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Bharat

The discrete component AF Amplifier file is in the FILES and PHOTOS sections of the BITX20 forum on Yahoo.com? You should be able to view and copy them from there.? If you have problems please send me an email and I will reply directly to your email with an attached file of the audio amp that I used.? Please be aware that I used a 741 Op-Amp in place of the LM-386 in my BITX20, and built the discrete component audio amplifier outboard in a speaker box.? With just the Op-Amp (configured for a gain of 100) there is plenty of audio to run headphones (32 ohm Walkman style headphones with the earphone elements in series).? The discrete audio amp is necessary only if you want to drive a speaker.?
If you want to eliminate need for an Op-Amp, you could probably just use a single transistor gain stage (much like Farhan's microphone amp circuit) and a simple emitter follower for your receive audio.? I will try to work up a circuit diagram and test that sometime in the next few days.

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CONSTRUCTION TIPS:

(1) Build both the RX audio section and the TX Microphone amplifier.? By powering up both simultaneously you can drive the RX amp with your TX amp and microphone.? This lets you test both audio sections before you build the rest of the circuit.

(2) Build the IF Filter and the BFO.? You can couple the output of your BFO (use a 3 pf capacitor) to the filter input and tune the BFO to determine your filter characteristics.? You will need a method of accurately measuring the BFO frequency, and a simple RF detector & high impedance DC meter to measure filter output.

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For the front-end Band Pass Filter of the BITX40 I used a pair of 10.7 MHZ IF Transformers from an AM/FM transistor radio.? These required 55 PF in parallel in order to bring them to resonance on 40 Meters.? I used the whole inductor and ignored the tap on these transformers.? The secondary winding of these was also ignored, but it might be interesting to experiment with coupling the input and/or output via those windings.? A copy of the schematic and picture of this bandpass filter is in the PHOTOS section of this forum.? Look in the directory showing my call sign (K7HKL).

NOTE: There was no particular reason for using the 10.7 IF transformers other than that I had them available and wanted to experiment with using them in a 40 Meter receiver.? You can also use Farhan's design for the front-end BPF (Band Pass Filter) and re-calculate & rewind the coils for 7.0 to 7.3 MHZ coverage.?

IDEA:? Maybe someone could experimentally determine the "inductance versus turns" for tap washers and publish it on this forum.? With that info we could easily scale Farhan's design to almost any band.

My VFO is still not in a final stage of construction.? I have built a PTO (also shown in the PHOTOS section) but am still trying to obtain exactly 10 KHZ per turn with good temperature stability.? So far, that has eluded me!? I have spread and compressed the coils to adjust the frequency coverage, but the turns on one end of the coil interact with the other end of the coil.? I seem to be getting close to my goal, but it is still not at the point where I have an acceptably even spread of frequency from one end of the band to the other.?

Your BITX40 VFO will need to tune from 3.0 to 2.7 to cover 7.0 to 7.3 MHZ RX & TX if you use the standard 10 MHZ IF & BFO.? Initial calculations indicate that this places all first-order spurs and all 2nd & 3rd harmonics outside the 40 Meter band.? I admittedly have not done a detailed analysis of complex mixing products to insure that there will be absolutely no unwanted birdies within the passband.
NOTE: VU2ITI has published a good article ( Ceramic_Resonator_VFO_by_VU2ITI_SPARK.pdf ) on using a ceramic resonator for VFO/VXO applications.? If you can find a copy of that document you might be able to apply something like this for your BITX40, assuming you can find a ceramic resonator that is in the 3.0-2.7 MHZ range.

At this point I also have not completed an acceptable linear amplifier section for 40 Meters.? Plans are for this to probably be a push-pull set of IRF-510 devices, but work on another transceiver (not the BITX40) has taken priority over that effort.? Obviously you can alter the inductors of Farhan's design to make it work on 40 Meters, but I want to go for a bit more power on 40, thus I will need to work up a different design for the linear.

For my BITX40 I have built two different IF Filter modules.? At this point I am not sure which to use.? The ladder filter using 10 MHZ crystals did not tune up well for me (the one for my BITX20 worked perfectly), probably because I used a set of older & larger 10 MHZ crystals with apparently much different characteristics.? I also built a lattice filter for 10 MHZ.? This seems to have less loss and a steeper slope for exclusion of the upper sideband energy.? However, I am still tinkering with both filters and am not ready to publish anything about them yet.? If you are contemplating near-future completion of your BITX40 you should probably closely follow Farhan's design for the IF Filter and use the same brand (KDS) crystals that he recommends.

In the FILES section of the forum you will find a spreadsheet program that calculates the VFO frequencies for BITX designs.? This will also let you explore a design that uses crystals other than 10 MHZ for the filter and BFO.? This might be handy if you are contemplating use of something like TV color-burst crystals (3.579 MHZ) for your filter and BFO (this is not recommended for a BITX40 though because your 3.579 MHZ BFO second harmonic would be at 7.158 in the 40 Meter band).? My longer-term plan is to use color-burst crystals for an LF (0.160 to 0.190 MHZ) version of the BITX, but that will have to wait until I finish the BITX40.

I will continue to publish portions of my BITX40 here on this forum as each section is finished and? tested.? If you have any particular problems please feel free to email me (I may be having the same problem...) and maybe we can possibly resolve them together.? The other members of this forum are also a good source of ideas and solutions.

73's
Arv - K7HKL
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On Mon, 2004-07-12 at 23:46, Bharat D Balsavar wrote: