On Wed, 23 Jul 1997 06:18:21 +0000 w8jitom@postoffice.worldnet.att.net
writes:
>> From: km1h@juno.com
To: <amps@contesting.com>
>> Date: Tue, 22 Jul 97 21:17:19 +0000
>
>> Perhaps someone would care to detail a proper test setup for
>evaluating
>> IMD with professional equipment available to many hams. By that I
>mean
>> test equipment from the 1960-80 era that is available at flea
>markets but
>> is still adequate. Never mind the cheap hams that believe life ends
>at
>> Radio Shack or the local Walmart or the other extreme being a modern
>HP
>> equipped lab.
>>
>> 73...Carl KM1H
>
>Hi Carl,
>
>First let me say I agree with Ian. Screen regulation is important, as
>well as adding a little negative feedback via an unbypassed cathode
>resistor.
Ian and I have agreed on the screen supply since our first chat a year
ago.
The problem with the cathode resistor Tom is that it does not adapt very
well at VHF and above.
The 20 Ohm or so cathode resistor does a credible job at audio and HF but
becomes a disaster at VHF when the criteria is a zero Ohm cathode ground
at DC AND RF in order to maintain stability.
Before you go off on a tangent...just remember that NO military or cavity
tetrode amp that I have ever seen used a cathode resistor. That stopped
at the 6360 or so power level.
Also note that this discussion STARTS at 144 MHz and goes up from there
and represents real world ham tubes....not 10KW.
>
>You can do IMD tests simply at home with a good selective receiver,
>and attenuator pad, and TWO separate transmitters. As you know I
>prefer a three tone test, since even a class C PA can look good in a
>two tone test.
>
>Couple the well shielded receiver to the dummy load via a pick up
>probe in the load, and put the well shielded attenuator pad at
>the receiver input.
I have a pair of TS940's and a nice 51J4 with all filters. Is this
adequate? Or would I be better to use another TS940....I have 4 of them,
all with IRCI filters? Phase noise issue? .
I also have a Ballantine 300 RMS AC Voltmeter and a HP 3400A version to
eliminate S Meter error; simple to plug into the audio line. Ancient but
damn accurate.
I am not against spending a few Pesos to build a jig that would yield
real results. NO...not for 10KW tubes but a few of the popular 400-1000W
(each) tubes. I think you know Tom that my primary interests these days
are VHF and above at serious power. Maybe in November I can get warmed up
to 160M again.
>Feed the PA with two signals of equal level with a known spacing (say
>two kHz) and tune the receiver to each of the two tones. Add enough
>pad, over 50 dB, to get the receiver down to some reference level
>on the S meter.
>
>Take the pad out and tune in the third order signal (it will be two
>times one frequency minus the other one), add enough pad until the S
>meter reads the same S reading as the carrier. The difference in
>attenuation is the IMD below one tone of a two tone test, and that is
>the commercial standard. The amateur radio standard is you add six dB
>to the results, making everything look better.
>
>A three tone test is normally accomplished by modulating the drive
>levels with a third low frequency signal, but it is difficult to read
>an S meter doing this.
>
>Repeat the same procedure for higher IMD products.
>
>By the way, this method, if you can shield the receiver well enough,
>can be more accurate than most lab analyzers. Many receivers have
>better gain compression specs and lower IM distortion than common
>spectrum analyzers.
>
I have noticed that my 51J4 hears absolutely nothing with the antenna
port terminated. The TS-940 is good but not like the old Collins.
A few guys have suggested the HP141T system but I have heard all horror
stories about them. Any comments?
I have a nice HP8568B available at times but that is pure overkill for my
needs. I dont mind spending a few $$ for something that will give me a
few years of trouble free use. From what I hear, the HP141T is unreliable
and parts are no longer available.
What about some of these off-shore DC-1300MHz Spectrum Analyzers? Any
experience or comments?
Tnx agn Tom...
73...Carl KM1H
>73, Tom W8JI
>
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