Gwinn

The AD9901 is an XOR phase detector. It has horrible ripple and drift. See Fig 10. AD9901 Output Waveform. Page 7,

"And even there, the 9046 has corrected that for good."

"I would really like the 9046 if I could switch off its VCO. I do not want an unneeded frequency on my board."

The MC4046 has the same deadband problem as the MC4044. The pullup and pulldown prop delays are not the same.

The standard Dual-D phase/frequency detector has no output ripple when the phase error is zero. This minimizes sideband spurs in synthesizers. I used the MC4044 as an example of how it can get deadband. This is corrected in modern chips.

Your comment that the dual-D phase/frequency detector has no gain at zero phase error shows a stunning lack of comprehension of how phase detectors work. Quote:

"I have the impression that you mix something with the CD4046 and its ilk. That has the problem that the charge pumps deliver no gain Kp when there is no phase error. That can be mostly healed with a 1 Meg bleed resistor."

The phase detector gain (Kp) is measured in Volts per Radian, not at zero phase error.

See Eq(36), Page 23, Texas Instruments, Theory of an Analog Phase-Locked Loop (PLL),

Kp = (VOH - VOL) / 4pi [V/Rad]

Where: VOH = maximum output voltage VOL = minimum output voltage

For other types of phase detectors, the phase detector gain can be determined in the same fashion.

Also see Eqn. 48 Page 13, Phase-Locked Loop Design Fundamentals by Garth Nash:

Nowhere near physicsy enough. ;)

Cheers

Phil Hobbs

I'm pretty sure the 20 C thing is the glass transition. Interestingly, teflon capacitors show virtually no effect whatsoever--the change in epsilon cancels out the dimensional change. Propagation delay in cables has a different functional dependence, so it doesn't cancel completely.

Cheers

Phil Hobbs

It's the transition between two crystalline phases, versus crystalline to amorphous.

I gather that the glass transition temperature of Teflon (PTFE) is

There is an Amorphous Teflon that has no Teflon knee:

.

This has a minimum glass transition temp of 160 C.

Joe Gwinn

True. A real physicsy tunable delay line would eliminate the jitter of the two comparators, and really resolve the jitter of the flop.

I used to be impressed by nanoseconds. Then picoseconds. We're working in femtoseconds now.