All you need is a coupler of some sort.
It does not have to have a flat frequency response nor expensive.
In the old-days we made directional couplers for homebrew SWR meters by
stripping the outer insulation off some coax and slipping some enameled wire
under the shield. One end was terminated with a resistor ( in your case 50
Ohms) and
the other end was the output. You will only need a few inches of enameled wire
to
get enough power into your spectrum analyzer to make your measurements.
It will not be calibrated in terms of attenuation nor will it be flat but good
enough.
Also a simple capacitive probe would work as well. But it also needs a 50 Ohm
terminator right at the probe to match the input impedance to your input
attenuator.
If it is a mismatch the spectrum analyzer's input attenuator will not be
accurate.
73
Bill wa4lav
________________________________________
From: Amps [amps-bounces@contesting.com] on behalf of Robert Groh
[rgroh@swbell.net]
Sent: Monday, November 04, 2013 10:11 AM
To: Dave Trainor; amps@contesting.com
Subject: Re: [Amps] Measuring IMD
Dave,
You definitely want to reduce the power input to the spectrum analyzer down to
something in the 0 dBm to +10 dBm level. I am not familiar with the HP 8924()
family but I have had, in my prior life as a radio design engineer (I am now
retired), extensive experience with spectrum analyzers, IMD measurement, etc.
The required power attenuation can be obtained in a number of ways - use a
'probe' (e.g. like Bird wattmeter's do), use small capacitor for decoupling,
use a power attenuator, etc.
The probe, capacitor, etc techniques are frequency dependent (e.g. the output
of the sampler decreases as frequency does down) but that won't affect the
accuracy of your IMD measurements because your are measuring IMD over a very
small frequency range (i.e. a couple of kHz).
If you use a good power attenuator (e.g. Bird) then you don't have to worry
(much) about frequency variations at all. As a side note, you can pick up high
power 50 ohm attenuators (on eBay, etc) for not a lot of money - I have a 40
dB, 250W Bird attenuator I got for $60 or so - a 40 dB attenuator is a darn
decent load all by itself and the 250W rating is a continuous duty rating - the
peak rating is determined by the voltage breakdown of the connectors and load
which is quite a bit higher than 250W (figured that out once but have forgotten
the details).
If you designed the system for (example) 2,000 watts and want a maximum input
signal to the SA of 0 dBm then you need 63 dB of attenuation (i.e. 2,000 watts
= +63 dBm = +33 dBw).
As a side note, I would worry a bit about the 'linearity' of the Heathkit load
when being pounded as you are talking about doing.
Much more to say but I won't burden the forum with it at this time. If you
have any other questions, you can contact me directly and I'll be glad to give
you some more info. You can also call via landline if you want (my address is
in the Callbook)
73
Bob Groh, WA2CKY
________________________________
From: Dave Trainor <dave@n8zfm.com>
To: "amps@contesting.com" <amps@contesting.com>
Sent: Sunday, November 3, 2013 8:51 PM
Subject: Re: [Amps] Measuring IMD
So here is a newby question, Let's say I have a HP8924C SA and it has a max of
60W input, and I want to check IMD on a 1.5KW amplifier. Do I do this with a T
connector in the line to the dummy load? Or do I need to simply pick off a
sample of the RF with a mica cap from the dummy load? I have one of the old
"Cantenna" dummy loads I use and could add something to pick off a sample. What
I don't want to do is somehow burn out my analyzer by making a silly mistake.
I usually use it for repair/alignment work on VHF and have never tried to
measure IMD on an HF amp but since I am in the process of collecting the parts
I need for a tetrode amp, measuring the IMD to be sure it's clean is now
something I'd like to do, and I am not quite sure how to safely measure that
amount of RF, or if picking off a sample would not be a valid testing method.
73 - Dave N8ZFM
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of peter chadwick
Sent: Sunday, November 03, 2013 4:26 PM
To: amps@contesting.com
Subject: Re: [Amps] Measuring IMD
The reference given to Pappenfus gives the equations for determining the level
of 3rd and 5th order products from the curvature of the line.
Noise testing is good, provided you are sure about the peak to average ratio:
to that end, noise with a notch in the middle of the AF passband leading to
measurement of the IMD power in that notch is good. The technique was used over
50 years ago in measuring FDM telephone systems, and was applied to
multichannel HF SSB tx's.
The question at the end of the day is 'How much use is the information about
more accurate levels of IMD measurement except as an academic exercise?' More
to the point is the IMD power in a SSB bandwidth at various offsets - as SM5BSZ
has measured. Plus the fact that the vast majority of amateurs neither
understand or care about how bad their tx is to the last few dB!
73
Peter G3RZP
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