I will be glad to share what I have done, but my spare time is a bit limited right now (I am in the midst of being hired for a new position...un-retired for the third time!). Sharing will require that I draw up schematics and take a few pictures. That will be done just as soon as all the design & construction is done, and as time allows.
My original BITX was the standard 20 Meter model, but as soon as it was completed it was "borrowed" by my son (also a ham). But, as Chris alluded to, my second BITX is quite different from the original design.
Here is an overview of what I did with or to Farhan's original design in building my BITX40:
- The VFO has been changed to a PTO (like WA6OTP's product), with a J-FET (MPF102) oscillator, a 2n3904 emitter follower, and a 2n3904 buffer amp. Most of the effort here involved mechanical design work to keep the knob equidistant from the front panel whilst allowing the threaded brass PTO shaft to move forward & back as it rotates. The semi-final design involves use of two loops of thin metal that make up a flexible shaft coupling and allow both shafts to rotate while the threaded brass one moves forward & back to adjust the frequency. Eventually, I expect metal fatigue to be a factor with this design, so I am still tinkering with it.
- The Rx AF Amp is made up of discrete devices, as shown in my earlier postings on this forum. I provided board space and connectivity options for a narrow bandpass AF filter, but that unit is not yet completed.
- The PTT switch or relay has been replaced with a 3-transistor power switch circuit that takes care of switching voltages from Rx to TX circuitry. Simply, the PTT lead (or the VOX circuit) pulls the base of a 4A 40V NPN to ground to lower the Rx voltage. Lowering of the Rx voltage takes the forward bias off a 2n2222 NPN which allows its collector voltage to rise, forward biasing another 4A 40v NPN which provides the Tx voltage. This changeover circuitry induced a loss of about 1 volt on Rx and 1.2 volts on Tx, so I adjusted my power supply for 14 volts instead of 12.6 volts output. I added three 1K resistors and associated LEDs to indicate (a) Green = presence of DC voltage, (b) White = Rx-mode, & (c) Red = Tx-mode. NOTE: I leave the last stages of the RX AF amp powered up during TX. This allows sidetone injection for CW, and eliminated a nasty "CLICK" on TX-to-Rx transitions. On suggestion from Chris, I added additional shielding between the RX front end and the TX PA to eliminate a "chirp" on the TX-to-RX transition. I also lowered the 12 volt supply bypass capacitor in the microphone amplifier from 47 mfd to 15 mfd while trying to eliminate the above chirp, and have not restored this to 47 mfd yet.
- The RX-TX antenna changeover circuit is presently a 20 pf capacitor and a pair of back-to-back 1N4148 diodes, with additional Rx circuit protection provided by a forward biased 1N4148 to ground. There appears to be a slight signal loss caused by this arrangement, so I will continue to work toward something better (but it has to be simple).
- Q14 is now a 2N7000 MOSFET. There is no particular reason other than I had some in my junk box and wanted to see if they would work in this application. It works great, but did require application of some miniature ferrite beads to tame a parasitic oscillation.
- The Rx front-end bandpass circuit was made up using two 10.7 MHz IF transformers. This has been posted and pictures shown earlier on this forum.
- My linear amplifier presently is a pair of Push-Pull 2N7000's, driving push-pull IRF-510's. I am not completely satisfied with this linear (too many very high impedance devices and some instability at certain settings of my antenna tuner). I will probably re-design it before I publish anything more regarding linear amps for the BITX units.
- CW mode is currently done with an external AF oscillator that plugs into the microphone jack. The keyed AF tone operates my VOX circuit to provide semi-break-in operation. Currently the VOX has to be re-adjusted when going from SSB to CW or CW to SSB. This seems to be because the hold-over timing for CW needs to be greater than the hold-over timing for SSB, and by the difference in switching voltage that is generated by speech versus CW tones. Eventually, th CW tone oscillator will be mounted inside the BITX itself and switched in-circuit by unplugging the microphone connector.
- As mentioned above, the VOX is working, but not yet perfect. Anti-VOX uses phase-reversal and injection of Rx audio into the VOX amplifier, but this is very problematic. This may be due to my room acoustics, or to some gain versus bandwidth difference between anti-VOX AF and the AF returned from the speaker to the microphone. I am still working on this.
- I added Rx RIT (Receiver Incremental Tuning) using a fairly complex arrangement of voltages that switch between Rx and Tx mode. I may publish this later, but in the interim I am looking for a simpler way.
Work yet to be completed on this BITX40 may include:
- Building a better Tx linear amplifier.
- Completing and installing the narrow AF bandpass filter for CW-mode operation.
- Building the CW-mode tone oscillator inside the BITX chassis, and making it switch in-circuit automatically when I unplug the microphone.
- The antenna changeover circuitry still could use improvement, as mentioned above.
- The VOX still needs quite a bit of design analysis and corrective work.
- The RIT circuit could use some simplification, but it works for now.
In addition to all this, I am exchanging emails with Hans (G0UPL) about adding a simple frequency display.
The VFO is quite stable, but it might be nice to add a Huff & Puff type stabilizer...at some later date.
I have an old PC-XT computer power supply opened up on my workbench with the intention of significantly altering it such that it will provide 12-14 volts for the BITX and 24-32 volts for the linear amplifier. Yes, I am considering going for QRO with this unit if everything else goes as anticipated. Think about push-pull pairs of push-pull IFR510 amplifiers and you will see what I am considering. This idea is not new, as it has been demonstrated in several ham publications over the past 10 or so years.
OK, Chris...is that enough for now? There will be more as I find the time to work on it.
On Tue, 2004-09-14 at 11:28, vdberghak wrote:
why not sharing the circuits that you use and explain why they are
better? Or did you already had them from other projects? ;-)
> I cannot provide meaningful measurements beyond the BFO mixer
> circuit from the filter to the antenna is quite different from
> original design.
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