[time-nuts] DMTD Terminations II
Brian Kirby
kilodelta4foxmike at gmail.com
Thu Mar 4 02:30:51 UTC 2010
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.
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