[time-nuts] Symmetricom 1111C OCXO question

Javier Herrero jherrero at hvsistemas.es
Thu Apr 3 22:21:03 UTC 2014


Yes, I was thinking something like that, but was not able to find it. 
After analyzing somewhat more, I'm finding that the problem seems more 
subtle. Since the gain of the operational is >1, and there is a positive 
feedback (the diode bias from the output), the output voltage should 
build up until reaching the zener threshold and then stabilize. But it 
only reach 2V, and remains there.

The zener is biased using a 2k resistor. A further examination reveals 
that the zener also supplies a reference voltage to three sections of a 
X9241 digital potentiometer, through resistors (1k for each of two 
sections, 10k for the other). The X9241 receives a 5V supply from a 
MIC5205-5 regulator, that takes its input also from the output of the 
main regulator. The X9241 is non-volatile, and has the I2C bus accesible 
from outside the oscillator.

When the output voltage is locked at ~2V, the output of the MIC5205 is 
also ~2V and in this condition is seems that the resistance to ground of 
the X9241 sections (nominally 10k each) is a lot lower, so they load too 
much the reference diode, and makes the voltage at the zener not to go 
up more. So I suspect that the output of the MIC5205 is activated too 
early, and when ramping up, it creates the lock-up. I've tested to put a 
1000uF capacitor at its output to try to delay its turn-on, and then the 
circuit starts up OK - the voltage at the reference diode is the 
expected, the output of the regulator is ok, the output of the 
oscillator is ok.

The MIC5205 has an enable pin, but it is tied to the input, and also has 
a bypass pin in order to connect a bypass capacitor to enhance its noise 
performance and power supply noise rejection, and this capacitor also 
has an effect on the regulator turn-on time. There is in fact a ceramic 
capacitor connected there. So tomorrow I will test to add some 
capacitance in parallel with it and see what happens. Normally I would 
doubt that the ceramic capacitor is bad, but I would also be surprised 
that the design of the circuit is so marginal that a slightly faster 
turn-on time in a secondary regulator is able to create this havoc :)

Best regards,


On 03.04.2014 23:41, Bob Camp wrote:
> Hi
> Normally that sort of circuit has a “boot strap” pull-up resistor that weakly biases the diode to get things running at start up.
> Bob
> On Apr 3, 2014, at 11:08 AM, Javier Herrero <jherrero at hvsistemas.es> wrote:
>> Hello all,
>> I've a 1111C OCXO (inside a Symmetricom 5045A that is inside an Oscilloquartz OSA 5585) that refuses to start. It seems that one voltage regulator is not operating, with only 2V at its output while it should have (I think...) around 12V. I've partially analyzed the circuit, and it is nothing from other world: a 6.9V reference diode, driving an operational amplifier, driving a NPN transistor. But the 6.9V reference diode is biased from the regulator output, so... if the regulator does not provide output, the zener is not biased, and then... the regulator does not provide output. So it must be some kind of start-up circuit, that makes the transistor to conduct a bit, or to bias the diode, when power is applied.
>> I've been looking around, but not found it yet. I have make a quick test by placing a 6k8 resistor between the collector and the base of the transistor to make it conduct, and then the regulator starts, provides adequate output (12.5V), and also the 5MHz output appears.
>> Any idea is welcome :)
>> Best regards,
>> Javier
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