[time-nuts] DMTD Mixer Terminations

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Feb 28 20:53:41 UTC 2010


My simulations indicate that terminating the Mixer IF port in an RF 
short (with both RF and LO ports saturated) increases the beat frequency 
zero crossing slope by more than a factor of 2 (exact value depends on 
mixer component characteristics) but doesnt significantly increase the 
beat frequency amplitude over that with a high value resistive 
termination. To achieve this the IF port termination impedance needs to 
be high at the beat frequency and its significant harmonics. The value 
above which the impedance is considered high depends on mixer details 
such as transformer turns ratio, RF source impedance, diode 
characteristics and RF input levels, etc.

Bruce

Bob Camp wrote:
> 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>_______________________________________________
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>>>>>
>>>>>            
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