-----Original Message-----
From: Steve Harrison <ko0u@os.com>
To: Steve Thompson <rfamps@ic24.net>
Cc: amps@contesting.com <amps@contesting.com>
To: <amps@contesting.com>
Date: 15 February 2000 05:23
Subject: Re: [AMPS] IMD Question
>At 11:27 AM 2000-02-15 -0800, Steve Thompson wrote:
>>On one amp I did a comparison between long hand measurements and the
network
>>analyser on amplitude sweep. I got good agreement.
>
>You have to be very careful when using any kind of broadband sweep
>generator to measure gain compression, as many of the older ones do not
>have good harmonic suppression. Some later models utilize PIN diode LPF
>switching to reduce harmonics from the generator; but when the test results
>are critical to the project, one should always check the generator for
>harmonic energy. For example, the good old standby YIG oscillators used in
>the 8350-series sweeper microwave plugins typically show as much as -20 to
>-25 dBc harmonic energy. A network analyzer with a tuned receiver will
>reject these responses; but at -20 dBc harmonic energy into the
>Device-Under-Test, gain compression can easily appear soft.
Yes, spectral purity is rarely top of the spec. list. In the case in
question, there was a driver amp with lots of selectivity but I've been
fooled in the past just as you describe.
>
>It never hurts to be cautious and certain.
For sure.
>
>>I've not noticed that FETs generate more odd order harmonic than other
>>devices.
>
>See the application notes in the older Motorola RF Transistor handbooks for
>examples of higher 3rd and 5th order distortion by FETs than bipolars
I do see the difference in odd order IMD products between FETs and bipolars,
but I've not picked it up as a general trend in actual harmonic levels in
either tuned or broadband circuits. I'll be watching from now on...
Steve
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