VCH-1006 PHM maintenance and operation



March 10, 2024

Little is known in the West about the production, care and feeding of commercially-available passive hydrogen masers -- only what the predominant Russian manufacturers have deigned to disclose to their customers, which isn't much. When it comes to high-performance time and frequency standards, though, passive H-masers provide stability second only to their active cousins which are even harder to come by. Consequently they are all worth preserving in good operating condition whenever feasible.

This unusual eBay find is an example of the VCH-1006 maser from Vremya-CH in Nizhny Novgorod, probably an early-production model around 20-25 years old. It was still in working condition when it landed on my bench but was starting to show its age. This page will describe measurements, repair notes, and general observations on this particular unit.


Resources

VCH-1006 user guide
VCH-1006 maintenance and service manual (such as it is)

VCH-1006 sell sheet published by Oscilloquarz SA, dated 2006 and labeled OSA-VCH 1006, specifying 2E-12 @ t=1s
OSA-3700 sell sheet OEM version marketed by Oscilloquartz SA circa 2009 with 7E-13 @ t=1s advertised stability
Russian-language sheet from Vremya (2014?) with 2E-12 @ t=1s specification matching the OSA-VCH 1006 brochure above
Most-recent specifications downloaded from vremya-ch.com (March 2024, 7E-13 @ t=1s)

Assorted historical references and research papers

.CSV data logger Runs under Windows, includes C source code. (Requires DB-9 straight-through cable between PC and maser)
.CSV data plotter Python source based on pandas and matplotlib



Interior components (top)




Interior photo (underside)



Visible above are the hydride canister, two gas lines connecting the canister to the pressure sensor and physics package, a PSU module, and some empty space reserved for two 12V SLA backup batteries (FG20271).



Oven controller PCB





Hydride canister and pressure sensor


Could use some help identifying the gas fitting used on the pressure sensor. Anyone recognize it?



Notes on internal controls and components

Two interesting features visible on the oven-control PCB are a toggle switch adjacent to the Cyrillic characters Б and М and a trimpot near markings that translate as "Hydride."

Hydride temperature adjustment trimmer

The unit's hydrogen pressure was reported at the minimum 1.5-atmosphere level when the unit arrived. Adjusting the 'Hydride' trimmer to increase the temperature of the lanthanum nickel hydride canister brought the pressure up to the 2- to 2.5-atmosphere range. This may have improved the second-harmonic level a bit, but parameters were changing so frequently at the time due to various factors, including recovery from long-term storage and the dissociator issue described below, that it was hard to be sure. (There is also a significant change in operating parameters that occurs when the maser's top cover is removed, further complicating the tweaking process. This effect is accompanied by small frequency excursions.)

Subsequent temperature increases didn't appear to raise the H2 pressure much, suggesting that the hydrogen will soon need topping up. (Another possibility: the pressure sensor is suffering from embrittlement or other age-related effects that degrade its accuracy.) It's possible that the intended setting for the 'Hydride' trimpot is at a point that maintains the H2 pressure level near the 1.5-atmosphere minimum. There appears to be little or no upside to running the maser at higher H2 pressure, assuming the purifier loop acts to maintain the same low-pressure setpoint regardless. Lowering the ~2 atm pressure back to 1.6 atm had only small-scale effects on most parameters (.CSV file here.)

Toggle switch on oven control PCB

The mysterious toggle switch originally arrived in the downward 'М' position (i.e., towards the front panel as seen in the photos). Flipping it to 'Б' caused a transient that affected nearly all parameters, similar to what happens when the top cover is removed but significantly more pronounced. Lock was lost for about a minute while the operating parameters returned to their previous values. The transient had to be removed from the data set for this screenshot, as it corrupted the RS-232C output from the controller.

Ultimately, the switch's only noticeable effect was that the maser's output frequency was about 1.58E-8 higher (0.158 Hz at 10 MHz) with the switch in its upward position. Despite this substantial frequency error, stability was unaffected at taus up to at least 100s. Flipping the switch back down restored the previous operating frequency with little or no apparent retrace error once the transient settled. The switch may control some sort of degaussing feature, and/or may cause the maser to lock to a spectral line at a different Zeeman sublevel, although the literature indicates that the Zeeman resonances at 1420 MHz are much more closely-spaced.

I haven't traced the schematic of the neighboring circuitry yet, but the cable going from the PCB to the physics package uses a rarely-encountered DB-26 connector. There are clearly several more wires than would be needed for the side/bottom oven heaters and thermistors alone. At minimum there is presumably a Zeeman coil connection and another coil that provides the quantization-axis field, and it seems likely that the switch has something to do with one of these.


Dissociator (HFO) repair

Some intermittent locking behavior was traced to the same issue that Tom Van Baak encountered in his CH1-75 active maser some time ago. The fix was also the same -- replace the homebrew 3-pF capacitor in the Clapp oscillator (aka series-tuned Colpitts) with another one.

I upgraded the ~0.1" semirigid coax to UT-141 to raise the breakdown voltage a bit:


There's a quick sketch of the schematic here but it may not be especially accurate. The current sensor wiring in particular doesn't make much sense, given that the divider used for sensing the voltage drop across the 0.3 ohm resistor should be on the downstream side of it.

The repaired dissociator yields a beautiful view of the entire Balmer series through 100-micron slits:

(Not to be confused with the less Nobel-worthy Ballmer series)



Data capture

March 10, 2024 Some data traces from a .CSV file captured following the dissociator repair:

Points of interest: The .CSV file can be downloaded here (4 MB) and displayed with the Python program above.



Performance assessment

Updated March 18, 2024 Cross ADEV measurement of the maser using two HP 5065A rubidium standards as references:

It's difficult to measure the performance of a maser without using another maser as a reference (preferably two of them!) Lacking that luxury, the plot above was taken using two modified HP 5065As as reference sources for a cross-Allan deviation measurement (.TIM file). This technique can be a good way to trade time for measurement headroom, but in this case the HP 5065As are influenced by shared environmental factors. They may as well be treated as a single imperfect reference at taus beyond 1500 seconds, since the measurement is dominated by their correlated behavior beyond that point.

The result is trustworthy enough below t=1500s, though. It can be seen that the VCH-1006 is outperforming the 2E-12 @ t=1s spec, but it falls short of the more aggressive 7E-13 @ t=1s spec from some of the other sales literature. On one hand, if a way can be found to increase the 2nd harmonic level, I'd expect the short-term performance to improve... perhaps substantially. On the other hand, this VCH-1006 is an older example, with component date codes ranging from 1991 to 2002. Subsequent design improvements may have been made after the unit left the factory.



Measurement examples

March 10, 2024 Three HP 5065A rubidium standards equipped with Corby Dawson's 'Super' modification, measured simultaneously against the maser:

This is another example of a measurement that would be impossible to make with mundane reference sources. All three HP 5065As are fitted with optical bandpass filters for improved S/N performance, placing them well ahead of most commercially-available standards at tau intervals less than a few hours.

In fact, the VCH-1006 isn't good enough to assess the short-term performance of the Rb1 and Rb2 units by itself. A three-cornered hat measurement is informative: while still susceptible to environmental influence as mentioned above, it shows that Rb1 and Rb2 can approach the maser's best factory specs at taus below 1000s. Only the maser, however, can tell us how the HP 5065As are behaving beyond that.

The slight ripple near t=1s is something I've seen before in measurements made with Tom Van Baak's Kvarz CH1-76 PHM. Not clear what's going on there, or if it's the same phenomenon at all.

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