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

[Bruce Griffiths]

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

Another technique is to null the AC magnetic field at the transformer 
using a set of coils and a servo loop.
However this isnt always a particularly practical/inexpensive/simple 
solution.

The AC field modulates the transformer inductances and hence the phase 
shift.
Thus in the absence of shielding or field nulling a transformer with a 
ferromagnetic core will suffer from this problem.
Ferrite core chokes will also suffer from inductance modulation by the 
AC magnetic field.

One is either left with using RC coupling which makes it difficult to 
achieve power gain from a CB stage and the dc load resistors can 
contribute significant close in phase noise.
One may need to resort to using an emitter follower to drive the load.
The key factor in keeping the close in phase noise down at low offset 
frequencies is to keep the dc gain from input to output low.
Where input means not just the actual RF input but includes any active 
bias regulation circuits.
It is relatively easy to ensure that the dc gain and low frequency from 
the input transistor base to the output transistor collector load 
resistor is relatively low by using a large value resistor (capacitively 
bypassed for RF) connected in series between the input emitter follower 
and the first common base stage. With such a resistor one can dispense 
with the bias regulator transistor and use a divider tap  buffered by a 
low frequency emitter follower if necessary to determine the dc base 
voltage of the input transistor. The drawback with such an approach is 
the increased power supply voltage and the dissipation in the bias and 
load resistors. With a 200 ohm collector load resistor and 45mA 
collector current there will be a 9V dc voltage drop across it (its will 
also dissipate about 400mW).

If the isolation amplifier dc collector current were increased to 
80-90mA or so (may require paralleling the outputs of 2 amplifiers) then 
it can easily drive 1V rms into a 25 ohm load.
This would allow a capacitively coupled 50 ohm load to be driven whilst 
ensuring the output stage reflection coefficient is relatively low. 
Since the collector current regulation circuit ensures that the output 
current that the dc gain from any of the amplifier transistor bases to 
the output transistor collector is low, it is well worth retaining the 
bias regulation transistor. However the dc and low frequency gain from 
the bias regulation transistor base to the outpout transistor collector 
load will be about 1X. This can be reduced by increasing the bias 
current regulator circuit effective reference voltage (about 1V if a RED 
LED is used). Using a light shield or encapsulating the led in opaque 
epoxy is probably a worthwhile precaution to avoid photocurrents and 
consequent incidental phase modulation at twice the mains frequency and 
its harmonics.


Alternatively narrowband techniques (tuned circuits with air core 
inductors) could be used however the resultant phase shift tempcos may 
be unacceptable.

Transformers aren't entirely ruled out as one could use an air cored 
Guanella balun transformer to couple the output CB transistor collector 
to the load.
The bandwidth of such a transformer is significantly less than a 
transformer with a ferrite or other ferromagnetic core but may be 
acceptable for a frequency distribution amplifier.
Such transformers tend to be bulky and are thus more practical at 100MHz 
than 5MHz or 10MHz.

Bruce