[NLRS] [BC'ers] Linearizing surplus Mitsubishi 902 modules

[tosca005 at umn.edu]

Bruce, I am probably being too optimistic (simplistic) in my thinking here, 
but here goes...

The spec sheet says that the part is actually designed to work either 
nonlinear class c for FM use, or linear modulation use, so it should be 
possible.

What it says about linear modulation is:

Linear operation is possible by setting the quiescent drain
current with the gate voltage and controlling the output power
with the input power

...and also...

b) Linear AM modulation:
By RF input power P in .
The gate voltage is used to set the drain’s quiescent current for the 
required linearity.

>From the graphs in the spec sheet, it looks like optimal linearity is at 
-20 dBm input maximum, where gain stays 60 dB, so a -20 dBm signal gets 
amplified to 40 dBm or 10 watts (10,000 mW). So I would set up any test rig 
to measure a maximum of 20 watts out and set the drive to -20 dBm (0.01 mW 
or 10 microwatts) which could easily be measured with many HP power meters. 
This would be the starting condition to aim for, then make other 
adjustments to suit. The graphs of power vs. drain voltage Vdd at 900 MHz 
seem pretty linear all the way up to the graph limit of 15 v, so I would 
probably use the regulated 13.8v power supply for that lead.

Looking at the graphs in the spec sheet, it **APPEARS** to me that you want 
to clamp the Vgg carefully at 4.4v or so, the part will continue put out a 
highly linear signal. The third graph on page 4 of the PDF file is the one 
I am looking at to make this conclusion, since it comes closest to our 
operating frequency of interest, at 900 MHz, instead of the graphs at 880 
or 915 MHz. At 4.5v Vgg, the linearity is starting to diminish (graph of 
output power vs. Vgg is not a straight line), but the output is maybe 18 
watts. If dialed back to 4.4v, it looks straighter (more linear), with 
output at least 15W. **GRANTED**, this is **NOT** the graph that would best 
tell the tale. What you really want is a graph of output power vs. input 
power at a variety of Vgg levels, to see if output linearity is as linear 
with input power as it is with variation in Vgg, but my thought is that if 
the output power stops rising as the Vgg rises above 4.4v, it seems that 
output-to-input linearity would also diminish as the Vgg rises much above 
4.4v. Given the low cost of the parts, it might be worth just setting up 
one of these bricks with regulated 13.8 Vdd and a variable (but regulated) 
Vgg (I have an adjustable power supply that could easily do that), set up a 
transverter with appropriate attenuation to a max output of -20 dBm on the 
peaks, and transmit into it with the output into the brick and the output 
of the brick into a 50 ohm load. Listen to the quality of the output for 
distortion as you talk into the transverter and see how bad it gets as you 
approach or exceed 4.4v, and maybe also check the Pout as you go. Maybe the 
output linearity is OK above 4.5v if the power gain is worth a little bit 
of distortion. Or maybe not -- maybe we should be happy to get the 10 watts 
out of one of these beasties in linear mode.

But someone who REALLY knows should feel free to intervene here with more 
truth than I am able to muster. :)

73 de W0JT


On Mar 24 2010, Bruce Richardson wrote:

>
>
>That's interesting and I'll have to re-read your email a few times
>to ponder it.
>
>More what I was getting at is, in the Mitsubishi spec sheet, (link
>provided earlier) it talks about how to bias one of the pins into
>the module to get Class AB.  I didn't take the time to reread it
>lately, but given what it says, how might a guy come up with the
>proper voltage to follow the instructions in the spec sheet in a
>practical manner. I'm asking because someone asked me.  Apparently
>the reader has the 903 transverter, would like to use one of the
>Mitsubishi modules as his output, but wants to try and take
>advantage of any linearity the module will provide.
>
>(about to go read the spec sheet after all.....)
>
>Thanks,
>Bruce
>
>
>"When the going gets weird, the weird turn pro." - Hunter S.
>Thompson
>
><Jerry's response snipped>
>
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