[time-nuts] HP5065A C-field mods and optical unit mods

Bob kb8tq kb8tq at n1k.org
Wed Nov 22 12:31:31 EST 2017


One interesting claim made “back in the good old days” was that certain 
pots had near zero TC when used as radiometric devices. Essentially the
metal on one side “always” had the same characteristics as the metal on 
the other side of the wiper. People would go to great extremes to exploit
this “well known fact”. Oddly enough I never really saw much proof that 
this was true ….You still see people doing things this way though. It was
a major “design item” with a certain company that made both potentiometers 
and OCXO’s ….


At some point, the magnetic field of the car in the garage gets into
the act on your Rb standard. There *is* a point where all sorts of 
things start to be questioned. Shielding is only just so good. That’s
true if it’s heat shielding, magnetic shielding or whatever ….

Once you get into the “part in 10^-15” range on an Rb, you are well past
the “show me” level of effects. You can (correctly) calculate that this or 
that is the impact. Demonstrating that it works that way at that level
… not so much. 

Why worry about demonstrating this or that? Often second or third order
effects pop up as you “null out” the first order stuff. Hysteresis in the TC
of a “compensated” standard is one very common example. At this point 
it’s not a big surprise when it shows up. That was not always the case ….

None of this is easy, there always is a lot more work involved that one might


> On Nov 22, 2017, at 12:04 PM, cdelect at juno.com wrote:
> HP5065A C-field mods and optical unit mods
> The contribution of the temperature coefficients of the fixed resistors
> and the pot (which is a 21 turn bulk metal foil 10PPM/Deg C pot) show up 
> as frequency errors as shown for several combinations.
> Note that the pot does not cover the entire 2X10-9 frequency range.
> There is a high, mid, and low switch setting that shorts both 1k,
> one 1k, or neither 1k resistor that in conjunction with the pot allows
> the entire range to be covered. This allows the pot to have a finer
> frequency resolution
> A- With only the 2.4K in circuit with the pot at zero Ohms a 1 Deg C
> change
> will cause a 2.8X10-15 frequency change.
> B- With the 2.4K in combination with the pot at 1K a 1 Deg C change will
> cause a 8.55X10-15 frequency change.
> C- With the 2.4K in combination with the pot at 1K and both 1K resistors
> in 
> circuit a 1 Deg C change will cause a 3.99X10-15 frequency change.
> If in example C you replace the pot with a 100PPM pot a 1 Deg C change
> will equate to a 2.45X10-14 frequency change.
> As you can see this circuit will work quite well in eliminating any power
> supply related C-field changes.
> Now the C-field coil is only partially temperature regulated by the 
> ovens in the optical unit as they  regulate the lamp and cell
> temperatures
> and the C-field is not co-located. 
> That is why I'm going to use the PC liquid cooler to actively regulate
> the
> temperature of the sealed enclosure I have placed the optical unit into.
> Cheers,
> Corby
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