> I asked:
> >I'm contemplating making the tuning automatic. Does anyone have any
> >ideas of how it can be done ? I faintly remember seeing something
> about
> >adjusting the tuning until the anode impedance is completely real
> (i.e.
> >at resonance)...
>
> John Lyles wrote:
>
> Yes, the text you want to take a look at is "SSB Circuits and
> Systems",
> 1964, by Pappenfuss, Bruene, and Schoenike, (all of Collins Radio).
> This
> classic book has several chapters covering linear amplifiers, with
> special
> emphasis on tetrodes with distortion reduction techniques. The
> autotuning
> technique of using samples of the grid tank current and plate voltage
> to
> drive a servomotor is discussed on page 166.
>
> It is stated there that you could also look at peaking screen current,
> to
> indicate maximum load impedance on the tube(s). This would indicate
> also
> maximum plate voltage swing. But low Q output circuits may exhibit
> errors
> of 10 - 20 degrees of phase shift off resonance. Also, tubes that have
>
> secondary emission on the screen can run negative current (we have all
> seen
> this!!), and would make such circuitry useless.
>
> The preferred method to adjust a resonance tuning capacitor (or
> inductor)
> according to the authors is to use phase detectors made up of diodes
> in a
> bridge. To also autotune the loading of the amplifer, one must assume
> linear tube operation, and set another bridge with diodes to detect
> the
> ratio of ep to eg. In this case, the fixed relationship is set (proper
>
> output impedance value = correct loading) by a servomotor driven from
> this
> bridge, which controls the loading control (output capacitor in a Pi
> network).
>
> On page 217 is one such autotuned amplifer, a 2 - 30 Mhz two stage
> tetrode
> feedback amplifier with (3) 4CX250B family tubes in parallel, driven
> by (2)
> 6CL6's. Collins Radio sampled the grid RF voltage and the plate
> voltage. In
> a properly tuned amplifier, these are 180 degrees out of phase. On
> page 219
> is an even more complex beast, a three stage 45 KW amplifier, again
> autotuned throughout using the same principles.
>
> Some other references on autotuning:
> V. DeLong, "Automatic Tuning Techniques for SSB Equipment, Proc. IRE,
> pages
> 1766-1774,Dec. 1956.
> V. DeLong, "Automatic Tuning for High Power Transmitter, Electronics,
> pages
> 134-137, July 1956.
>
> You do NOT want to look for plate current dip, with a computer
> control. For
> one thing, it is not level independent. If the drive level changes,
> then it
> looks like a dip might be occuring. Plus, you don't want your servos
> to be
> starting from off-resonance as you power up, and having to track
> across and
> find the dip. (quick way to make smoke and noise) I have done the
> converse
> of this using an HP Basic series 9000 workstation once, driving a
> sweep
> oscillator which fed power into a resonant cavity. I wanted to know
> the Q
> automatically for a dielectric measuring automatic tester. The
> algorithms
> must look at the slope of the peak (or dip) and determine when you are
>
> parked on the top of the curve (or bottom). Nowadays, Hewlett Packard
> has
> built-in firmware to read Q and bandwidth automatically in their
> network
> analyzers. These computer controlled algorithms are good when there is
> only
> one distinct resonance in the frequency span being measured, and when
> no
> noise or stray modes (resonances) show up on the side.
>
> Grid current peaking is a simple way to adjust an input circuit to a
> power
> amplifier, again we must be careful of noise, and allowing the
> amplifier to
> run across a range of input tuning to find this.
>
> FOR ANTENNA MATCHER AUTOTUNING (FEEDBACK):
>
> The later edition of "SSB Systems and Circuits" by Sabin and Schoenike
>
> (Collins Radio) is probably still available from McGraw Hill Inc. Mine
> is
> dated 1987. Since most transmitters are solid state by that time, the
> emphasis is not so much on autotuning in power amplifiers, but in
> antenna
> couplers. Chapter 16 discusses various automatic techniques, with HF
> discriminators for measuring the phase and loading. All HF
> practicioners
> planning to develop autotuners own themselves a look at this chapter.
>
> Reference:
> H. Landt, "Tracking Impedance Measuring System", US Patent 4,506,209.
> 1985.
>
> PRESET AUTOTUNING METHODS:
>
> Then there are the techniques using look up tables, as you mentioned.
> Of
> course, my ICOM AT500 does this with little preset pots. And a number
> of
> other amateur and pro manufacturers use variants on this idea.
> It's cheap and easy, as long as the load never changes.
>
> Delta Electronics made an HF tuner (some of you may have them - the
> Delta
> HFAS-4) for 10 KW PEP, to load a long whip or wire. It used preset
> pots
> also, on little plug in cards. When a station changed freq., the card
> was
> swapped out, or switched out, to a different set of pots, with
> corresponding different servomotor positions on a large inductor and
> vacuum
> variable capacitor. These types of systems, as you would guess, are
> only
> useful for an antenna or TX with a constant load. No bird nests or
> icing
> problems please. And would require adjustment for different
> frequencies
> also, although a straight line interpolation is usually possible, and
> can
> be automated, to track the frequency of the exciter. Using a look up
> table
> in EEPROM is the modern day analogy of the analog memories of 20 years
> ago
> (potentiometers).
>
> CONCLUSION:
> I personally would use the phase feedback methods if I had to
> autotune.
> BUT, I like to manually fiddle with the tuning and loading on power
> amplifiers, as long as it's not too early in the AM, and I have had my
>
> coffee. It sort of gives me a feel for the thing, and for any abnormal
>
> conditions, such as the quick surge as you tune across some parasitic
> oscillation, or the crackle of a melting fingerstock followed by a
> loud
> bang. The first time I tuned the plate tuner (sliding short) in our 3
> MW
> 200 MHz triode amplifiers, while operating near plate dissipation
> limits,
> was an unnerving experience. Took some getting used to: Tap the motor
> drive
> switch, watch three meters and scopes with only two eyeballs
> simulataneously, and then realize it went the wrong direction! I'm
> sure the
> folks at VOA and other high power TX sites had similar worries before
> autotuners. When your bottle costs over $100K, you don't want to see
> it
> burst in smoke and noise!
>
> John
> K5PRO
Thanks for that, I'll chase the references. Since my PA is a triode,
note a tetrode, screen current is not an issue. I have directional
couplers in the output (already in place) and input (not yet made), so
measuring phases of the drive relative to the output will be simple
using a mixer, with the input and output drive applied to the RF and LO
ports of a mixer. Measuring the relative phases at the tube terminals
(cathode and anode) will be more difficult. This is all clearly a lot
easier at HF, where the phase shifts down short lengths of cable is
virtually zero. At VHF this will not be true, and will be frequency
dependant, which may mean the system needs an elaborate calibration
technique - again fairly easy with a computer, but I dont fancy that.
--
Dave Kirkby G8WRB
Department of Medical Physics,
University College London,
11-20 Capper Street,
London,
WC1E 6JA.
Tel: 0171 209 6406 (International +44 171 209 6406)
Fax: 0171 209 6269 (International +44 171 209 6269)
--
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