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Author Topic: Question: Two-Tone xmitter test methodology plz  (Read 732 times)
KC9KEP
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Posts: 208


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« on: January 08, 2010, 08:28:04 AM »

Hello all,

I hopefully plan to do some two-tone testing of my
all-tube transmitter when it is complete.

I have been reviewing articles about two-test methodology
from past issues of ARRL handbooks.

Frequently, the signal set up is as follows:

- Two-Tone Generator source
- Transmitter
- Pickup-unit connected to output of transmitter, routed to a scope
- 50-Ohm Dummy load on output of transmitter

The pickup-unit system is frequently a few loops of wire
that are looped in conjunction with the output lead originating
from the transmitter.

This induced signal is then routed to a tank circuit for the sake of
peaking the induced signal’s amplitude.

Here’s where my question comes in.  

The signal from the pickup unit is shown routed to the VERTICAL PLATES
of the scope.

I’m not sure if many modern scopes even have vertical plates anymore
(because most scopes are now digital acquisition units with an LCD
type readout display.)

Why did they do that?  (i.e., route to the vertical plates?)

Could one simply route the induced signal to the vertical input of the scope
if the signal was attenuated enough as to not damage the front-end of the scope?

Thanks!

--Tom Nickel AKA KC9KEP
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WA3SKN
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Posts: 5559




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« Reply #1 on: January 08, 2010, 08:53:57 AM »

Just what are you trying to measure?
The "two tone test" I am familiar with consists of two separate audio frequencies at the same levels inputted via the mic jack and increasing their levels til you get sum and difference frequencies... where the non-linear amplification begins.  And you then adjust things so that your maximum audio levels do not reach that point.  You can just listen for the sum and difference frequencies on a receiver tuned-in nearby.
Note you pick the audio frequencies so that all four will be in the audio passband of the receiver.... say 800 and 1200 Hz will mix to 400, 800, 1200, and 2000 Hz at the non-linear point, all within most receiver's range.
No scope needed.

-Mike.
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VK1OD
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Posts: 1697




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« Reply #2 on: January 08, 2010, 11:09:49 AM »


Could one simply route the induced signal to the vertical input of the scope
if the signal was attenuated enough as to not damage the front-end of the scope?


There are two main ways that a two tone test is observed.

One is on a CRO, and yes, you feed a sample of the DUT RF output to the vertical input of a CRO. (No, it does not need to go directly to the deflection plates.) This method is a visual assessment of the onset of serious distortion, but does not quantify IMD products.

The other is using a narrow band measuring instrument to measure the level of each of the spectrum components. The most common instrument for this is a spectrum analyser, but I have already referrered you to an article showing how to use a HF receiver and PC sound card to perform quantitative measurments of IMD products. A Selective Level Measuring Set (SLMS) as was once used in carrier telephone systems is another instrument that suits the task.

Owen
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WW6L
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Posts: 7




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« Reply #3 on: January 08, 2010, 11:27:19 AM »

they are just using generic terminology to indicate that the signal from the device under test is routed to the vertical deflection of the oscilloscope for measurement. This is common as each o'scope may need some unique attenuation or specific attention paid to how it connected to source signal.  This may be due to voltage or grounding issues.
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KC9KEP
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« Reply #4 on: January 08, 2010, 12:10:34 PM »

OK, gentlemen, thank you for the responses.

The thrust of my question was why I repeatedly keep
reading reference to connections directly to the
"vertical deflection plates" rather than vertical
input of a scope.

I just wanted to be sure that I wasn't missing
anything.

I appreciate all the info!

--KC9KEP
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W8NF
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Posts: 53




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« Reply #5 on: January 08, 2010, 12:22:35 PM »

Before about forty years ago, most oscilloscopes could not display much past audio frequencies.  An RF signal would not make it past the front-end amplifier stages of an oscilloscope.

A transmitter often had enough RF voltage level to drive the CRT's deflection plates directly, and by feeding the signal directly to the plates, you bypassed the bandwidth-killing vertical amplifier.

I had, for a few years, only an ancient RCA "student" oscilloscope.  AC-coupled inputs, good for 100kHz max.  But I could feed the rig's output directly to the CRT plates and see all the way to 10 meters.

Dave W8NF
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W8JI
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Posts: 9296


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« Reply #6 on: January 08, 2010, 04:52:33 PM »

Before about forty years ago, most oscilloscopes could not display much past audio frequencies.  An RF signal would not make it past the front-end amplifier stages of an oscilloscope.

A transmitter often had enough RF voltage level to drive the CRT's deflection plates directly, and by feeding the signal directly to the plates, you bypassed the bandwidth-killing vertical amplifier.

I had, for a few years, only an ancient RCA "student" oscilloscope.  AC-coupled inputs, good for 100kHz max.  But I could feed the rig's output directly to the CRT plates and see all the way to 10 meters.

Dave W8NF

+++ on this reply!

One more thing of note Tom, IMO a trapezoid test would be a much easier way to see non-linearity. One or two tone generator to the audio input and one axis of the scope, and the RF output sample to the other axis.

Tom

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G3TXQ
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Posts: 1533




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« Reply #7 on: January 09, 2010, 03:14:44 AM »

As Owen said, narrow-band spectral analysis with an analyser or receiver would make it easier to quantify the distortion levels. However the 'scope can be useful in diagnosing the nature of the distortion - for example it's easy to spot the difference between "flat topping" and "cross-over distortion".

73,
Steve G3TXQ
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