[time-nuts] DMTD Terminations II

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Mar 3 11:03:55 UTC 2010


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





More information about the Time-nuts_lists.febo.com mailing list