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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

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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--- In BITX20@..., Rahul Srivastava <vu3wjm@y...> wrote:

Hi!

As far as BITX design is concerned it is well done. The IF gain

issue arises only due to varying characteristics of same devices
available worldwide.

Even within the USA there are poor spec parts problems and
availability issues.

Where noise figure is concerned under ideal components there is a

limitation to what we can get in a negative feedback config. BITX
nature and paralleling of the amps further effects the feedback and
consequently affects the noise figure on the whole.

Very true, for the IF stages the is not a big problem unless poor
devices are used. The Q1/Q13 stage has more than adaquate noise
figure for 20m or lower HF bands. For 15/17/12/10M bands I'd
want a better device than the 2n3904/2n2222 in the recieve path
a 2sc2570 or 2n5179 would do well there. At VHF we want better
as the band noise is much lower.

Another device that needs some attention is mixer driver Q7. At
above 25mhz the 2n3904 is getting near it's limits though a
2n4124 worked well at 41mhz.

Where BJT preamps are concerned a grounded base cofig provides

better performance though at a lesser gain. A Norton noiseless preamp
provides very low NF probably the best in BJT. Even the Icom IC7800
uses it.

Grounded base BJT is stable but low gain and generally requires
a propper input match for good noise performance. I've looked
at a Norton amp as they are quieter for the same BJT
though they tend to run lower gain. I can get a U310 common gate
fet to a below 1db noise figure and 12db gain with less effort.
However I'd need two stages to overcome the DBM and image filter
losses. Based on earlier work with DBMs at 6m if I want a good NF
and MDS a dual gate MOSFET with around 20db gain is successful.
The idea is enough RF gain to overcome the image filter and DBM
losses (typically -10 to -12DB) with a little extra. The only
problem is that is a more difficult amplifier due to high gain.
The positive is the IP for a dual gate mosfet is very good for the
power required.

Maybe we ought to look at the unique switching arrangement used in

RU design Desna already under files for improved NF.

I am considering splitting the path at the mixer with a optimized RX
path to mirror the TX. It's a trivial thing to use a diode switch
at that point. Building a second filter for the RX is trivial.

Other considerations is balancing power consumption as this is for
portable ops and making fit the intended package. That and keeping
it simple to reproduce at least in part. The PLL I use may be a
parts procurement problem for some.


Allison

at a Norton amp as they are quieter for the same BJT
though they tend to run lower gain.? I can get a U310 common gate
fet to a below 1db noise figure and 12db gain with less effort.
However I'd need two stages to overcome the DBM and image filter
losses. Based on earlier work with DBMs at 6m if I want a good NF
and MDS a dual gate MOSFET with around 20db gain is successful.?
The idea is enough RF gain to overcome the image filter and DBM
losses (typically -10 to -12DB) with a little extra. The only
problem is that is a more difficult amplifier due to high gain.
The positive is the IP for a dual gate mosfet is very good for the
power required.

>

---

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--- In BITX20@..., Rahul Srivastava <vu3wjm@y...> wrote:

Hi!

A DG MOSFET would certainly be an improvement. I had been thinking

on only BJT lines.

A good BJT can be excellent but most BJT designs are aimed at
high IP amplifiers. I've built one radio that used them MRF5xx
series in a norton push pull config with IP in the +35dbm range.
Nice but 100mA for the RF amp is high power drain for a portable.

At 6m a cascode connected pair of MPF102 jfets will work very well
with a good noise figure. I'm testig that to see if I'll use it.
The appeal is common jfets, easy to find, easy substitution.

Rather than purchase them from mkt I normaly salvage them from old

defuct TV tuners. The SMD BF989 or the 2SK series works well in most
cases.

My favortie source. Also old VCRs are a good source for balun cores
and PIN diodes.

Allison
KB1GMX