---------- Forwarded message ----------
Date: Thu, 24 Jun 2004 10:22:26 +0530
From: Ashhar Farhan <computercorp@...>
To: arvevans@...
Cc: farhan@...
Subject: RE: BITX20, 40 or 75 M versions?
dear arv,
you have spent a great deal thinking about the design and the possiblities.
it is wonderful to see that amongst us the natural need to push the envelope
is well and alive.
when i had declared to W7ZOI that the design is frozen, he was skeptical and
hopeful at once. he said, wait until someone decides to do things with it
that you hadn't even thought of. i guess the idea of an 'open source' design
like BITX20 is that you have a simple basic design that invites adaptation
and evolution.
very interesting points, you raised. let me give you my side of the story on
these:
(1) One could just change the frequency of the VFO and front-end
coils as you mentioned, but also the actual IF frequency could be changed
to minimize the possibility of 'birdies' from unwanted frequency mixing,
and to take advantage of whatever crystals were available cheaply for the
individual builder.
I didn't want to stray away from 10MHz crystals as they seem to be the
cheapest and most universally available of the lot. An interesting variation
is that you can use a 10.7MHz standard filter and the same VFO will cover 20
and 40 meter bands. Only the RF preselectors need to be switched.
(2) Use of an FET for the VFO as you mentioned in the web page is
also interesting, but the sometimes inexpensive availability of DDS chips
might make a digitally tunable unit possible for some hams (see SM0VPO's
simple frequency synthesizer for another cost effective approach).
this design was made primarily for Indian hams (although i dont know of any
who have started to build this in india yet). DDS chips are hard to come by.
The entire transceiver costs about 10 dollars to build in india. the DDS
chip costs much more.
this brings me to the issue of DDS. My personal taste is for purely analog
designs. It is a very personal choice. I spend most of my time writing voice
over IP software which is wholly digitally. For me radio is a way of getting
away from it all. I would rather spend the evening tempurature compensating
a low noise FET VFO rather than fight a noisy DDS. But again, it is a purely
personal choice. I have seen some pretty good cleaning PPL loops around a
basic DDS. If any, I am inclined to implement a Huff and Puff stabilizer for
the VFO as a add-on.
(3) A cheap microprocessor employed as a frequency counter (with
binary LEDs or 7-segment displays) would add digital readout capability.
On the BITX20@..., you will see that Hans Summers
(www.hanssummers.com) has a binary counter made of two 74HC4040s. This seems
ideal to me. I have avoided it from the basic design to keep the
construction challenge low. I personally use my lab frequency counter along
with my transceiver for frequency spotting. I keep the counter separate
because i found that unless they are well sheilded, the counter tend to
generate a lot of noise that back couples from the VFO into the receiver
front-end.
(4) Adding a dual op-amp to the transmit audio section could provide
some speech compression and/or audio filtering. I wonder if the same
compression and/or filtering could be applied to the receive audio with a
positive result? If obtaining a dual op-amp was problematic for some
builders, a couple of transistors could also be employed as a compression
circuit, and two more could be used for the speech filter.
the primary filtering is provided by the 10 MHz filter. that is why, the
modulator is so sparse. i think two back to back diodes could clip the RF to
provide better throughput. The trouble with this approach is that you will
have to switch this off for CW and PSK31 to preserve the gaussian shaping of
these keyed modes.
(5a) Conversion of the unit to include CW capability looks fairly
simple at first glance. Three options come to mind from just looking at
the schematic: (one) upset the balanced modulator via a keyed transistor
to load one side of it, or (two) use the key to drive a transistor switch
that bypassed some RF around the balanced modulator, or (three) key a
sinewave tone generator into the microphone input circuit. The latter
method seems more in keeping with the simplicity of the original design.
just upsetting the modulator balance might not give you a full carrier as
the carrier oscillator is set to the edge of the filter. a better idea might
be to add a sidetone audio oscillator and inject it into the audio amplifier
as well as the modulator. probably you can get the mic amplifier oscillating
with an RC feedback circuit.
(5b) Adding CW sidetone might involve leaving the receive audio
circuit powered up when transmitting CW and injecting sidetone audio to
that section. CW sidetone could be switched by the keying circuit, or by
detecting a small sample of the RF output and using that to "key" the
sidetone oscillator.
let me confess, most of my personal operating of BITX20 has been on CW and
PSK31. not ssb. I use a morse generating program running on my thinkpad feed
into the mic input of the BITX20. i do this because it allows to be leave
the BITX20 running at the other end of the table and continue my mng qth
work (i telecommute to work. my 'office' is a server in an underground
dungeon of rackspace in boston). if i hear an interesting station on CW, i
just have to switch windows and start a ragchew.
(6) Adding VOX capability could be accomplished by sending some of
the transmit audio to an amplifier, rectifier, and relay driver that
operated a small relay to replace the manual DPDT you used in the original
design. Anti-VOX circuitry would not be necessary if the ham were using
headphones instead of speaker for reception, or it could be added in the
form of a properly phased audio cross-over from received signal to the VOX
circuit.
i now have a two relays working in my own build. i control it through the
serial port of the my thinkpad. the thinkpad is programmed to do a lot of
things including run an AGC, generate morse, VOX and a bit of voice
compression. the code is messy and patchy. i will post this as soon as i get
some time from my mng qth.
(7) If one added CW capability and wanted QSK there are several
options for adding this function: (one) a CW sidetone could be injected
into an abreviated VOX circuit to operate a change-over relay, (two) The
key could simply operate a relay driver so that a small DPDT relay
performed the keying (use a hold-over timer to keep relay chatter to a
minimum), and (three) a more complex transistorized RX/TX switching scheme
could be devised to replace the manual DPDT PTT switch.
(8) If obtaining the LM386 audio amp were a problem for some BITX
builders, a comparable audio amplifier can be fabricated using a NPN & PNP
pair, driven by another NPN. About 1/4 watt of audio is available this
way...plenty for headphones or a small speaker.
quite true. and i should have done this. too lazy to solder an audio amp
together. there is one in the first chapter of EMRFD. i think i will post
that circuit on the site.
A BITX for 75 M also seems logical for US hams because a significant amount
of ragchewing contacts take place on that band. This could provide a
daytime local coverage method that augments today's use of 2 Meters for
local coverage.
Yes, 75M is too noisy in the tropics. In europe and usa, it seems to be the
preferred local band. plus, the indian cities are too congested for a 75M
antenna. 75M would mean using VFO at 13.5MHz or at 6.5MHz. If is always
preferable to have your VFO running on the higher side of the IF to reduce
the birdies but stability threatens. At 13.5MHz, you could use a VCO with a
10 turn pot to for tuning.
Since I also play around on the US LF band (the Lowfers group) the thought
occurred regarding a possible BITX for 160 KHz through 190 KHz. After I
get some experience with converting your design to 40 Meters I may try a
Lowfer version.
<Each band will require a different set of coils. If you have worked out
a
set, do send us the details. this information needs to be deciminated.
Thanks again for your reply and assistance. Your contribution of the BITX
design to Ham Radio is much appreciated. The layout represents an elegant
approach to a sometimes complex set of problems in the design and
construction of inexpensive but fully functional ham radio equipment. Have
you considrered starting your own company in India to construct and market
your designs? I suspect there might be a worldwide market for either kit
form or fully assembled single-band SSB transceivers if one could keep the
cost at a reasonable level. Your design also seems like a logical radio
component for interfacing with PSK-31, packet radio, or other computer
supported communication modes.
thanks for the kind words. i think far lesser credit should really be
attributed to me. most of these circuit blocks came off standard works like
Solid State Design and Experimental Methods in RF Design. W7ZOI gave me a
huge lot of encouragment and advice on making the rig better. For instance,
having a triple tuned circuit on the input was a small but very important
change (two capacitors and a coil) that remarkably improved the overall
receiver performance and transmission purity.
om hans summers has been an exceptional organiser who is marshalling us all
to gather together and get on with soldering. he is organising for
components, people and places. i am trying to source him with some key
components that are very cheaply available in india. thanks for dropping in
the email and do stay in touch.
73s,
- farhan
Thanks again & 73's
Arv
_._
-----Original Message-----
From: Ashhar Farhan <computercorp@...>
Sent: Jun 23, 2004 1:39 AM
To: arvevans@...
Subject: RE: BITX20, 40 or 75 M versions?
dear arv,
join up BITX20 group at . it is a forum of the
BITX20
builders.
No, as of now, i dont know anyone who has attempted to build it for 40
meters. but i dont see any reason why it should not work at 40Ms. i am
personally interested in a 40 meter version as most of the local ragchew is
on 40 in india.
it is a simple matter of substituting the RF filter and changing the VFO
coil to get it going to 40 meters.
- farhan
______________________________________
Ashhar Farhan
71, Huda Heights, MLA Colony,
Banjara Hills, Hyderabad.
INDIA 500034.
Phone - +91-40-23314077
______________________________________
From: arvevans@...
Reply-To: arvevans@...
To: computercorp@...
Subject: BITX20, 40 or 75 M versions?
Date: Tue, 22 Jun 2004 22:58:50 -0600 (GMT-06:00)
Mr. Ashhar Farhan
Has your BITX20 design ever been built as a 40 Meter or 75 Meter version?
I am considering attempting a 40 M model, and if that goes well maybe one
for 75 M. If anybody else has already done this I would be interested in
contacting them to see what results they had.
Thanks,
Arv - K7HKL
-<arvevans@...>-
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