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

Gerhard Hoffmann dk4xp at hoffmann-hochfrequenz.de
Wed Sep 24 20:44:30 EDT 2008


On Mon, 22 Sep 2008 15:52:19 +1200, you wrote:


>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.

A standard electrolytic? The ESR is less than stellar, see my musings in 

http://www.hoffmann-hochfrequenz.de/downloads/experiments_with_decoupling_capacitors.pdf

and BC transistors are AF types. 
OK, getting ft is like drinking from a hydrant in Si planar.


>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.

The question is more if the 100r * 100u do anything to de-noise
the LED voltage. Close to the carrier probably not. The problem is
that you nowhere get exact numbers to base a decision on.


>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.

Yes, but don't tell anybody about that, because Rohde claims he has
a patent for that IIRC (forgetting a capacitor :-) .

>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.

So be it custom. Anything less than 300 uH in par will ruin the flat frequency 
response, and the parasitics of a large L + coupling cap will do the same :-(
And with the flat response, the delay stability will be gone, too.

>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.

This was an area of doubt for me, upto now. 
Experimenting with LT-Spice brought some insights. 

(btw available for free from http://www.linear.com/designtools/software/switchercad.jsp
__Highly_recommended__! Even in the sense of free beer, not free/open source.)


Noise gain of a voltage fed to the base of common base stage is proportional 
to Zc/Ze. The Impedance at the emitter of Q3 is quite low, so the
noise voltage at the base reference divider is propagated to the output
just like an input voltage to the BFG196.  --> AF decoupling needed for Q3
(and the space is reserved on the board)
www.hoffmann-hochfrequenz.de/downloads/noise_base_Q3.gif
Generator input is 0 dB, solid lines = amplitude, dotted lines = phase.
The AF noise itself may not make it to the output, but it may be 
modulated onto the carrier.

The second cascode stage, Q4, is fed from the high source impedance of the
Q3 common base stage. The emitter current is forced into Q4 without
much influence of the Q4 base voltage. The noise voltage at the base
from the resistive divider is pretty much suppressed, so heavy 
decoupling of Q4.base should not pay.
www.hoffmann-hochfrequenz.de/downloads/noise_base_Q4.gif

Thinking about it, in this circuit only the second CB stage delivers
"full" isolation.

>It is also necessary to use some active filtering of the power supply if 
>one is to achieve low close in phase noise.

Yes, but that's a different board. I have built some regulators in the
style of Walt Jung of Analog Devices, and they look quite promising.
(clever idea, feeding the reference from the regulated output)
They feature 2 or 3 nV / sqrt Hz.

http://waltjung.org/PDFs/Regulator_Excels_In_Noise_and_Line_Rejection.pdf

www.hoffmann-hochfrequenz.de/downloads/IMG_0341__jung_pos_ref_1024_q75.jpg
(voltage reference and positive regulator)


>To maximise reverse isolation the individual amplifiers will need to be 
>enclosed in RF shields.

The space and the holes are there already, but I still need the access.

>Have you measured the reverse isolation?

not yet.

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

The next iteration will have to live with BNC because R&S smpd, hp8662A,
SNA-33 & friends all have BNCs and the holes in the 19" front plate of
my GPS disciplined XO are drilled already. 
OK, we'll get optional SMAs, too. (the singe channel proto has them already.)

>If you are using thick film resistors replace them with thin film 
>resistors if you want low close in phase noise.

Susumu NiCr 0.1% from Digikey, next delivery..

>Have you measured the phase noise?

Not yet. Still bootstrapping. If it's worth measuring, it
cannot be done with the spectrum analyzer.
I've got an offer from a 3rd party to have it measured
and I'll accept that :-)

>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.

output return loss is in the .pdf
35 MHz must happen to be the sweet spot in the eyes of the ZRB2 bridge.

Input is RC only, should be trimmable. The next version will be changed to
have equal input delays for all channels, so measuring input RL now
would be a waste of time.

thanks for the proposals!

regards, Gerhard, dk4xp

(The week doesn't have enough end. And it's half past 2, again. Good Night!)



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