My Bitx20 transceiver – a great little radio!

Posted on January 4, 2011

0


My Bitx20 transceiver – a great little radio!

BiTx 20 transceiver (emitator/receptor) realizat in casa.

Este un fascinant echipament dupa  o ideie a indianului inginer, radioamator, Ashhar Farham, VU2ESE, idee imbratisata, in lumea intreaga, de un numar mare de radioamatori. S-a constituit un grup impresionant, activ in http://groups.yahoo.com/group/BITX20 (funcţionează din 2004 cu peste 200 de membri).

Trebuie sa fii inregistrat. Acolo aflii cele mai mici si neasteptate detalii din partea celor care studiaza  sau realizeaza proiectul. Fotografiile de pe aceasta pagina sunt realizate de mine. Montajul functioneaza impecabil.
Ideea autorului a fost de a proiecta o schema usor de realizat, folosind piese ieftine, majoritatea din recuperari, cu valori necritice, dar mai ales cu un numar redus de bobine, spaima, in general, a constructorilor!
Ca in orice simplificare, trebuie acceptate unele compromisuri, acceptand parerile pro si contra acestui echipament.
Puncte tari, puncte slabe, general valabile nu am gasit la cei care deja au construit BiTX20! Informatii relevante pot fi culese de pe net, atentie insa ca sunt si o de mulţime de informaţii inutile. Din fericire nu exista o problema tehnica ce nu poate fi rezolvata!!! Nu necesita cunostiinte speciale, echipamente pe raft pentru reglari si configurare! Nu e greu sa gasesti componentele electronice, “no exotic components”.E nevoie, mare nevoie insa de PASIUNE! O recomandare: odihnitiva dupa fiecare etaj construit! Reveniti la reglaje doar odihniti! Dar nu peste 2-3 luni!

Fa mult cu putin!

Bitx= bidirectional (emisie-receptie).
20=banda de 20 m, respectiv 14MHz.
Multa vreme schema proiectata, a fost testata in 20 m dar au aparut variante/versiuni in celelate benzi, mergand pana in banda de 6m!
Din punct de vedere constructiv, fiecare dupa cum a crezut ca e mai usor sau dupa posibilitati. Personal nu am agreat varianta “in aer” parandu-mi-se inestetica si dificil la reglari. Am ales PCB pe o singura fata, vezi poza de mai sus..
Se pare ca toate variantele au produs rezultate bune. Am urmarit pe Youtube variante si variante!
Cum functioneaza?
BiTx 20 este tipic SSB, la receptie si la emisie.Principiul bidirectional al transceiverului permite ca unele module (parti) sa permita sa treaca atat semnalul la receptie cat si la emisie. Aceasta combinatie duce la reducerea numarului de componente, deci la simplitate.
O DESCRIERE SIMPLIFICATA a traseului semnalului.
Semnalul de radiofrecventa cuprins intre 14.000 si 14.350 MHz cules de antena, este selectat de flitrul de antena , filtru trece banda, care elimina semnalele nedorite, dupa care se amesteca cu frecventa oscilatorului variabil (4-4,5 MHz) -parte a transceiverului numita VFO, rezultand suma 18 MHz (14 MHz+4 MHz), iar diferenta 10MHz (14 MHz-4 MHz).Folosind un filtru ingust de latime de banda, frecventa intermediara (IF) in cazul nostru de 10 MHz, putem selecta semnalele utile intre 14.000 si 14.350 MHz.
Urmeaza o amplificare a semnalului selectat, dupa care se demoduleaza pentru a putea fi auzita componenta audio.
Acest lucru se realizeaza prin insertia frecventei fixe data de oscilatorul de purtatoare. Astfel dupa amestec semnalul audio este amplificat pentru a fi redat prin casti sau difuzor. In mare … aceasta este partea de receptie.

Partea de emisie.
Semnalul captat de microfon este amplificat de amplificatorul de audiofrecventa dupa care se amesteca cu semnalul de purtoatoare cu valoarea de 10 MHz, dupa care este injectat in modulatorul de balans unde se suprima o banda laterala. Semnalul cu banda laterala superioara trece prin filtrul de 10 MHz dupa care se amesteca cu frecventa oscilatorului variabil (VFO) 4-4,5 MHz (10.MHz +4.0 MHz-4,5 MHz) insumand 14.000-14.500 MHz ( limita fiind 14.350 MHz). VFO-ul permitandu-ne baleaj intre partea de jos a benzii si cea superioara.
Urmeaza amplificarea semnalului pana la 5 wati output spre antena.

—————Lista cu necesarul de componente————-

Capacitors:
-0.001u  2 buc
-0.1u  44
-100p  3 buc
-10p   4
-2.2p  2
-220p 5
-47p   3
-560p 3
-56p  1
-elec 1u    2 buc
-elec 50u  6
-var 22p   6

-var 365p  1ptr VFO
Urmeaza completarea.
Modificari de luat in seama, aici

IDEA LIST FOR MODIFYING YOUR BITX.. TRANSCEIVER:

 (1)  Add computer sound card connections to support computerized CW,
PSK-31, MSK, SSTV, RTTY, etc.  Take a signal from the top of the Rx
volume control and connect it via a jack to the sound card input.
From the sound card output use a 100:1 attenuator connected to feed
the microphone input of the BITX..

 (2)  Since the carrier oscillator is very close to 10.000 mHz it
would require only a few divider chips to generate marker signals
for use in calibrating your VFO dial.  Two divide-by-ten chips get
you to 100 kHz, and an additional divide-by-four results in a marker
for 25 kHz.

 (3)  Add a simple audio detector and DC amplifier / relay driver to
provide VOX capability.  This circuit would be fed from the AF output
of the BITX microphone amplifier.  This works for headphone operation,
but for speaker use it may be necessary to tap a small portion of the
Rx audio and feed it, phase inverted, into the VOX detector to counteract
microphone audio received via the speaker.

 (4)  For CW operation, build a keyed AF oscillator (about 700 Hz)
that feeds through a suitable attenuator into the mike input.  Part
of this audio could also be used as sidetone for monitoring what one
is sending.  If the VOX has already been built and is working properly
it might be utilized to provide full QSK operation.

  (5)  Use DDS (Direct Digital Synthesis) signal generation in place
of the VFO for digital frequency

control.

  (6)  Metering:
        (A)  Use a detector to sample the Rx audio (ahead of the
volume control) and amplify that to drive a small S-Meter.
[ S-Meter schematic posted in this folder 7-24-04 ]
        (B)  Run the antenna lead through a toroid inductor and use
the secondary of that transformer to drive metering circuitry for
showing forward and reflected power.
        (C)  Incorporate both A and B above to use one meter for
monitoring both transmit and received signals.

  (7)  Incorporate a switched attenuator at the input of the Tx PA
Driver to switch between QRP (1 Watt) and QRO (maximum power).

  (8)  The BITX design can be modified to operate on other bands.
For 40 Meters, it seems possible to do this by simply changing the
VFO frequency range.  For other bands it may be necessary to change
the IF frequency as well.  Just be sure that the set of frequencies
you have chosen does not cause unwanted 'birdies' in the transmitted
signal, or in the receiver coverage range.

  (9)  Using transverters it should be possible to put your BITX.. on
VHF, UHF, or even Microwave frequencies.

  (10) Alternatives to the 'two-stage' tuning of Farhan's original
design are possible.  Use your favorite VFO circuit and mechanical
tuning adjustment method.  The BITX40 being constructed by K7HKL
uses the WA6OTP PTO design (www.wa6otp.com/pto.htm) that is tuned
by threading a 1/8 inch brass screw into/out-of the oscillator inductor.

  (11) Speech compression could be added to the microphone amplifier.
Do this using your favorite compression circuit.  

  (12) For headphone-only operation, the LM-386 AF amp may be replaced
with a simple Op-Amp.  Connect the headphone jack such that the two
32-ohm elements of popular headphones are connected in-series.  This
raises the impedance to a level that the Op-Amp can easily drive to
adequate volume.

  (13) For analog-tuned VFO circuits you could add the "Huff-and-Puff
Stabilizer" circuit to make this unit tune in controlled steps.

  (14) At least one BITX20 builder has shown his BITX system using a
microprocessor chip to measure VFO frequency and drive an LCD frequency
display.  It seems that one could also do this using divider chips and
a PC to sample the frequency and display that on the computer screen.  

  (15) To measure antenna current, run the feedline through a toroid
core.  Then connect an LED accross a two turn secondary winding.
Adjust the number of secondary turns for best indication at your
particular power level.

  (16) For some DX operation it is desireable to use split-frequency
operation, where the transmitter is on a different frequency than
the receiver.  It is possible to do this with the BITX20 by building
two separate VFO sections and switching between them when changing
between Rx and Tx.

  (17)  Several suggestions have been posted regarding conversion of
the BITX front-end to facilitate SDR (Software Defined Radio)
functionality.  These suggestions involved:

       (A)  adding synchronous detection (using 10.000/4 for phase
detection)
       (B)  incorporation of phasing method for SSB generation
       (C)  elimination of both Rx and Tx audio circuitry and relying
on a computer sound card entirely for these functions.

  (18)  Use of simple binary dividers driving LEDs to display the VFO
frequency ( in binary ) can provide very cost-effective display of
the BITX operating frequency.  

  (19) The linear amplifier section designed by Farhan is not the only
design that will work for your BITX system.  Several other configurations
have been suggested and/or used:

       (A) use two push-pull IRF510 or IRF511 and higher supply voltages
to obtain a significant increase in output power.
       (B) use a pair of medium signal transistors to build the BITX..
and its linear as a QRP rig having up to 1 watt of power output.
       (C) use a pre-existing linear amplifier (even a valve-type unit)
as a linear amp.

  (20)  The BITX RF Mixer is singly balanced and has no DC return path the IF
side of the circuit.  PA3CRX has suggested a grounded 1 mh RFC to provide
the DC return path.  Farhan-VU2PEP has discussed the possibility of converting
this section to a fully balanced mixer (requires just an additional balun).

  (21)  If you add a microprocessor chip to augment some of the BITX
functions, it might be advisable to use the 10 MHZ BFO signal to clock
your uP.  This could help avoid unwanted 'birdies' from the uP oscillator
getting into your BITX circuitry.

  (22)   Use a transistorized DC switching layout to switch power between Rx
and Tx modes.  Use a diode-based antenna switch that is driven by the switched
RX/TX DC voltage changeover.  

  (23)   Include built-in metering for Forward & Reflected RF Power (or just
a simple LED-based RF Antenna Current indicator).  

  (24)   Power your BITX20 from the 12 Volt leads of an unmodified computer
power supply.  Alternatively, modify the computer power supply to deliver higher
current 12 Volts from what originally was the 5 Volt section.  Then use the old
12 volt output (now operating at 24 volts) to power the IRF-510 linear to much
higher output levels (probably something like 25 to 100 watts using multiple
IRF-510 MOSFETs).

  (25)   Add band switching to operate the BITX unit on two, or more,
ham bands.

  (26)   For BITX's that have been modified for CW operation, include a
transistor based iambic keyer circuit inside the BITX enclosure, thus
making it a fully integrated CW station.

  (27)   Add an LED and threshold detector to indicate when you are
operating the Tx audio at the most efficient level.

  (28)   Add RF blocking between the BFO mixer and the Rx audio amplifier
to improve audio quality at high volume levels. [ thanks Chris Pa3CRX for
this suggestion]

  (29)   Add BFO switching and modifications to the BFO mixer to support AM
mode operation.

  (30)   For CW mode add a second BFO oscillator and keying circuit that is
centered on the IF filter passband.

  (31)   For some operators, the LM-386 audio output may seem a bit low powered.
This can be corrected by replacing the LM-386 circuit with a discrete component
AF amplifier.  Schematics for this have been posted in the FILES/MODIDFICATIONS/ section of this forum.

  (32)   There are two different schematics of the BITX20 available on the
Internet.  One of these has no attenuator at the AF side of the BFO Mixer,
while the other drawing shows an attenuator using two 220 ohm resistors and
one 22 ohm resistor.  The circuit with the attenuator is the prefered circuit to build.  Go to  to see the latest
circuit from Farhan's web page.

  (33)   ???
Author: ?
Sorry.
Publicat de Costi la Monday, October 18, 2010
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