[AMPS] Network analysis of suppressors

[Steve Iezzi]

>
> > It just so happens that I was building new suppressors for a Clipperton
> > that I am returning to service and the parasitic topic comes up again.
> > Since I have access to some very nice equipment, I can build the
> > suppressors and measure them very accurately.  I thought I would pass
> > along my findings.
>
> Can you measure S21, and have you measured it?

S21 for this network is 46 + j74 at 50 MHz.

>
> > My final network has eight turns of nichrome about .25 inches
> in diameter
> > along side of a Matsushita 100 ohm 3 watt resistor.  The trace reveals a
> > very nice characteristic with a self resonance of 1.5 Gigahertz.
>
> What is a "nice characteristic"? What was the goal?

The goal was to dissipate about 1 to 2  watts per suppressor at 29 MHz.
572B tubes have significant feedback capacitance and this particular amp
arced and sparked with the original suppressors (6 turns of bus wire
concentric with a 2 watt 100 ohm carbon resistor).  You could also tell that
original suppressors were at one time very hot as the coating on the
resistors had bubbled.  It was time for some new ones.  The network analyzer
showed that the new network behaved as an inductor in parallel with a
resistor all the way past 1 Gigahertz and the impedance was consistent with
my dissipation goal.  This is what I meant by "nice characteristic".

>
> > The nichrome behaved as advertised and lowered the Q of the
> resonance by a
> > factor of about 10.  At these very high frequencies, your
> results may vary
> > so pay attention to the setup.  Attempts to duplicate the
> resistance wire
> > with an additional series resistance did not work as the body of that
> > device added additional resonance albeit also at very high frequencies.
>
> What frequency did you determine was involved in the problem you
> saw? How did you determine the frequency of the problem?
>
> > The construction technique was also critical.  Wrapping the wire around
> > the body of the resistance added capacitance lowering the resonance
> > frequency. Winding spacing also changed the resonance.
>
> What frequency are you talking about?

This network behaved as a resistor in parallel with an inductor all the way
to 1.5 Gig at which point the intrinsic capacitance of the resistor and the
winding resonating with the inductance.  The frequency that I refer to is
the self resonance of the circuit.  One of my goals was to move this self
resonance as high as possible.

>
> > Differences between the two types of wire were only evident at very high
> > frequencies where it lowered the Q of the self resonance and very low
> > frequencies where the network behaved as a pure resistor.
>
> 1.5 GHz is ultra high frequency. Why are you concerned about the
> system at that frequency?

The network would have probably worked fine even if it resonating at 500
MHz.  My natural tendency is to optimize.  I was just noting that with
proper construction, one could build a network that behaved as intended to
very high frequencies.

>
> > With the network installed in the amplifier, one could identify a loop
> > that included the plate capacitor, the capacitance of the plate
> choke, the
> > high voltage bypass capacitor and the suppressor that could resonate at
> > VHF frequencies.
>
> What was the impedance these "problems" presented right at the
> anode of the tubes? How much did you change the anode
> impedance with you mods, as measured right at the anode?
>
> Why is resonance bad? Can't it be good also?

The anode output impedance at the operating points is about 6K ohms per
tube.  Even at 29 MHz, the network added no more than 26 + j50 to the anode
impedance.  However, the small change was enough to operate this amp on the
10 meter band.  Something that I couldn't do before.

>
>  Because of differences in construction, this resonance
> > frequency may and will vary from one amplifier to another.  If
> you happen
> > to get one that resonates at a harmonic of the exciter frequency or at a
> > frequency where there is sufficient gain from the tube, a parasitic may
> > ensue.
>
> What do harmonics have to do with parasitics?
>
> If any oscillator has more regeneration than loss, it will oscillate. If
> it doesn't, it won't. External excitation has NOTHING to do with
> this, since even random shot or thermal noise in the circuit will
> start the oscillation.
>
> You have an oscillator, or you don't.

Circuits designed with poles on the Y axis will not intrinsically oscillate
but will ring for long periods of time if excited at their resonance
frequency.  We use this type of circuit for high frequency (Gigahertz) clock
recovery and other related problems.  It is very common for these circuits
to be designed to ring at a harmonic of the excitation frequency.  It works
kind of like a poor man's phase locked loop.

>
> Now certainly gain has something to do with a parasitic, but it also
> requires feedback of the correct phase and the correct impedance
> at the proper spots are critical to this.
>
> What did you do to define the problem? That would be valuable in
> understanding if the cure was logical...or just an exercise in looking
> at one narrow parameter that has little to do with the overall
> problem.

This type of problem is very commonly encountered in the designs of high
frequency switching power supplies.  The fundamental of the switch may be 1
Megahertz but the ringing can be as high as 100 Megahertz and is the result
harmonic excitation.  Since there is insufficient load at these high
frequencies due to component inductance, the ringing manifests as radiated
RF.  Something the FCC frowns upon.  Careful attention to design and the
addition of lossy networks reduces this activity to acceptable levels.

To be honest, I did not go to the trouble of predicting why this amplifier
misbehaved at 29 Meg.  In the interest of expediency, I introduced a lossy
network and cured the problem to my satisfaction.  If I had the luxury of
redesigning the entire chassis with better components and geometry, I may
have been able to use less lossy suppressors.

>
> > The fact that many amplifiers work fine with the stock
> suppressors or even
> > no suppressors doesn't surprise me.  The variation in construction may
> > move the resonance to frequencies where there is insufficient gain to
> > cause a problem.  This would make some systems more likely to fail than
> > others.
>
> I can make any stock Clipperton, or any Clipperton blessed with
> nichrome, oscillate by misterminating the input and/or output, and
> adjusting the controls "properly" provided nothing is done to
> address the HF feedthrough problems.

Maybe so.  The additional 1 ohm of resistance dissipates less than 1 watt at
160 meters where the resistance wire is the dominant characteristic of the
network.

Steve


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