[time-nuts] Reference input on HP counters
kb8tq at n1k.org
Sun Jan 7 13:03:41 EST 2018
> On Jan 7, 2018, at 12:55 PM, Jerry Hancock <jerry at hanler.com> wrote:
> As far as lazy, I will check my equipment.
> When the others that replied using the term “lock” or “locked”, what do you mean by that? That for instance, the external reference is multiplied up directly to the internal frequency (IF I suppose) or is it phase locked, for instance? I’m sure there are other cases.
> I’m trying to differentiate between two cases:
> 1) I apply an external reference at 10Mhz (assumed all the devices require 10Mhz at a certain level) and using a switch of some type (electronic or manual) the reference is now multiplied and manipulated directly to feed stages of the device. In this way, the phase noise of the device is very much dependent on the external reference. So if a noisy external is used it impacts the device across all time values.
> 2) I apply an external reference and another oscillator, maybe the default or supplied internal 10Mhz (like a 10811, etc) is now EFC adjusted to phase lock to the external reference. In this way, I would think the phase noise of the device would have been impacted less by the external reference (under the time constant of the PLL or EFC loop.
> Either way, an oscillator with phase issues used as the external reference is going to have some impact on the device, correct? I’m thinking of in the case where you use a GPSDO that doesn’t have a very good master oscillator.
In the classic HP approach, you have a VCXO at (say) 50 MHz. It is fed by a PLL. The reference to that PLL is either the OCXO in the
counter or the external reference input to the device. The loop bandwidth of the PLL is (hopefully) narrow enough that the bulk of the
far removed noise is rejected. You always will see the close in ( 1Hz) stuff multiplied up. Since the input to the PLL can be any of a
number of frequencies, the gain of the loop (in radians) varies. That impacts the loop bandwidth. Thus saying anything precise about
what the cutoff is can be a bit tough …..
> Jerry Hancock
> jerry at hanler.com
> (415) 215-3779
>> On Jan 7, 2018, at 8:05 AM, jimlux <jimlux at earthlink.net> wrote:
>> On 1/7/18 7:21 AM, Jerry Hancock wrote:
>>> I’m being a little lazy as I can check the schematics, but in the general case, do HP counters, frequency generators, etc, switch to the external reference when one is available or discipline the internal oscillator? Are there brands of test equipment that generally discipline vs switch?
>> By the each.
>> Particularly for newer equipment it can be somewhat of a hybrid - the Keysight 33622, for instance, seems to have some sort of frequency locked loop (according to the manual and emails from Keysight). It stays on the internal reference, but the frequency is adjusted to match the external reference (whether they adjust the oscillator, or reprogram a DDS, I'm not sure).
>> I've seen test data, though, that seems to imply there's something else going on - the phase noise is reduced when fed from a quiet external source, and if it was a simple frequency locked loop, I don't think that would happen.
>>> Third question, again generally, when you discipline an oscillator, what impact does this have on the oscillator's phase noise? I assume this third question depends on the discipline loop (if I’m using the correct term).
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