Re: RF power chain mods and improvements..

ajparent1/KB1GMX
 


Gain and power line up with a few possible solutions listed:

*Mixer output at full tilt about -10 dbm.
Go higher and the spur gets bigger so lower output here is better.

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*Q90 no mods at 80m about 16db at 10M about 10-11db  and not more than 3mW power.
Replace it with a really hot transistor.  BFR106 has a FT of 5ghz and can do over 100mW.
This should keep the stage gain at about 16-18db from 80 through 10M and deliver the 3mw!
It is the easiest stage to make flat and high gain as this get more difficult at higher power out.

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*Pre-driver takes that 1mw and boost it to about 40mw, We hope.  What I saw was about 45mW
 at 80m and barely 8mW. note gain at 80m is about 16db and at 10m maybe 9db.

We need the gain again to be more stable and higher as well.  Transistors tried that worked better
included the TO18 2n2222A, 2n3866, and 2n5109.  The 5109 was best even though I used only 1.
Second best was tie between a single 3866 and two 2n2222.  MPSH10 was disappointing.
*** I plan to try an oddball 2n6661A, [at 14$ each most will choke] a VMOS fet has potential
and requires many circuit changes[Bipolar bias to MOSFET].  They worked really well in
another project at VHF.

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*Driver take that 40mw and make about .4W at 80 and sinks to barely .08W at 10M Gain for that stage
is supposed to be about 16db and likely is at 80mbut at 10M measured for different 3904s 8 to 11db.

This is where the 2n3904 really fails. Its bandwidth at high currents actually does down.  The 2n2222A
has the reverse, the gain and bandwidth increased with increasing current.  While not an ideal part
it does work better.  The ideal part dow be two devices push pull [not 4] and higher FT.   I havent
tried 5109 here but its a solid bet.  I wish 2n3553s, 2SC2166, 2SC799, 2sc1306 and others were
still around [real ones].

*** I plan to try an oddball 2n6661A here too as its good for up to 6W.
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*Finals IRF510 at 80M approaches 15-20W with a gain over 17db, at 10M its about 13db and but
the drive is so low that yields maybe 2W.

If there is enough power at the gate it does the job we ask.   RD16HHF does not offer more gain or
bandwidth as MOSFETS do not have the equivilent of FT.  IT does offer a handy groundable tab.
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To go from -10 dbm[.0001W] to +40 dbm[10W] we need 50 db of gain over all.  That includes
losses to transformers and other circuit features.  So we need more gain than that by maybe
6 to 10DB to allow some latitude on the gain adjustment pot.

So with four stages and the last being limited to the IRF510s.  We set a few rules.

*One we never run a transistor wide open as one may exceed and another fall short.
For 2n3904s that happens and the lowest common denominator for them is around
10db. Use better transistors and lots of feedback.  More likely to work for everyone
than a bet on Monte Carlo.  

*Two we only have four stages!  The board is layed out that way.  Reality sucks.

*Three:   IRF510 or RD16HHF you get about 13-16db of gain, period.  More
is wishful thinking or running wide open and risking stability.  Hint IRF510s blown
about 2$, RD16HHFs blown 10$.    Going to IRF520 and 530 not better [for 10W]
as the internal capacitances are significantly higher and it doesn't make the drivers
job easier.

*Four Based on total gain needed and what the finals can be reasonably expected to do...
 The three prior stages must deliver 45 to 47 db of gain.  Its also a lot so attention to
 stability is everything.

*Five  the result must be stable.  Oscillation will kill the finals.

Item four is the killer as its basically asking for 16db from all three stages.
You need good parts for that.  Also the interstage coupling must be up to the job
while not introducing uncontrolled losses.  Tall order.  So far I have only partial
answers.

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Dump the 3904s and use 2n22222(to18),  for those interested in 80-17M it works remarkably
well and give a boost up high too.  If you go to 2n5109s you may need 1 is predriver and 2 in driver
besides they are big and the space small.  Whatever is use SHORTEST leads possible.

Replace or parallel R941,R911, R96,R942 to get 11 ohms each (I paralleled 22ohms across them).
Lower emitter resistance helps gain and power out to finals.

Replace Q90 with BFR106,  note R81 has to be increased to between
2K and 2.7K for this part.    (for those making suggestions I tried 2n2369 in
SMT, it was better but not great).  Besides mouse has the BFR106 for a
whooping 38 cents each.

Change C81 to 470 pf,  This flattens the 80 and 40M runaway power and helps the higher bands.

There are many changes to transformers possible but for the moment the above are best bang for the buck.
Aslo changing the transformers might be a handful for some.

Warning every time I tried the 2;4 turn transformer with any ferrite the finals were heating a lot and the
stage efficiency was well under 40% [terrible with IRF or RD16].  The 2:3 was better at high power
but below 50%.  This is still in the grinder...

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Allison

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