[time-nuts] DMTD Terminations II

Bob Camp lists at rtty.us
Wed Mar 3 12:04:33 UTC 2010


Hi

These days you can get (for a price) NPO caps up above .1 uf. That makes super high value coils less important in a low frequency beat note system. If you are trying to measure phase noise at 100 KHz then 0.1 uf to ground at multiple places likely is not a real good idea.

Bob


On Mar 2, 2010, at 11:28 PM, 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
> 
> 
> 
> 
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