[AMPS] 2 tone intermod test

[John Fielding]

Hi Dave,

I would like to second Rich's comments and just mention that the subject was aired
extensively a while back on this list.  G2DAF amplifiers et al.  Ian White G3SEK and
others (myself included) have performed many 2 tone tests on "seemingly good" 
amplifiers but has found that speech is a real killer when it comes to intermod and
splatter. 

The basic problem is that as Rich correctly states is that the traditional 2 tone test 
(which incidentally is performed by summing 2 audio tones of, say,1000Hz and 1500Hz
together of equal amplitude and feeding into the mic input of your tcvr, the tones must
NOT be harmonically related) does not adequately exercise the system.  (My ARRL 
handbook contains a 2 tone generator circuit for this test). 

Using such an input stimulus tends to make non-linear portions of the amplifier (screen
supply or grid supply voltage stabilisers etc.) go up and down slowly and so tends to
mask the sudden variations which speech - or Rich's African Grey parrot - does sooooo
much better.  

At work we use the 2 tone test method as it is well accepted as a "true test" but it
helps to hide the "funnies" which we don't want the customer to find out about in our 
systems!!!!!

A far better test is that used for TV transponders which uses 3 unequal level tones or
carriers, this really gives the amplifier a good workout and shows up any
non-linearities.

My 2 cents worth.

John	ZS5JF

----------
> From: measures <measures@vcnet.com>
> To: Dave D'Epagnier <DAVED@ctilidar.com>; 'amps@contesting.com'
> Subject: Re: [AMPS] 2 tone intermod test
> Date: Tuesday, November 23, 1999 8:38 PM
>
> The 2-tone test does not simulate genuine rotten splatter.  For this you 
> need a complex waveshape.  A human or an African gray parrot is such a 
> source.  The splatter test is done by tuning a double-filter Rx (such as 
> a TS-830) about 3.7KHz above and below the test signal to isolate imd 
> products from the fundamental) and then comparing the signal strength to 
> the fundamental.  Since S-meters tend to be inaccurate, a 1db step 
> attenuator.should be used to measure relative signal strength.  
> >
> cheers, Dave
> 
> - BTW --   the only splatter that matters to the FCC is what goes beyond 
> the Amateur Radio Service band.  
> ..
> 
> -  Rich..., 805.386.3734, www.vcnet.com/measures.  
> 
> 
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