[HBR] HBR2K -- Chapter 14 -- Large Signal Performance, Part 6

[hbr@mailman.qth.net]

> In the next few days I'll test my combiner, which is the *handbook* 
> device.   (Should be easy  -- just compare third harmonic output from the
> combiner vs. a 6 db attenuator, using a general coverage receiver.)    If
> it's not good I'll either improve it or look for a commercial combiner. 

"It's the combiner, stupid" -- or so it seems.

I couldn't, of course do the proposed third harmonic test because the 
signal generators have tons of third harmonic energy.  But I set the 
generators to 3.5 and 6.5 Mcs and ran them through the combiner to 
the receiver, tuning it to 10 Mcs.   Wow ... at full generator output of 
100,000 uV with 28 db of attenuation after the combiner, I had an S-9 
distortion signal at 10 Mcs.   Adding 20 db attenuation to each 
generator *ahead* of the combiner dropped the signal nearly to 
inaudibility, even with *no* attenuation after the combiner.   So the 
thing is wildly non-linear at power levels below +6dbm.   

Where's the bag of toroids?   By stacking cores I had in twenty 
minutes a bifilar transformer with 2-1/2 times the core area and in a 
couple minutes more, a combiner that is 27 db cleaner at 100,000 
uV, giving an S9 distortion signal with 1 db of post-combiner 
attenuation, rather than 28 db.   *That* should make better 
measurements possible ... we'll see.  

I'll order some larger cores later today; it ought to be possible to 
make a combiner that will handle the full generator output with little 
or no non-linearity.   The price will be high end performance -- the 
*Handbook* design I was using is supposed to be okay to around 50 
Mcs and with more distributed capacitance a high power unit 
probably won't get there.   But then, I don't *want* to go there.

Walt Hutchens
KJ4KV