The circuit in the ARRL Handbook was intended to establish the static load-line
when tuning up,
but by adding an oscilloscope it could also be used for indicating dynamic
linearity.
Similar circuits were used in many manually and servo-tuned amplifiers.
73/
Karl-Arne
SM0AOM
----Ursprungligt meddelande----
Från: audioguy@q.com
Datum: 2013-11-03 15:24
Till: "Karl-Arne Markström"<sm0aom@telia.com>, <g8on@fsmail.net>,
<amps@contesting.com>
Ärende: Re: [Amps] Measuring IMD
A similar linearity-detecting scheme was used in a 3-1000Z design from
the 1970 ARRL Handbook. "The SS-70 2-KW Amplifier", pp 204-208.
It used diode RF detectors in the cathode and plate circuits, a
"balance" pot and a 50-0-50 uA front-panel meter.
I have no idea how well it worked, but this thread rang a bell and I had
to go look it up. It would be interesting to try it. If the mechanical
meter turned out to be inadequate, a connector for an external scope
could be added.
73,
Jim N7CXI
On 11/3/2013 3:10 AM, Karl-Arne Markström wrote:
> It is sad to read about the death of Warren Bruene. One of the really great
> in RF engineering have left us.
>
> Regarding the use of linearity monitoring with diode detectors at the input
> and output, I recall this being a part of the
> servo-tuning system used in the AEG-Telefunken 20 kW transmitters that I
> worked with in the 80' and part of the 90's.
> (Picture at the bottom of this page: http://www.seefunknetz.de/sdj.htm)
>
> By first tuning for resonance in all circuits, then loading for the proper
> load line in the PA and finally adjusting the drive level
> to a point just below where the transfer function started to be curved, a
> good linearity (at least - 35 dB IM3 relative one tone) was assured.
>
> It also was part of the envelope-feedback linearisation system that made the
> last few dB's improvement of IMD suppression possible.
>
> By using this form of linearity monitoring, the most offensive cases of
> overdrive or misadjustment are possible to identify at a glance.
> More accurate measurements require a spectrum analyser or a selective level
> meter capable of measuring adjacent channel suppression either with
> multi-tone driving signals or noise.
>
> 73/
> Karl-Arne
> SM0AOM
> 73/
> Karl-Arne
>
> ----Ursprungligt meddelande----
> Från: g8on@fsmail.net
> Datum: 2013-11-03 11:28
> Till: <amps@contesting.com>
> Ärende: Re: [Amps] Measuring IMD
>
> A very simple method which is adequate for most amateur use is to couple
> diode detectors to the input and output of the amplifier. The input detector
> goes to the X input of an oscilloscope and the output to the Y input. Adjust
> the coupling to get more or less equal voltages. The oscilloscope only needs
> to work at audio so an old tube Heathkit scope will do! When the line on the
> screen as you talk is straight, the linearity is adequate. With a 2 tone
> test, you can see by the curvature of the line such things as too low a
> standing plate current, poor grid circuit regulation and overdrive, and by
> making voltage measurements on the graticule, you can calculate 3rd and 5th
> order distortion. See the 1964 edition of 'Single Sideband Principles and
> Circuits' by Pappenfus, Bruene (who died the other month) and Schoenike.
>
> With a solid state PA, a spectrum analyser is really needed because you have
> to consider the much worse high order IMD products (7,9,11) which tend not to
> be negligible. Even with LDMOS, which is as bad as bipolar on linearity.
>
> 73
>
> Peter G3RZP
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