[time-nuts] homebrew 13 dBm distribution amplifier based on NIST design 5 to 100 MHz

[Bruce Griffiths]

Bruce Griffiths wrote:
> Tom Clark, K3IO wrote:
>   
>>    Gerhard -- the discussion between you and Bruce has been very
>>    interesting. I asked a VLBI colleague to look over your design and he
>>    had this comment.
>>    In VLBI,  H-Maser frequency standards used to generate local
>>    oscillators at microwave frequencies. We have problems with amplitude
>>    modulation being converted to phase modulation when hum is present.  I
>>    notice that Bruce also uses transformers. Comments from both of you
>>    will be eagerly awaited!
>>
>>          Thanks Tom. I notice the Gerhard Hoffmann circuit has a
>>      transformer on the outputs. Wenzell also uses transformers and we
>>      have found them to be a problem if there is any stray AC mag. field
>>      around. We have added some magnetic shielding to the UpDown
>>      converters to reduce the 60/120 Hz modulation which results if the
>>      UpDown is close to a another piece of electronics with a AC fan or
>>      AC transformer. I don't think 60/120 Hz is a problem for VLBI2010
>>      but it can be a problem for mmvlbi.
>>
>>    To help to decipher some of our "code words":
>>      *  the UpDown Converter is a wideband frequency converter that takes
>>        an arbitrary chunk of 1-20 GHz RF and mixes it to a more convenient
>>        & standardized frequency to feed a polyphase filter bank.
>>      *  VLBI2010 is design prototype effort we are doing that will use
>>        (relatively) small antennas operating over the entire ~2-15 GHz
>>        spectrum to produce geodetic measurements accurate to mm-levels on
>>        global baselines (up to ~10,000 km).
>>      *  mmvlbi refers to VLBI at mm wavelengths (like 100-500 GHz) for
>>        astronomical measurements. Most recently, the mm observations of
>>        the size/structure of  radiation from the area around the black
>>        hole in the center of our galaxy are really exciting.
>>
>>    Regards, Tom
>>   
>>     
Tom

Attached partial circuits showing the output coupling circuit using 
either air core Ruthroff or Guanella Transformers show that these 
devices short out the 200 ohm output resistor in Q4 collector for DC and 
low frequencies. This is desirable as it reduces the dc and low 
frequency gain from any of the isolation amplifier transistor bases 
(including the bias regulation transistor) to the output transistor 
collector to near zero, minimising the close in phase noise contribution 
from these noise sources. Another advantage of using air core 
transformers is that they do not saturate so that dc flowing in the 
windings merely heats them a little due the finite dc resistance of the 
windings.

The Guanella configuration is bulkier but is likely to have a much wider 
bandwidth.

The air core transformers will need to be shielded to minimise crosstalk 
between isolation amplifiers, however shielding is required in any case 
the shields just need to be a bit larger.

To maximise physical separation between amplifier outputs whilst 
minimising the physical separation of the amplifier inputs a star layout 
where the individual isolation amplifiers radiate from the central input 
may be useful.

Bruce
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