Re: Searching for IMD


Ashhar, I basically did this visually, using the Scope as a quick check between the two versions.

 I am aware of the Rule of Thumb about the IIP3 being 15-16dB above the 1db compression point.  

I will do a more accurate check using the SA tomorrow when i have more time available.


On Sat, Sep 15, 2018 at 04:05 PM, Ashhar Farhan wrote:
glen, you can do this,
find out, accurately, the gain of the amps at very low input levels.
Now, keep increasing the drive level until you see the gain dip by 1 db.
that is the compression point. For bipolar device amplifiers, the IIP3 will be about 15 above that point.
- f

On Sat, Sep 15, 2018 at 10:14 AM, Glenn <glennp@...> wrote:
Post 59439 is partly incorrect.  Just after I posted mentioning the visible clipping I realised, stupid me, was measuring gain during clipping which of course compresses the result.

I deleted that post. And have attached the new plots where you can see the  gain difference between uBITX and Hayward version. This obviously partly accounts for the difference in input levels before output clipping occurs. The biasing arrangement gives different Iq also.  Gain is reduced about  4dB in the Hayward version I built over uBITX. It now accords more closely also with  his test results of 15.5dB I get 16dB at 30MHz.

Haywards paper gives some values for varying the gain to other values also.

deleted this:-
On Sat, Sep 15, 2018 at 12:16 PM, Glenn wrote:
Hi Ashhur,
I just built up a proto of both versions on vero board.

As a rough check of IP3, since I am not set up to do proper measurement of IP3, I found that the 'visible distortion' of the Hayward version occurs about 10dB higher than the uBITX version.  ie the point of clipping (negative side in this case)  
~-22dBm for uBITX
~ -13dBm for Hayward.   This would then imply IMD is better.

In my set-up I found gain to be similar and pretty flat. I used junk-box parts of mixed tolerances. I built only one side of the Bi-Amps.
uBITX:-   ~ 12dB gain, flat over the range 3-45MHz flat within 0.8dB
Hayward:- ~ 10db gain, flat over same range within 0.6db




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