I'm wondering, in a quiet, museing, sort of way, whether its a
reasonable prospect to use the IF strip as a driver to VHF using a
transverter in place of the PA and RX BPF. I have suitable
transverters to try it with but first I have to build the BITX.
A good idea, if its possible, to add a MODS section. I guess that
no one is going to build their second BITX without adding
personality to the build.
atb Mike W
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On 17 Jul 04, at 11:31, Arv Evans wrote: --=-eSRYBxmSXrAkn9U7Gr+H
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Chris, & others...
Please take a closer look at the BITX IF string without any crystal
filter. There would be no control of the passband and thus no way to
tune a specific frequency, other than as a very broadband DC RX with a
converter ahead of it. There would also be little rejection of image
frequencies. The crystal filter is the only frequency determining
part of the IF amplifier string, so at least some minimal bandwidth
control is probably necessary. I have to admit that my first working
receive-only mock-up of the BITX20 did use a pair of 10.7 IF
transformers that were peaked at 10.0 MHZ in place of the crystal
filter. It sort of worked (very, very broad tuning and prone to
oscillation) so I purchased the necessary 10 MHz crystals and rebuilt
it per Farhan's instructions.
Use of a single-crystal IF filter is interesting for QRP activists who
may not need SSB. The IF filter loss would be significantly less than
the 10 to 12 db of the ladder filter, and with a variable capacitor
(maybe a varicap) it should be possible to adjust the bandwidth to
some extent. Farhan & myself have discussed using the BITX for other
modes, including CW by using a computer sound card or by making the
microphone amplifier into a keyed oscillator (probably easier than
unbalancing the modulator to get a keyed carrier).
The most intriguing part of the BITX design philosophy is that it is
relatively easy and inexpensive to duplicate Farhan's design, and
then to customize it for whatever special features you might want to
try. Farhan uses his BITX on CW and PSK-31 as well as SSB, so we know
it is capable of those modes. Listed below are a few of the more
obvious modifications for the basic design:
(1) Consider the possibility of adding VOX by using a small relay
for
the T/R changeover and driving that relay with an amplified and
rectified portion of the microphone audio. Anti-vox could be
incorporated by integrating a small portion of the RX audio (in proper
phase) with the VOX audio amplification. For CW the same VOX
circuitry could automate RX - TX changeover.
(2) If you use computer driven modes of operation, you should
probably include a connector for your computer on the back of your
BITX-xx. This connector might include the coupling attenuators that
are necessary to bring the sound card output down to the correct level
to match your microphone amplifier input, as well as tapping the RX
audio ahead of the volume control so that adjusting speaker volume
would not disturb your computer RX signal level.
(3) Adding a simple FWD & REF power meter or SWR meter seems
obvious
and rather easy to do. The same meter could also be used as an
audio-derived S-Meter with a one-transistor amplifier and a couple of
diodes.
(4) Depending on the type and impedance of your antenna, it might
be
possible to include the antenna tuner inside the BITX enclosure for a
nearly self-contained station.
(5) There are many options for the VFO, even building two VFOs for
offset frequency operation. For my BITX40 I built a PTO instead of
using the two-capacitor design of the BITX20. If one has built a DDS
(Direct Digital Synthesis) unit, then that could become the VFO for
your BITX.
(6) At least one BITX20 builder has shown pictures of his unit that
incorporates an LCD frequency display, and another has shown his BITX
with either 7-segment LED and /or binary frequency displays.
(7) While Farhan's original design does incorporate a linear
amplifier section, it is possible to substitute any of several good
linear amplifier circuits ( even a tube-type linear if you have one
lying around ! ). I expect to soon hear of BITX designs that run much
more power using combinations of parallel and push-pull IRF510 finals.
This amazing potential for altering and/or augmenting the basic unit
is part of the magic associated with Farhan's original design. Maybe
we should start a "BITX MODIFICATIONS" sub-section as part of the
forum...? Hans, is it possible ( or even advisable ) to build a
MODIFICATIONS sub-directory in the FILES section of
BITX20@... that would support multiple users adding their
circuits to that area?
73's
Arv - K7HKL
_._
On Sat, 2004-07-17 at 05:03, vdberghak wrote:
Hi,why the intention to add one x-tal as a filter if you want to
generate DSB? Just remove the current IF filter from the design and
you have already DSB! By doing this, you can also investigate (for
example during receiving) the impact of the IF filter. If the
received signal looks good without filter and is horrible with the
filter, someting is wrong with the shape or position of the
passband. Good lock, Chris.
Would
it be possible to use a single 10 MHz (or other frequency) crystal
as
the IF filter in the BITX-xx design and simply center the carrier in the
IF passband. This would result in DSB (Double Sideband) instead of SSB
(Single Sideband) transmission, but it might work to get a system on the
air. One possible problem with this is that single-crystal IF filters
tend to be very narrow banded, in the order of 200 HZ, and have a steep
roll off on one sideband only. I am not sure what this characteristic
would do to the audio quality of a DSB signal. I guess that someone has
to try it for us to know for sure...?
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Chris, & others...<BR>
<BR>
Please take a closer look at the BITX IF string without any crystal
filter. There would be no control of the passband and thus no
way to tune a specific frequency, other than as a very broadband DC RX
with a converter ahead of it. There would also be little
rejection of image frequencies. The crystal filter is the only
frequency determining part of the IF amplifier string, so at least
some minimal bandwidth control is probably necessary. I have to
admit that my first working receive-only mock-up of the BITX20 did use
a pair of 10.7 IF transformers that were peaked at 10.0 MHZ in place
of the crystal filter. It sort of worked (very, very broad
tuning and prone to oscillation) so I purchased the necessary 10 MHz
crystals and rebuilt it per Farhan's instructions.<BR> <BR> Use of a
single-crystal IF filter is interesting for QRP activists who may not
need SSB. The IF filter loss would be significantly less than
the 10 to 12 db of the ladder filter, and with a variable capacitor
(maybe a varicap) it should be possible to adjust the bandwidth to
some extent. Farhan & myself have discussed using the BITX
for other modes, including CW by using a computer sound card or by
making the microphone amplifier into a keyed oscillator (probably
easier than unbalancing the modulator to get a keyed carrier).<BR>
<BR> The most intriguing part of the BITX design philosophy is that it
is relatively easy and inexpensive to duplicate Farhan's design,
and then to customize it for whatever special features you might want
to try. Farhan uses his BITX on CW and PSK-31 as well as SSB, so we
know it is capable of those modes. Listed below are a few of the
more obvious modifications for the basic design:<BR> <BR>
(1) Consider the possibility of adding VOX by using a small
relay for the T/R changeover and driving that relay with an amplified
and rectified portion of the microphone audio. Anti-vox could
be incorporated by integrating a small portion of the RX audio (in
proper phase) with the VOX audio amplification. For CW the same
VOX circuitry could automate RX - TX changeover.<BR>
<BR>
(2) If you use computer driven modes of operation, you
should probably include a connector for your computer on the back of
your BITX-xx. This connector might include the coupling
attenuators that are necessary to bring the sound card output down to
the correct level to match your microphone amplifier input, as well as
tapping the RX audio ahead of the volume control so that adjusting
speaker volume would not disturb your computer RX signal level.<BR>
<BR> (3) Adding a simple FWD & REF power meter or SWR
meter seems obvious and rather easy to do. The same meter could
also be used as an audio-derived S-Meter with a one-transistor
amplifier and a couple of diodes.<BR> <BR> (4) Depending
on the type and impedance of your antenna, it might be possible to
include the antenna tuner inside the BITX enclosure for a nearly
self-contained station.<BR> <BR> (5) There are many
options for the VFO, even building two VFOs for offset frequency
operation. For my BITX40 I built a PTO instead of using the
two-capacitor design of the BITX20. If one has built a DDS
(Direct Digital Synthesis) unit, then that could become the VFO for
your BITX.<BR> <BR> (6) At least one BITX20 builder has
shown pictures of his unit that incorporates an LCD frequency display,
and another has shown his BITX with either 7-segment LED and /or
binary frequency displays. <BR> <BR> (7) While
Farhan's original design does incorporate a linear amplifier section,
it is possible to substitute any of several good linear amplifier
circuits ( even a tube-type linear if you have one lying around !
). I expect to soon hear of BITX designs that run much more
power using combinations of parallel and push-pull IRF510 finals.<BR>
<BR> This amazing potential for altering and/or augmenting the basic
unit is part of the magic associated with Farhan's original
design. Maybe we should start a "BITX MODIFICATIONS"
sub-section as part of the forum...? Hans, is it possible ( or
even advisable ) to build a MODIFICATIONS sub-directory in the FILES
section of BITX20@... that would support multiple users
adding their circuits to that area?<BR> <BR> 73's <BR> Arv - K7HKL<BR>
_._<BR> <BR> On Sat, 2004-07-17 at 05:03, vdberghak wrote: <BLOCKQUOTE
TYPE=CITE>
<FONT COLOR="#737373"><I><TT>Hi,why the intention to add one x-tal
as a filter if you want to <BR> generate DSB?<BR> Just remove the
current IF filter from the design and you have <BR> already
DSB!<BR> By doing this, you can also investigate (for example
during <BR> receiving) the impact of the IF filter.<BR> If the
received signal looks good without filter and is horrible <BR>
with the filter, someting is wrong with the shape or position of
the <BR> passband.<BR> Good lock,<BR> Chris.<BR> <BR>
>Would<BR> > it be possible to use a single 10 MHz (or other
frequency) crystal <BR> as<BR> > the IF filter in the BITX-xx
design and simply center the carrier <BR> in the<BR> > IF
passband. This would result in DSB (Double Sideband) instead
<BR> of SSB<BR> > (Single Sideband) transmission, but it might
work to get a system <BR> on the<BR> > air. One possible
problem with this is that single-crystal IF <BR> filters<BR> >
tend to be very narrow banded, in the order of 200 HZ, and have a
<BR> steep<BR> > roll off on one sideband only. I am not
sure what this <BR> characteristic<BR> > would do to the audio
quality of a DSB signal. I guess that <BR> someone has<BR>
> to try it for us to know for sure...?<BR> </TT><BR>
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