[time-nuts] Strange oscillations

Robert Aurand bob at aurand.com
Fri Sep 3 08:07:34 EDT 2004

This could very well be due to a small oscillation in the proportional heater contol circuit, as you say.   I've seen it many times before in OCXO's.   One technique we used to use in production to confirm this was to monitor the oven current simultaneously with the frequency.   Then overlay graphs of the two quantities using the same time scale.   Excel works great for this.   You may need to increase/decrease the scale of one quantity as needed.

Where did you get this OCXO?   A friend recently bought some OCXO's  of different manufacture from EBAY.   At first they seemed OK but every one eventually had jumps.   Most likely they were pulled from equipment for this reason.

----- Original Message -----
From: Alberto di Bene <dibene at usa.net>
Date: Thu, 02 Sep 2004 14:54:02 +0200
To: time-nuts at febo.com, tacgps at lists.tapr.org
Subject: [time-nuts] Strange oscillations

Re: I would like to ask your opinion about some strange oscillations.
Re: I built a GPSDO, not a replica of the Shera's project, a new one,
Re: and monitoring the EFC to check its behavior, I noted that, after
Re: it stabilizes, there are some pseudo periodic oscillations, which
Re: I don't think are caused by an instability of the PI algorithm used
Re: for controlling the EFC.
Re: I have graphed them below, capturing via a serial port the EFC values,
Re: and converting them to frequency deltas, knowing the sensitivity of
Re: the EFC input of the OCXO used, an Isotemp OCXO-134, using Matlab
Re: for the conversion and the graph.
Re: My suspect is that they are caused by a thermal cycling of the oven,
Re: given the period (about 5 minutes), albeit I am pretty sure this
Re: oven has a proportional control system, not bang-bang.
Re: This suspect is also confirmed by the fact that blowing energically
Re: on the oven causes a major disruption of that cycling.
Re: I would know your opinion, especially from those who have a greater
Re: experience than me on this stuff,  thanks.
Re: 73  Alberto  I2PHD
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