Hi Putting The C on the feedback R in a positive gain setup is only going to take the "roll off" gain down to 1. Doing the same with an inverting amp or using a series R / cap to ground will drop the gain a lot more in the roll off region.
I would worry about any resistor that's marked as 10K and reads 20K. It's likely noisy. A typical DBM has a loss of 5 to 7 db when not in compression. With a +7 to +10 dbm drive that should give you an output of 0 to 2 dbm . The mixer output should be in the .6 to .8 V p-p range into 50 ohms. You should get about twice that on the beat note running into a load > 500 ohms. A gain of 20 should be plenty. That would give you .6 x 2 x 20 = 24 V p-p out of the amp. If you "rf short" the output of the mixer you may double the beat note again (total of 4X the 50 ohm value). Net would be a 2.4 to 3.2 V p-p beat note. Anything much over a gain of 10 would be a problem then. This is one of the cases where 2 X 2 probably does not = 4, so measurements are indeed in order. Bob On Feb 28, 2010, at 1:01 AM, Brian Kirby wrote: > The values in the schematics are wrong for the op amp gain. The drawing was > from an earlier drawing where I made a preamp to start checks on the mixers, > and I sent it to you (Bruce G). Thats when you determined I did not have > enough gain to get near the noise floor. The THAT1512/1646 ICs were ordered > to make a new preamp for the future measurements on the mixers. > > When I use the scope and check the outputs of the IC, I have 20 volts peak to > peak, sine-wave. I know from previous readings I see about 500 mv p-p out of > the mixer. > > I went down to the bench and the resistors I used were still there (I bought > several taped reels of Dale RN55D resistors when a local business went out). > I used 294 ohms and 14.9 kilo-ohms, for a gain of 50 (the power rails are > +/- 15 volts). Also not shown on the schematic is a 0.47 uF cap around the > 14.9 kilo-ohm resistor. I think I was trying to limit the bandwidth to > around 15 hertz. > > Also the resistor going between the op amp and the limiting diodes was marked > 10K, its 20K. The diodes are 1N4148. Corrected drawing attached. > > This is what happens to time nuts who can only play on the weekend and stay > up all night....and my employer just thinks I party too hard.....for Monday > mornings. > > > > Brian KD4FM > > > > Bruce Griffiths wrote: >> The LT1037 is shown with a gain of ~1690x, if this amplifier is used to >> amplify the beat frequency signal, it will saturate. >> Opamp recovery from saturation is poorly documented and may be very slow. >> It would be better to use some diodes in the amplifier feedback network to >> limit the large signal gain to 5x (so that the LT1037 remains stable as it >> isn't unity gain stable). >> This will ensure a somewhat faster recovery from overload as the LT1037 then >> avoids saturation and the opamp input stage remains in the linear region. >> >> Bruce >> >> Bob Camp wrote: >>> Hi >>> >>> Assuming that the junction of the back to back diodes goes trough a chunk >>> of coax to get to the counter: >>> >>> You are forming a low pass filter with the 10K resistor and the coax >>> capacitance. The LT1037 is quite happy driving a 600 ohm load. You could >>> easily drop the impedance at that point below 300 ohms. That should give >>> you a faster edge into the counter. >>> >>> You also should check the slew rate performance of the 1037. You don't want >>> the op amp to be slew rate limited. >>> >>> Bob >>> >>> >>> On Feb 27, 2010, at 12:41 PM, Brian Kirby wrote: >>> >>> >>>> I am in the process of designing a DMTD system. As an experiment to do >>>> basic measurements on the chosen mixer, I used a capacitor (0.01 uF) in >>>> series to ground with a 47 ohm metal film resistor. Where the capacitor >>>> and resistor meets, another resistor is attached (390 ohms) that goes to >>>> ground. The idea is to provide a 50 ohm termination at 20 Mhz and a >>>> lighter termination at audio frequencies. I seen this is a NBS note and I >>>> can say, its a starting point for my experiments. >>>> >>>> This (my) system is designed for 10 Mhz, using a 10 hertz beat. A >>>> schematic is attached of what I am experimenting with at the moment. A >>>> HP5370B is the recording instrument. The noise floor from 1 days >>>> observations show 2x10-11 at 0.1 seconds, 2x10-12 at 1 sec, 5x10-13 at 10 >>>> sec, 6x10-14 at 100 sec, 7x10-15 at 1000 sec, and 7x10-16 at 10,000 secs. >>>> It will be interesting when the project is completed to see how much >>>> improvement there will be. >>>> >>>> As I understand (or learning..) mixer performance is the key to the DMTD >>>> system. It occurs to me that maybe a capacitor designed for 50 ohms at >>>> 20 mhz may be a better termination (for the IF port) for this mixer. A 16 >>>> pF capacitor is 50 ohms at 20 mhz, and for comparison at 10 hertz, it >>>> would be 100 meg-ohms, which would give maximum amplitude at 10 hertz. >>>> As I understand, a capacitor terminated mixer will give a triangle wave >>>> output, which is very beneficial to the design - as the end result is to >>>> get maximum slope out of the mixer. I would say, unqualified as I am, the >>>> capacitor termination matches the 20 mhz signal, and helps attenuates the >>>> harmonics of the mixer, and has no , or very little effect on the audio >>>> frequencies that we are interested in. >>>> >>>> And saying/rambling on... that if maximum slope is needed, its needed on >>>> the 10 hertz beat signal - so maybe a capacitive termination on the 10 >>>> hertz signal only and something resistive on the 20 mhz >>>> signal........another idea use the 16 pF direct off the mixer, then a >>>> series resistor for isolation and then a large capacitor on the 10 hertz >>>> beat for maximum slope. >>>> >>>> At the present, I am awaiting parts to build a low noise preamp base on >>>> the THAT1512 so I can make better measurements on the mixer. Bruce has >>>> provided a lot of good suggestions and helpful comments on my project and >>>> Ulrich has provided me quite a bit of user support on his program, >>>> Plotter. Thanks to all. >>>> >>>> Comments ? Brian KD4FM >>>> <DMTD_Plans.pdf>_______________________________________________ >>>> time-nuts mailing list -- [email protected] >>>> To unsubscribe, go to >>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>> and follow the instructions there. >>>> >>> >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >>> >>> >> >> >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> > > <DMTD_C_Plans.pdf>_______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
