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

[Bruce Griffiths]

Gerhard Hoffmann wrote:
> Hi all, 
>
> after reading "A low noise 100 MHz distribution amplifier for precision metrology"
> by M. Siccardi, S. Römisch, F. W. Walls, and A. De Marchi (NIST),
> I have implemented a homebrew version of their design.
>
> Circuits, simulation & measurement data are contained in:
>
> http://www.hoffmann-hochfrequenz.de/downloads/distri.zip
>
>
> Suggestions & ideas are welcome. 1:1 board layout of the next
> iteration will be available as .pdf or Gerber.
>
>
>
>
> regards, Gerhard, dk4xp
>
>   
Gerhard

RF grounding all three electrodes of the BC860 is not good practice.
The BC860 will tend to oscillate when you do this unless the ESR of the 
100uF base bypass cap is large enough.
Some early microprocessor reset circuits buffered the RC reset timing 
circuit with an emitter follower which burst into RF oscillation as the 
RC network charged up.
The cure is simple omit the 100uF base bypass cap or at least leave an 
unbypassed resistor of sufficient value in series with the BC860 base.

You can improve the phase noise somewhat if the time constants 
associated with the BC860 base and collector bypass circuits are reduced 
so that the BC860 reduces the collector current noise of the BFG196 in 
the dc - 100kHz or so spectral region.

The major problem with this circuit is the large dc current flowing in 
the transformer primary increases the output distortion significantly.
This may make it difficult to extend the frequency response down to 5MHz 
without using a large custom wound transformer.

Minicircuits and similar transformers tend to have a coupling 
coefficient much greater than 0.99.

The purpose of the heavy bypassing of the bases of the BFG31 transistors 
is to reduce the low frequency noise at the BFG31 bases, this reduces 
the amplifier close in phase noise.
It is also necessary to use some active filtering of the power supply if 
one is to achieve low close in phase noise.

To maximise reverse isolation the individual amplifiers will need to be 
enclosed in RF shields.
Have you measured the reverse isolation?

For maximum phase stability the BNC connectors should replaced by 
threaded connectors such as TNC, SMA , N etc.

If you are using thick film resistors replace them with thin film 
resistors if you want low close in phase noise.
Have you measured the phase noise?

The output device will saturate or at least have increased distortion if 
the output isnt terminated in 50 ohms whilst the input is driven at +13dBm.

Have you measured the input and output VSWR or reflection coefficients?
With real transformers the value of the 200 ohm resistor may need to be 
adjusted to minimise the output reflection coefficient.


Bruce