Warren My Spice model of the dc levels in the oscillator indicate a current more like 4mA in the 510 ohm resistor.
Bruce WarrenS wrote: > I just saw your schematic of the rest of the buffer circuit. Sorry I missed > it early. > > The Osc Vcc is shown driven from a 510 Ohm resistor, and with the total > current thru the 510 res at under 2 ma, > Looks like Osc Vcc should be at more like 4.0 volts instead of 2.49 volts. > If I'm correct about that (this time) then, It has got to be one of three > things. > The 510 ohm is wrong, the 5 volts is wrong, or the load on the 510 ohm is > wrong. > It should be easy enough to measure which one it is. > The voltage at the junction of the two 4.7K resistors are WRONG. > Looks like it is caused by an excess current draw on the Grn lead. > Check the direction and leakage of the voltage controlled cap diode and > bypass. > If all else is right, Look for the cause of the extra loading current on the > 510 ohm, > > WS > > **************** > > ----- Original Message ----- > From: "Bruce Griffiths" <[email protected]> > To: "Discussion of precise time and frequency measurement" > <[email protected]> > Sent: Friday, July 03, 2009 9:55 PM > Subject: Re: [time-nuts] Broken Ovenaire OSC 85-50 > > > >> WarrenS wrote: >> >>> A couple more thoughts; >>> >>> Is the VCC (Red wire) current draw correct?. >>> Should be 1.3 ma (per the schematic values) with the 470 Ohm and >>> more like 7 plus ma with the 47 ohm. >>> >>> AND I don't remember seeing, IS the Freq real close AND can it be tuned by >>> the cap and Green wire voltage? >>> If both answers are YES, at least most things are working correctly. >>> >>> As Bruce said: >>> >>> >>>> "if the resistor values are correct this is the voltage(s) that one should >>>> get" >>>> >>>> >>> With the resistors values shown, I agree that all the other voltages are >>> correct given the 2.49 volts, >>> BUT the 2.49 volt is NOT controlled by any of the values on that schematic, >>> it's just an input. >>> AND the output amplitude is not yet being controlled by current cut off. >>> >>> Something that needs to be asked is are all these the correct values. >>> It sounds like you got them by measuring and not from a "should be" >>> schematic. >>> >>> One last, way out thought. >>> It was my belief that very good osc have some sort of AGC to control the >>> amplitude, >>> ANY chance that the VCC is used as the input to control the amplitude of >>> the Osc output? >>> If so, could it be that the external AGC that is what broken and supplying >>> too low a VCC voltage to the circuit? >>> >>> ws >>> >>> >>> >> The Colpitts crystal oscillator limits the crystal current by being >> periodically cutoff for part of the cycle. >> There is an optimum duty cycle (usually something like 25%) that >> corresponds to minimum phase noise. >> The oscillator dc collector current determines the crystal current and >> hence the output amplitude. >> It is possible to perform AGC by varying the oscillator Vcc but the >> output amplifier schematic appears to have no circuitry for this. >> Some circuits actually do this, but the oscillator transistor is still >> cutoff for part of the oscillator cycle. >> The oscillator VCC is fed from a tap in a resistive divider network >> (located on the external (to the oven) buffer board) connected between >> the 5V supply and ground. >> Usually one just varies the emitter (or collector) dc current by >> selecting a resistor value. >> >> Bruce >> >>> ***************** >>> ----- Original Message ----- >>> From: "Bruce Griffiths" <[email protected]> >>> To: "Discussion of precise time and frequency measurement" >>> <[email protected]> >>> Sent: Friday, July 03, 2009 8:21 PM >>> Subject: Re: [time-nuts] Broken Ovenaire OSC 85-50 >>> >>> >>> >>> >>>> WarrenS wrote: >>>> >>>> >>>>> Here is my two cents worth >>>>> 20 mv output, sure sounds like something is broken. >>>>> It should be fixed before it is modified. >>>>> The 2.49 volts on the Red input voltage seem LOW, More Vcc maybe. >>>>> >>>>> >>>>> >>>> Not so, if the resistor values are correct this is the voltage that one >>>> should get. >>>> The question that remains: Is the voltage what the designer intended? >>>> With the Colpitts oscillator used limiting in the oscillator transistor >>>> (usually by current cutoff) is used to limit the amplitude. >>>> Just increasing the voltage without checking the resultant crystal >>>> current will be within permissable limits can be counter productive. >>>> >>>> >>>> >>>>> The "Grn" labeled wire, FreqCtrl input should be about 1/2 VCC for >>>>> testing. >>>>> If you do 'need' to modify the gain, >>>>> It would seem better to bypass the 470 ohm resistor with a cap in series >>>>> with the 47 ohms. >>>>> >>>>> ws >>>>> >>>>> >>>>> >>>> Bruce >>>> >>>> >>>>> ******************** >>>>> ----- Original Message ----- >>>>> From: "Mike Monett" <[email protected]> >>>>> To: <[email protected]> >>>>> Sent: Friday, July 03, 2009 11:50 AM >>>>> Subject: Re: [time-nuts] Broken Ovenaire OSC 85-50 >>>>> >>>>> >>>>> >>>>> >>>>> >>>>>> > An update on the Broken Ovenaire OSC 85-50. >>>>>> >>>>>> > I prepared a 'schematic' of the Output Board and the Oscillator >>>>>> > Board (attached) and I have lots of pictures of the external unit >>>>>> > and the insides if anyone is interested. >>>>>> >>>>>> > I resoldered all connections and replaced all transistors on the >>>>>> > Output Board and the Oscillator Board all to no benefit. I >>>>>> > measured all the components with an LCR meter and found the 0.01 >>>>>> > uF bypass on the 330 ohm resistor in the emitter circuit of the >>>>>> > output transistor of the Output Board to be low and with a high >>>>>> > ESR. I replaced this with about a 20% increase in output amplitude >>>>>> > but still inadequate. I replaced the rest of the 0.01 uF caps on >>>>>> > the output board with no additional benefit. I transiently >>>>>> > disconnected the Red wires from the Oven Controller board and >>>>>> > there was no increase in output or significant increase in voltage >>>>>> > to the Oscillator Board. >>>>>> >>>>>> > Therefore, it appeared that a 'low output crystal' (if such a >>>>>> > thing exists) was the only logical explanation that I could come >>>>>> > up with. That seeming to be the case, there appeared to be only 4 >>>>>> > options. 1. Toss the OCXO (sorry, too much effort so far). 2. >>>>>> >>>>>> > Build an external amplifier (seemingly too much additional >>>>>> > effort). 3. Try to adjust on the bias of the oscillator transistor >>>>>> > to achieve a higher output (seemed too 'iffy'). Or 4. Lower the >>>>>> > value of the resistor in the emitter circuit of the Oscillator >>>>>> > Board to get more gain out of the last stage in the Oscillator >>>>>> > Board. >>>>>> >>>>>> > I replaced the 470 ohm resistor with a 47 ohm resistor and the >>>>>> > amplitude increased to about 0.4 V P-P into a 50 ohm load and was >>>>>> > sufficient to make it a usable OCXO again. >>>>>> >>>>>> > I reassembled, resealed with Epoxy and all seems well so far. >>>>>> >>>>>> > If anyone wants pictures or other info, please let me know. >>>>>> >>>>>> > Thanks for all the suggestions and help. >>>>>> >>>>>> > Joe >>>>>> >>>>>> Joe, >>>>>> >>>>>> Congratulations on getting your system to work! >>>>>> >>>>>> A couple of things. First, trying to measure the currents in the >>>>>> circuit with a ferrite toroid won't do you much good. You don't know >>>>>> what the currents should be, and the secondary of the toroid >>>>>> transformer requires a termination resistor. The value changes with >>>>>> the turns ratio. >>>>>> >>>>>> Just from looking at the circuit, the RF currents will be extremely >>>>>> low. This requires a large number of turns on the secondary, which >>>>>> will probably resonate at or below the 10MHz operating frequency due >>>>>> to stray capacitance from the connection to the scope. So it is >>>>>> unlikely you will get any useful progress in this direction. >>>>>> >>>>>> However, from the values on your schematic, the output tank circuit >>>>>> resonates at 9.602MHz with a Q of 9.6. So the tank is already well >>>>>> below resonance, which attenuates the output voltage. >>>>>> >>>>>> Any stray capacitance you add to the circuit will bring the resonant >>>>>> frequency lower, further aggravating the loss in signal. >>>>>> >>>>>> The output tank is tapped with the 75pF and 91pF in series. This >>>>>> further attenuates the signal. >>>>>> >>>>>> I'd change the circuit to a single capacitor across the tank with a >>>>>> small trim capacitor to tune it to resonance. >>>>>> >>>>>> To get the signal into 50 ohms for distribution, I'd add a limiter >>>>>> if you can tolerate a square wave output, or a good emitter follower >>>>>> if you need a sine wave. Take the output from the collector of the >>>>>> 2N2369 to get the maximum signal amplitude. >>>>>> >>>>>> Your original post mentions an output amplitude of 20mV. If the >>>>>> normal amplitude is around 2V, this represents a loss of 40dB. This >>>>>> is a huge loss in signal. The circuit obviously worked at one time, >>>>>> so there may well be some other hidden problem. >>>>>> >>>>>> It is possible the crystal is damaged, but this seems unlikely. A >>>>>> crystal oscillator probably won't even start if the signal level is >>>>>> down 40dB. >>>>>> >>>>>> You can check the oscillator and crystal in SPICE. Normally, the >>>>>> high Q of the crystal will make the analysis very slow. It could >>>>>> take many hours for the simulation to begin oscillating and >>>>>> stabilize at the final amplitude. The transient analysis requires a >>>>>> very fine time step for accuracy, and you could run out of memory >>>>>> before the simulation was complete. >>>>>> >>>>>> I have developed a much faster way of analyzing a crystal oscillator >>>>>> in SPICE. Instead of requiring tens or hundreds of thousands of >>>>>> simulated cycles, this method gives accurate results in only a few >>>>>> dozen cycles. For more information, please see "SPICE Analysis of >>>>>> Crystal Oscillators" >>>>>> >>>>>> http://pstca.com/spice/xtal/clapp.htm >>>>>> >>>>>> You can estimate the value of the crystal ESR by finding the Q of >>>>>> your crystal and working backwards. >>>>>> >>>>>> I'm attaching a gif of your schematic for reference. This is rotated >>>>>> 90 degrees and enhanced in LView Pro to improve the contrast. >>>>>> >>>>>> Please let me know if you have any questions. >>>>>> >>>>>> Thanks, >>>>>> >>>>>> Mike >>>>>> >>>>>> >>>>>> >>>>>> >>>>> _______________________________________________ >>>>> time-nuts mailing list -- [email protected] >>>>> To unsubscribe, go to >>>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>>> and follow the instructions there. >>>>> >>>>> >>>>> >>>>> >>>> >>>> >>>> >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >>> >>> >>> >> >> >> >> > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
