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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Sep 24 22:08:51 EDT 2008


Gerhard Hoffmann wrote:
> 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.
>
>
>   
There usually not too much point in low pass filtering the LED voltage 
as the noise of a forward biased LED is very low unless of course the 
LED has poor reliability (high LED voltage noise is a very good 
predictor of poor reliability).
Of course the finite slope resistance of the LED will allow some of the 
power supply noise to appear across it.
>> 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 :-) .
>
>   
Its a bit more than omitting a base bypass cap as it has been 
recommended for decades (well before Rhode came up with his not too 
original idea - this was obvious once NIST demonstrated that extending 
the bandwidth of a current regulation loop helped suppress low offset 
frequency phase noise) that a base bypass cap for the current regulator 
transistor not be used to avoid oscillation.
>> 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.
>
>   
You are likely to find that its only practical to cover the 80-120Mhz 
region as the NIST and Spectradynamics distribution amplifiers do.
>> 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. 
>
>   
You need to read the phase noise references listed at the bottom of:
http://www.ko4bb.com/~bruce/IsolationAmplifiers.html 
<http://www.ko4bb.com/%7Ebruce/IsolationAmplifiers.html>
Essentially noise voltages between collector and base modulates the 
collector output capacitance and hence the modulates the output signal 
phase shift.
It can also modulate the emitter current and hence the transistor ft 
which also phase modulates the output signal.
Other transistor parameters can also be modulated by noise voltages from 
the power supply, resistors etc.

> (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.)
>
>   
It has some severe limitations for most of the simulations I have done.
> 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.
>   
Not true it still modulates the collector base capacitance of Q4.
> www.hoffmann-hochfrequenz.de/downloads/noise_base_Q4.gif
>
> Thinking about it, in this circuit only the second CB stage delivers
> "full" isolation.
>
>   
Typically a CB stage has 40dB or more reverse isolation (at low 
frequencies with a low impedance connection from base to ground) whilst 
an emitter follower may have 10db less isolation (depends on hfe at the 
frequency for which the reverse isolation is measured. Using a 
darlington or Sziklai pair will improve the reverse isolation over that 
of an emitter follower).
>> 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)
>
>
>   
These need to be supplemented with on board filtering as they aren't 
quite as quiet as you need.
Either the NIST style darlington buffered RC low pass filter (one per 
amplifier) and/or a modified (stabilises the shunt transistor re by 
making its collector current approximately PTAT) version of Wenzel's 
active power supply noise filter can be used.
>> 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.)
>
>   
A TNC connector should fit in the BNC holes.
TNC to BNC adapters are available when you need to connect to a BNC.
>> 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..
>
>   
1% metal film would suffice.
>> 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 :-)
>
>   
You can easily measure the phase noise for low offset frequencies using 
a low noise mixer with appropriate (not 50 ohm) IF termination followed 
by a low noise (audio frequency) preamp driving a sound card. A 24 bit 
sound card is ideal, however 16 bit sound cards just need a little more 
preamp gain. No need for a PLL just split the output of a low noise OCXO 
or similar source drive the mixer LO port with one output and the 
isolation amplifier with the other whilst the isolation amplifier output 
drives the mixer RF port. You will need to adjust the phasing between 
the LO and Rf signals so that they are approximately in quadrature by 
using a suitable length of coax or other means. You can even take 
advantage of the 2 channel (stereo) sound card inputs to do get well 
below the mixer noise and/or sound card noise floor by using cross 
correlation techniques.

For higher offset frequencies (>100kHz) a similar setup with a 50 ohm IF 
port termination plus a high gain low noise amplifier can be used to 
drive the input of a spectrum analyser to measure the phase noise.
>> 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!)
>
>   
Bruce





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