[time-nuts] DMTD Terminations II
Bruce Griffiths
bruce.griffiths at xtra.co.nz
Wed Mar 3 04:58:12 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
NP0/C0G caps are a good idea as they arent voltage dependent and have
lower phase noise than other types.
If you use a choke its a good idea to select one whose first self
resonance lies somewhat above the sum frequency.
Suitable chokes are a little scarce if one uses the 100uH, 220uH or
330uH chokes that NIST have often used.
EPCOS has a range of suitable chokes which consist of a single layer
winding on a ferrite rod core.
These are available from Farnell or their US distributor (Newark, IIRC).
Bruce
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