[time-nuts] DMTD Terminations II

[Brian Kirby]

Bruce Griffiths wrote:
> Brian Kirby wrote:
>> I got to correspond with a person at NIST today about DMTD - about 
>> the HP 10514B Mixers.  They pointed me to a application note at 
>> http://www.thegleam.com/ke5fx/stellex/5952-8217.pdf (hey - thats one 
>> of our time-nuts!) that shows driving the 10514 mixer at +13 dbm for 
>> the LO port and 0 dbm for the RF port.
>>
>> This application note is a good read.  HP recommends terminating the 
>> mixer into a 3900 pF cap and 50 ohms in series for the RF 
>> termination.  ANd they use a LC filter for their VNA set for 90 
>> Khz.   I also looked at the schematic of the 8407A were I got the 
>> mixers from.  They terminate the IF into a 1100 pf cap in series with 
>> a 51 ohm resistor, and then it passes thru a 2.7 uH choke going into 
>> an amplifier - the IF I beleive is 298 Khz for this unit.
>>
>> I also did a reconfiguration of my temporary DMTD experiment that has 
>> also improved the noise floor.  Drawing attached.  Also changed the 
>> drive levels.
>>
>> For reference the previous configuration showed 2x10-11 for 0.1 sec; 
>> 2x10-12 for 1 sec, 5x10-13 for 10 sec, 6x10-14 for 100 sec, 7x10-15 
>> for 1000 sec. for the noise floor.
>>
>> The current configuration (of the drawing attached) shows 8x10-12 for 
>> 0.1 sec; 8x10-13 for 1 sec, 8x10-14 for 10 sec, 8x10-15 for 100 sec 
>> and 8x10-16 for 1000 sec.  I used a little more attention in 
>> sheilding and spacing to eliminate crosstalk.  Note the noise shows 
>> some linearity this time....plus more drive....
>>
>> Brian - KD4FM
>>
> Is that circuit stable?
> The LT1037 is only stable for gains of 5 or more.
> When either set of diodes is forward biased the gain drops to a little 
> above unity so does the opamp then oscillate?
>
> Bruce
>
>
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I am not seeing any signs of oscillation.  I have went back to the old 
circuit, to see if everything duplicates , for repeatability reasons.  
SO running some more test to compare to each other.