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

Sat Sep 27 00:49:00 UTC 2008

Gerhard
>
>   
>> 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.
>>>>         
>
> Yes, but capacitors have their problems, too. Ceramic ones will
> misbehave like piezos, converting vibration to voltage. C0G or NP0
> will be to small for AF; foil caps will be inductive over a wide range;
> electrolytics won't be there if you need them in winter; 
Yet another reason for temperature control?
> Sanyo's
> contraindication list for OSCONS brings chemotherapy to mind. 
>
>   
For low voltages one could look at supercaps for filtering as some of 
these have relatively low esr and low leakage.
However these are somewhat bulky.
>   
>>> (LTspice)   
>>>       
>> It has some severe limitations for most of the simulations I have done.
>>     
>
> I'd like to hear more about this; I could evade to ADS.
>
>
>   
I'll look at this again when I finish some fibreglass work on a 24" 
telescope and sort out simulation of software implementations of sigma 
delta and MASH DACs.
>> 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).
>>     
>
> In simulation with LTspice, a CB stage brings about 60 dB, which is 
> optimistic because parasitics in the decoupling caps are not modeled (yet).
> The CC stage brings close to nothing, let's say 6 dB, because the 
> forward biased BE diode does its best to keep the BE voltage drop 
> at 0.6 V constantly. So most of what is delivered to the emitter will 
> make it to the base, too. 
>   
The simulated reverse isolation depends critically on accurate 
transistor models.
This is one aspect that can vary widely between simulators.
60dB is indeed optimistic.
S21 for some of the older HP microwave transistors in CB configuration 
is listed at 0.01 to 0.005 at 100MHz on the datasheet.

Surely the CC stage has somewhat higher reverse isolation than that.
When the emitter current is modulated by a signal connected to the 
output of a CC stage via a resistor (50 ohm) then the base current is 
usually  somewhat smaller than the emitter current.
Ib = Ie/(hfe+1).
Even at 100MHz at 4GHz ft transistor has a current gain of about 40 or 
so (provided this doesn't exceed the dc gain) so the reverse isolation 
of a CC stage at this frequency should be around 32 dB.
At frequencies somewhat below ft the reverse isolation of a CB stage is 
limited by the Early effect which produces a varying Vbe for a fixed Ic 
and varying Vcb.
This can be reduced by regulating the emitter voltage with another 
transistor, however this reduces the high frequency reverse isolation.
This technique is very useful with a CB input stage as it also 
significantly reduces the distortion (provided the collector load 
impedance isnt too large)
> Total reverse isolation in simulation was abt. 130 dB with ideal 
> capacitors. Maybe I need another CB stage.
>
>   
Adding another CB stage will increase the phase noise floor noise slightly.
> In real live, I measured abt. 100 dB reverse isolation from output
> to input upto 20 MHz this evening. Above that, it gradually became worse
> and at 60-80 MHz there was some kind of pole / notch. That was completely
> without shields and with coax cables that were not really network analyzer quality.
>
>   
Spectradynamics achieve about 110dB reverse isolation over the 80-120Mhz 
band using a similar amplifier topology in their HPDA100 distribution 
amplifier.
> Splitting the bias resistors in two and decoupling in the middle brought nothing,
> neither in simulation nor in measurement; but it might make a difference
> when the shields are soldered in.
>
>   
My simulations indicate otherwise when the parasitics of real components 
are included.
You could try measuring the attenuation of the bias network itself when 
RF is injected at the transistor base taps.
Do this without the transistors present.
>>
>> 1% metal film would suffice.
>>     
>
> There isn't much choice. NiCr SMD is meant for precision.
> 1% resistors are thick film, usually.
>
>   
Digikey list 1% and 0.5% Susumu thin film surface mount resistors in 
their catalog.
Physically larger resistors can be used in the voltage divider string.
>> 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.
>>     
>
> The PLL is already here. And the preamp, highpasses etc in the
> Wenzel appnote style. I changed it to true differential to fight offsets,
> with the obvious inflation in 2SK369 FETs. Matching these was no fun.
> This must be replaced with something more repeatable. The power consumption
> of the relays does no good to the offset, either.
>
>   
If you don't need to go below 0.1Hz or even 0.01Hz then you can either 
use an integrator or AC coupling between stages to eliminate the 
amplified offset from the 2SK369's.
A differential input stage makes it much easier to reject low frequency 
(<1Hz) power supply noise than when using a single ended input stage.
You should disable the PLL for component phase noise measurements and 
just adjust the phase with a length of cable or similar.

There is some evidence that the flicker noise of JFETs as well as BJTs 
can be much lower than specified on datasheets if one shields the 
devices from air currents and stabilises its temperature (temperature 
regulation for frequencies below 1 mHz , large thermal time constant 
enclosure for frequencies above this). Have you measured the preamp 
noise spectrum in the flicker region?
You could use latching relays to overcome the dissipation problem as 
these only need to be powered long enough to latch in the new state.

Since the IF port impedance of a mixer (even if terminated in a 
capacitor for very low noise) is relatively low, a bipolar input preamp 
using an SSM2220 should work well just add an integrator to control the 
~1mV residual input offset . NB for low gains you may  want to connect 
a  resistor in series with a capacitor between the collectors rather 
than reducing the collector load resistors as Enrico suggests.
> http://www.hoffmann-hochfrequenz.de/downloads/IMG_0009__1000_q50.jpg
>
> I have crammed 100 meters of Aircom-plus cable into a 6 HU 19" box for
> 407 nsec delay. I can barely lift it :-). 
> The _variable_ delay is under construction. Coax relays with
> cable delays in binary steps under computer control. Still a lot to do.
> Two 16 bit 2.5 MSPS digitizers are left over from an earlier project.
> Does anybody out there have Matlab or C code for the three-cornered-hat?
>
>
>   
> To return to the amplifier: I'll probably go the macho way without
> output transformer. With twice the bias current I should be able
> to develop half the voltage into 25 instead of 200 Ohm. It works
> in simulation. I'm approaching the BFG31's limit of 100 mA here.
> Maybe I need 2 in parallel. I searched today for fatter alternatives
> to the BFG31: nothing. 
>
>   
Power dissipation in the transistors may be a problem at 80mA unless 2 
amplifier outputs are connected in parallel.
> Having a wideband 25 Ohm load is a drawback at AF; the load could be
> paralleled by a choke. As long as the choke is fairly high impedance, 
> it will do much less harm at the 25 Ohm than at the 200 Ohm level. 
> I think there are 1uH chokes w/o ferrite in 1206. 
> A few of them would be enough and transformer coupling does not matter.
>   
You would probably achieve better choke performance if you wind you own.
If the frequency were higher a 1/4 wave shorted 50 ohm transmission line 
could be used to short out the 25 ohm resistor for dc and low frequencies.
> Gerhard
>
>   

Bruce