6M power chain working

ajparent1 <kb1gmx@...>
 

This morning I tested the 6m power chain. Results are very good
with more power than expected. When integrated on the bench
a two tone SSB signal nets 4.5W. I made the first local contact
after connecting to the antenna. Reports were good as expected as
I first tested with a local reciever before putting it on the air.
A check on the spectrum analyser says harmonics and spurs at least
-50dbc. That will require a bit of minor work to the output low pass
filter get it in the range I consider clean. Also I think a bit of
ALC for TX would help. The ciruit for that would be rectify the RF and
use that voltage to throttle the gain of the microphone amplifier.
This will help prevent overdrive.

Reciever proved to be a bit weak during testing. The problem was
not an IF gain issue. The 6m band requires a lower noise front
end than HF. The changes were using a 2n5179 for Q1 initially.
I also tried a MAR-6 MIMIC (20dbgain 3db noise figure), it did
not improve matters and verified the bandpass filter loss
was significant. Later I added a U310 JFET preamp before the
bandpass filter. The loss through the filter (at 6m) is great
enough that the noise figure of the reciever is compromized without
12db of gain before it. Measured MDS was -130dbm with the preamp
without it it was a very weak -118Dbm. For 6M operation _my_ goal is
-137Dbm or better. Either way this will recieve some work as there
is to much wide band gain before the crystal filter. This is
important on 6M because of the sometimes very strong signals that
appear during band openings. I also have commercial television
broacast less than 10 miles away on US CH-2 (56mhz) so intermodulation
performance is important. A better layout would be preselector
filter, RF amp, image filter, then mixer. The preselector filter
would be low loss and the image reject filter would be the higher
order bandpass. The revised topology would permit applying AGC
to the RF amp if a dual gate MOSFET such as 2sk122 or BFR988 is
used. I am also considering cascode MPF102 JFET.

Experiments with the mixer indicate the two transformer doubly
balanced works better at 6m and substituting a commercial DBM
(Minicircuits TUF-1) gave similar performance. Transformers
were wound using two hole ferrite balun cores using 6 turns of
#32 wire x3 (trifilar). Diodes used were 1n4148 type matched
for same forward voltage at 1mA.

This makes the second operational BITx. I need to fine tune the
design for best performance on 6M and package it so it's portable.
I tend to be fussy about reciever performance and transmitted signal
quality but so far this version is behaving like the 20m version
allowing for the differences needed for 6M operation.


Allison
KB1GMX

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