Interesting Dave to hear what can go wrong in spec measurements. Here a long
forgotten idea to simplify routine measurments:
For routine measurements, has anyone of you tried the very simple instrument
for routine measuring the total rms signal-to-intermodulation distorsion
ratio described on page 359 in the old very good stand-by on SSB: SINGLE
SIDEBAND PRINCIPLES AND CIRCUITS by Pappenfus, Bruene and Schoenike?
Published in 1964 by Mac Graw Hill and later on in a paperbound edition, I
believe?
One single reading represents the sum of all distorsion products at any
given power level similar to audio distorsion meters. I will now do my best
to describe this very simple instrument and partly quote from the book:
You feed your twotone signal into a product detector via an attenuator and
mix it with a fixed crystal oscillator with a frequency off about 25 to 50
cps off the frequency that would be midway beetween the two test
frequencies to avoid an lf beat in the meter indication. After the mixer you
can choose beetwen going straight to your rms meter that has an attenuator
before it or via a simple high-pass filter that separates all the distorsion
products from the two signal frequencies.
You can then compare the sum of all distorsion products to the sum of all
signals since the power in the distorsion products is such a small fraction
of the signal power that it can be safely neglected. As an example the book
talks about a signal of say, 20 db total distorsion power below the signal
power, the error is only 1 per cent in power or 0.04 db!
I hope this idea attracts all the future DAF experimenters, hi! And again,
for the third or fifth time: is it not a shame that so many good ideas will
be forgotten unless someone saves it in a book or on a CD?
Hope that you do not have a hangover after this fantastic millenium
happening? In Stockholm we had the biggest firework ever and everybody was
out in streets despite the minus 10 centigrade.
Prosit de Hans SM5KI
>Från: Dave <dhaupt@bewellnet.com>
>Till: amps@contesting.com
>Ämne: [AMPS] DAF Spectrum plots
>Datum: lör 1 jan 2000 17.55
>
>
>Just an observation from looking at
>
>http://home3.swipnet.se/~w-30548/g2daf/pa5.gif
>
>I believe the spectrum analyzer controls were set improperly. The
>resolution bandwidth was set for 1 kHz. This is too wide for properly
>analyzing an SSB signal produced by a male voice. The male voice
>fundamentals are below 100Hz, and thus the spectrum is a sequence of
>tones separated by about that amount (or double, depending on the
>existence of odd and even order overtones). These tones are not visible
>in the display.
>
>Remember that a spectrum analyzer's RBW filter is not a receiver
>filter. A spectrum analyzer's resolution bandwidth filter is Gaussian
>in shape, so as to preserve pulse fidelity. As a result, it has very
>gently sloping skirts. One kHz in this case represents the 3dB down
>points. The apparently-triangular shape in the spectrum analyzer plots
>is the shape of the spectrum analyzer filter. As you can see, the
>filter is so wide that it covers many of the probable IMD products. My
>suspicion is that an RBW of 100 Hz or less is needed. VBW can be set
>quite wide (>10X RBW) because VBW filtering is not needed for this
>peak-hold based measurement.
>
>Also, the noise floor appears too high. The block diagram shows 98.5dB
>of attenuation between the amplifier and the analyzer. And the analyzer
>screen shows an internal 30dB attenuator is turned on, so you have a
>total of 128.5dB of attenuation from the amplifier to the spectrum
>analyzer's mixer in the front end. If the amplifier is delivering 1kW,
>or +50dBm, then the analyzer's mixer is receiving only -68.5dBm peak
>power. While this assures you won't blow the front end of the analyzer,
>it also means you'll be noise-floor limited. Better to set the mixer
>input level between -20 and -40dBm (at least for the 8590) to improve
>the dynamic range of the analyzer. Also, an investigation should be
>made of what the real attenuator values are. Based on the analyzer
>display, the peak power delivered by this particular amplifer is +66dBm
>or 4kW. I can't tell from the photographs what tubes are used in this
>EBK-1, but could they possibly produce 4kW peak? So it would appear
>something is amiss in the attenuator chain.
>
>Remember, when you're doing IMD measurements, you have not achieved a
>useful result until you can actually see the IMD. All this plot shows
>is that the IMD is below the 8590 filter skirts. Whether that is
>acceptable IMD or not is not certain. Let's say there was an IMD
>product 20dB down from peak and in the next adjacent channel (3kHz
>away). It could not be seen on this plot because the filter skirt is
>covering it.
>
>Unfortunately, all my ham gear is stowed, otherwise I'd pull out a rig
>and pop it on an analyzer and see what sort or settings seem to be a
>good compromise between speed and display of IMD.
>
>I'm making no comments on the G2DAF circuit itself. I've been following
>the thread with a bit of curiosity and admit some temptation to build
>one just to see what it can do. But the reality of a job with lots of
>travel and fixing up a sixty year old home in California tend to put a
>serious clamp on hobby activities!
>
>Regards,
>
>Dave W8NF
>
>
>--
>FAQ on WWW: http://www.contesting.com/ampsfaq.html
>Submissions: amps@contesting.com
>Administrative requests: amps-REQUEST@contesting.com
>Problems: owner-amps@contesting.com
>Search: http://www.contesting.com/km9p/search.htm
>
>
----------
>Från: Dave <dhaupt@bewellnet.com>
>Till: amps@contesting.com
>Ämne: [AMPS] DAF Spectrum plots
>Datum: lör 1 jan 2000 17.55
>
>
>Just an observation from looking at
>
>http://home3.swipnet.se/~w-30548/g2daf/pa5.gif
>
>I believe the spectrum analyzer controls were set improperly. The
>resolution bandwidth was set for 1 kHz. This is too wide for properly
>analyzing an SSB signal produced by a male voice. The male voice
>fundamentals are below 100Hz, and thus the spectrum is a sequence of
>tones separated by about that amount (or double, depending on the
>existence of odd and even order overtones). These tones are not visible
>in the display.
>
>Remember that a spectrum analyzer's RBW filter is not a receiver
>filter. A spectrum analyzer's resolution bandwidth filter is Gaussian
>in shape, so as to preserve pulse fidelity. As a result, it has very
>gently sloping skirts. One kHz in this case represents the 3dB down
>points. The apparently-triangular shape in the spectrum analyzer plots
>is the shape of the spectrum analyzer filter. As you can see, the
>filter is so wide that it covers many of the probable IMD products. My
>suspicion is that an RBW of 100 Hz or less is needed. VBW can be set
>quite wide (>10X RBW) because VBW filtering is not needed for this
>peak-hold based measurement.
>
>Also, the noise floor appears too high. The block diagram shows 98.5dB
>of attenuation between the amplifier and the analyzer. And the analyzer
>screen shows an internal 30dB attenuator is turned on, so you have a
>total of 128.5dB of attenuation from the amplifier to the spectrum
>analyzer's mixer in the front end. If the amplifier is delivering 1kW,
>or +50dBm, then the analyzer's mixer is receiving only -68.5dBm peak
>power. While this assures you won't blow the front end of the analyzer,
>it also means you'll be noise-floor limited. Better to set the mixer
>input level between -20 and -40dBm (at least for the 8590) to improve
>the dynamic range of the analyzer. Also, an investigation should be
>made of what the real attenuator values are. Based on the analyzer
>display, the peak power delivered by this particular amplifer is +66dBm
>or 4kW. I can't tell from the photographs what tubes are used in this
>EBK-1, but could they possibly produce 4kW peak? So it would appear
>something is amiss in the attenuator chain.
>
>Remember, when you're doing IMD measurements, you have not achieved a
>useful result until you can actually see the IMD. All this plot shows
>is that the IMD is below the 8590 filter skirts. Whether that is
>acceptable IMD or not is not certain. Let's say there was an IMD
>product 20dB down from peak and in the next adjacent channel (3kHz
>away). It could not be seen on this plot because the filter skirt is
>covering it.
>
>Unfortunately, all my ham gear is stowed, otherwise I'd pull out a rig
>and pop it on an analyzer and see what sort or settings seem to be a
>good compromise between speed and display of IMD.
>
>I'm making no comments on the G2DAF circuit itself. I've been following
>the thread with a bit of curiosity and admit some temptation to build
>one just to see what it can do. But the reality of a job with lots of
>travel and fixing up a sixty year old home in California tend to put a
>serious clamp on hobby activities!
>
>Regards,
>
>Dave W8NF
>
>
>--
>FAQ on WWW: http://www.contesting.com/ampsfaq.html
>Submissions: amps@contesting.com
>Administrative requests: amps-REQUEST@contesting.com
>Problems: owner-amps@contesting.com
>Search: http://www.contesting.com/km9p/search.htm
>
>
--
FAQ on WWW: http://www.contesting.com/ampsfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|
