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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