Tuned input circuit

Tuned input circuit

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Why
is a tuned input
necessary?

An RF power
amplifier’s tuned
input circuit is
more than just
impedance matching.
While reasonable
impedance matching
can be important for proper exciter operation, the
tuned input circuit
of a grounded-grid
or cathode driven
radio frequency
power amplifier
provides multiple
important functions for the amplifier:

• The tuned
  input system
  prevents harmonics
  generated in the
  PA tube from
  passing back to
  the exciter
  • This prevents false high SWR or exciter power readings, since cathode
    harmonics show as reflected power       
• The tuned
  input circuit
  provides a stable
  low impedance to
  the cathode of the
  PA tube
  • This helps amplifier linearity and reduces IMD, and can improve PA
    efficiency
• The tuned
  input circuit
  matches the
  cathode impedance
  to the exciter
  system impedance
  • This lets the exciter work into a proper SWR

A tuner between the amplifier and radio,  a matching circuit some
distance from the cathode, or a high pass or broadband network will not do all
of the above.

Harmonics

Current in the cathode shares the same path through the tube as current in
the anode, with an additional current that flows in the grid-cathode path in
class sub-2 amplifiers. (AB1 amplifiers have no grid
current, while AB2 amplifiers have grid current.) The
cathode of a vacuum
tube essentially
contains the same
time-varying current
as the anode, and if there is any grid current that additional current is also
harmonic-rich current.

This is a spice
model of a grounded
grid pentode tube
operating class AB2.

Drive voltage is
35 volts RMS, or 24
watts.

Output power is
112 watts.

Cathode harmonics
across resistor R4:

Harmonics at
exciter are negligible thanks to low-pass filtering by the tuned input:

It is important
to use a low-pass input
matching network
that presents a low
impedance for harmonics at the
tube cathode. This
keeps cathode
current transitions
between conduction
and non-conduction
very sharp. A sharp transition into and out of conduction and reduction of
cathode harmonic energy keeps efficiency
high.

Type of Tuned
Inputs and the
Effects On Amplifier
Performance

The tuned input
should always be located as close to the cathode of a
grounded-grid amplifier as possible.  The input circuit should never be a
large fraction of the wavelength away from the cathode at harmonics of the
highest operating band. This probably relegates the safe distance to a few
inches for an HF amplifier, although it is tube and operating class sensitive.
There is also a way to work around issues, any distance will work as long as the
cathode “sees” a relatively  low impedance at harmonics.

I learned this lesson well when designing a pair of 3-500Z’s for Heathkit.
After struggling with low plate efficiency on ten meters and poor IMD
performance on ten and fifteen meters, I found the problem was in the cable
length between the tuned input network and the cathodes of the 3-500Z. Changing
that cable from 52 ohms to 18 ohms increased efficiency from 30% to 60% on ten
meters, and improved IM3 from -31 dB PEP to -38 dB PEP on fifteen meters. The
reduction in cable impedance added capacitance to the tube cathodes and
significantly lowered cathode impedance on harmonics. The reduced cathode
harmonic impedance sharpened the transition into and out of conduction, and
greatly reduced even-order harmonics in the cathode system.

This tuned input is
a high-pass “T”
network. It
presents a variable,
but generally high,
impedance to the
cathode at
harmonics.
The harmonic
frequency impedance
at the cathode is
greatly affected by
the length of the
cable between the
amplifier and the
exciter, and by the
exciter’s “reverse
impedance” at
harmonic
frequencies.

Amplifiers using
this type of input
will be critical for
cable length between
the exciter and
amplifier. This
is because the
length of the coax
between the exciter
and amplifier
has a very large
effect on cathode
impedance on
harmonics. The
harmonics can
also back-feed the radio,
causing an apparent
high SWR even though
the fundamental
frequency is
matched!

This system is a
low-pass “T”
network. Even though it is a
low-pass, it presents a
high impedance to
the tube on
harmonics. Because
it is a low-pass
filter, cable length
and exciter output
port “reverse
impedance” have very
little effect on
amplifier
performance. While it
sometimes works and
does make the
amplifier immune to
changes in input
cable length, it
can also cause a
reduction of
efficiency and an
increase of IMD.

This is the best
system for tuned
inputs. It presents
a low impedance on
harmonics,
especially the even
harmonics, at the
tube cathode. This
improves efficiency
and IM performance,
or at least
eliminates the
chance of problems
in efficiency and
IMD performance as
exciters and cable
lengths external to
the amplifier are
changed.

The input network
should be installed
as close to the tube
as possible, with
less than one foot
of lead at ten
meters. If the input
circuit has to be
installed remote
from the tube, C7
can be moved to the
cathode area or the
coaxial line between
C7 and L1 can be
reduced to a very
low impedance. Using
a low-impedance line
has the same effect
as moving capacitor
C7 closer to the
cathode.

Input Impedance

The driving impedance of a cathode driven amplifier always
varies some amount with power level and tuning. We can say the input impedance
of a cathode driven stage is dynamic,
with the
tube’s driving impedance changing with parameters like drive power and how the
amplifier is loaded or tuned.

The input impedance changes dynamically because the output
system is in series with the input system. The same plate current flows through both
cathode and anode systems. Varying anode load impedance, because the same very
same current flows through both, changes
input impedance. In general the lower tube mu, the greater the effect of
output tuning and power level on driving impedance. This is because a low-mu
tube generally has a high driving impedance, making cathode impedance a
larger percentage of anode impedance. The higher cathode impedance increases negative feedback,
and increased negative feedback causes the input impedance to vary more with
anode load impedance changes. The tuned input’s resonance or “flywheel
effect” does not stabilize this impedance variation, but
additional losses brought into the cathode system by tuned input circuit components
helps dilute
or “swamp out” dynamic changes in driving impedance. An attenuator pad
would do the same of course, but would not filter harmonics or sharpen
conduction point transitions.

The dynamic cathode impedance, where cathode impedance changes
with drive level, prevents us from accurately adjusting the tuned input system of a
typical cathode driven amplifier at very low power. This prevents accurate use
of an antenna
analyzer to adjust the tuned input while using the operating PA tube as a load. The input system should always be adjusted near full output
power under normal operating conditions, or with a suitable dummy load replacing
the tube’s dynamic impedance.

A reasonable approximation of tube input impedance, for
circuit adjustment purposes, would be a small non-inductive resistor from each
cathode pin to a control grid pin at each tube. This resistor should be the
approximate value of the tube input resistance when driven near full power with
normal tuning and loading. Filament power and HV should be off and the tubes
left in place, and the input driven normally as if the amplifier was running but
with a low power antenna analyzer substituting for the the normal exciter.

For various tubes the following cathode resistances are used
at each tube:

Using Tuners in
Radios, or External
Tuners, for Matching

• If the
  amplifier contains
  a proper low-pass
  tuned input, and
  impedance is close
  but not quite
  right for your
  exciter, there is
  nothing wrong with
  using an external
  tuner.

• If the input
  network is
  missing, or is a
  high-pass network, relying on an external tuner might cause
  IMD or efficiency problems. There is no guarantee the system will have a
  problem, but there is also no guarantee the system will not have a
  problem.

• Without a proper tuned input, system behavior will vary with the exciter
  type, the band being used, cable lengths, and other variables.