[time-nuts] Re: James Miller/Brooks Shera 10 MHz GPSDO

[Bob kb8tq]

Hi

If you check the 10811 schematic, the varicap is biased at roughly +6.8V internally.
The other side of the diode is what the EFC is driving. This gives them the +/- 5V
range they specify on the 10811 data sheet.

Bob

> On Aug 30, 2022, at 5:39 AM, paul swed via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> Pretty clever approach to establishing an offset. But doesn't it matter how
> the vari-cap is hooked up in the oscillator circuit? If the veri-cap needs
> a negative voltage to work that may indicate the cathode is at ground and
> the positive voltage would forward bias the vari-cap. Just thinking about
> the possible impact.
> Though the cost of experimenting is $0. No harm.
> Regards
> Paul
> WB8TSL
> 
> On Tue, Aug 30, 2022 at 1:22 AM Mervyn Thomas via time-nuts <
> time-nuts at lists.febo.com> wrote:
> 
>> Stephen,
>> 
>> An idea I may try -thanks for your response.  The current involved in
>> the EFC line to the oscillator is extremely small-button cells would
>> last years.
>> 
>> Merv
>> 
>> On 29/08/2022 8:42 pm, Stephen C. Menasian via time-nuts wrote:
>>> Merv,
>>> 
>>> If I understand your problem, the 10811's VCO control input voltage has
>>> drifted above the output range of the GPSD driver board. If the VCO
>>> control terminal current is low enough, you might be able to quickly
>> solve
>>> this problem by putting 1 or 2 Lithium Ion Coin cells in series with the
>>> control lead. If only a few microamperes are involved, this solutin might
>>> last several years.
>>> 
>>> Stephen Menasian
>>> 
>>> On Mon, 29 Aug 2022 15:56:52 +0800
>>> Mervyn Thomas via time-nuts <time-nuts at lists.febo.com> wrote:
>>> 
>>>> Hi Bill,
>>>> 
>>>> Unfortunately the 2 HP 10811's I have have no way to poke anything thru
>>>> the can to adjust a capacitor.  They are completely enclosed in an
>>>> aluminium can with 2 power leads coming out with 6 pin female
>>>> connectors on the end which go to the power board as used in the HP
>>>> Z3801 GPSDO and 2 thin coax cables with SMB connectors - one marked as
>>>> 10 MHz and the other as EFC.  I may have to replace mine with the model
>>>> that has the tuning capacitor.
>>>> 
>>>> Thanks for your response.
>>>> 
>>>> 73   Merv
>>>> 
>>>> On 29/08/2022 12:45 pm, Bill Beam via time-nuts wrote:
>>>>> I also run a Brooks Shera/HP 10811. Still going after many years.
>>>>> You must live with the limited steering voltage.
>>>>> The solution is to adjust the HP 10811 tuning capacitor to get it
>>>>> back near a frequency that the EFC can handle.
>>>>> I have had to do this more than once.  Also if replacing the
>>>>> oscillator its tuning capacitor may need adjustment.
>>>>> 
>>>>> On Mon, 29 Aug 2022 11:25:26 +0800, Mervyn Thomas via time-nuts wrote:
>>>>> 
>>>>>> Hi All,
>>>>>> I'm new to time nuts but here goes.
>>>>>> I built a GPS disciplined-  10MHz osc about 15 + years ago.-  It
>>>>>> uses an HP 10811 D/Oven Oscillator and the Brooks Shera PCB and
>>>>>> software V402N.- The physical construction article was per James
>>>>>> Miller G3RUH. This unit has worked flawlessly for all these years
>>>>>> but now is unable to hold the freq to precisely 10MHz - it is
>>>>>> constantly .29 to.60 Hz high.- The DAC output which controls the EFC
>>>>>> into the osc outputs a negative only adjustable voltage between -3v
>>>>>> down to 0V and is settable by varying a 10K potentiometer.-  The
>>>>>> Osc. now needs abouit +0.46V to bring it down to 10MHz but the DAC
>>>>>> does not seem able to output a positive voltage.-  The DAC is an
>>>>>> AD1861 (obsolete) and has a +5V applied to pin 16 and a -5V applied
>>>>>> to pins 1 and 8. My first reaction was the HP osc. had reached the
>>>>>> end of it's life but the datasheet states the EFC control voltage
>>>>>> can be between a plus 5V and a minus 5V which varies the freq by 1Hz
>>>>>> so it seems it is just over half way through it's life.
>>>>>> I feel the Brooks s/ware would have provided some method to ensure
>>>>>> the DAC output could be adjusted to swing the full full 5V or at
>>>>>> least 3V?- I have the source code but I'm no programmer so would
>>>>>> appreciate any help someone can provide please.
>>>>>> 73 de Merv-  VK6BMT
>>>>>> _______________________________________________
>>>>>> time-nuts mailing list -- time-nuts at lists.febo.com
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>>>>> Bill Beam
>>>>> NL7F
>>>>> 
>>>>> 
>>>>> 
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