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

Gerhard Hoffmann dk4xp at hoffmann-hochfrequenz.de
Fri Sep 26 19:39:19 EDT 2008


On Thu, 25 Sep 2008 14:08:51 +1200, you wrote:



>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; Sanyo's
contraindication list for OSCONS brings chemotherapy to mind. 


>> (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.


>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. 
Total reverse isolation in simulation was abt. 130 dB with ideal 
capacitors. Maybe I need another CB stage.

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.

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.

>>> 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.

There isn't much choice. NiCr SMD is meant for precision.
1% resistors are thick film, usually.

>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.

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?


> 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.

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. 

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.

Gerhard



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