[hpsdr] IMD in Ham transmitters

Helmut, DC6NY dc6ny at gmx.de
Sun Jan 20 00:27:34 PST 2013

Hi guys,

first of all you have to know at least the -1 dB compression point of your
amp, better the transmission characteristic. Makes no sense to measure in
the area of saturated power. This defines your max. PEP-power. With 2 tones
this level is already achieved with -6 dB power per tone. This has nothing
to do with the distance to the IM products. They are defined as distance in
dBc relative to the two tone levels.

73, Helmut

-----Ursprüngliche Nachricht-----
Von: hpsdr-bounces at lists.openhpsdr.org
[mailto:hpsdr-bounces at lists.openhpsdr.org] Im Auftrag von John
Gesendet: Sonntag, 20. Januar 2013 01:50
An: Keith Gooley
Cc: hpsdr at lists.openhpsdr.org
Betreff: Re: [hpsdr] IMD in Ham transmitters

***** High Performance Software Defined Radio Discussion List *****

Years ago I used to measure IMD on Collins equipment.  Their specs at 
that time were always in dB below each of two equal tones.  If you think 
about it the peak of the envelope with only two equal tones is twice 
each tone peak value, so Collins spec was more conservative than other 
manufacturers of that time.

In those days the spectrum analyzer and two tone oscillators took up a 
full single bay rack (on wheels) called a 478R-1.

Sorry I incorrectly sent my answer to your post as an answer to Lyle's 
in my last Email.

John  WoGN

On 1/19/2013 4:38 PM, Keith Gooley wrote:
> ***** High Performance Software Defined Radio Discussion List *****
> Hi All,
> If the commercial suppliers are claiming an extra 6 dB for PEP then 
> that doesn't seem right in my view. The IMD level increases at a rate 
> greater than the increase  in power, 3rd order IMD which is mostly 
> what we are talking about, increases at 3 times the rate of increase 
> of the 2 signals generating the IMD product. So if the output power is 
> doubled (3 dB increase) the IMD increases by 9 dB. Hence the IMD level 
> relative to the main signal is 6 dB worse.
> 73 de
> Keith

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