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HI Folks, I've just found this url in a newsgroup that may be of
interest to someone out there :-)



Using AVR's in both projects, with free development software, free
samples from Amtel and a simple programming interface this
looks, to me, like an interesting prospect.

all the best, Mike W, G8NXD

Hello,

I have just uploaded a file with the results of my measurements of
the IF filters passband.

The set-up was a DDS-VFO as a signal source and a HF millivoltmeter
as a detector. I have measured the IF Filter including one amplifier
on each side. That are transistors Q11 and Q12 und Ashhar circuit
diagramm.

A previous measurement of just the filter terminated with 56Ohm
resistors on each side gave a far worse result (less bandwith, >3dB of
bumpiness in the passband), which I think comes from wrong termination of
the filter. I did not know, how to terminate it correctly, thus I used the
amplifiers of the circuit.

I am impressed with the shape I have obtained, but the passband seems
a little wide.

Building continues towards the audio end of the BITX. I will then
repeat measurements with feeding audio in the mic and measuring
response in the IF.

73 Heinz, OE5EEP

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MILAREPA@.............

It would appear from your chart that the higher frequency crystals may not be oscillating, or that your frequency counter may not be capable of counting above 10 MHz.
Lets explore the first possibility (that the higher frequency crystals may not be oscillating):

? (1)? Check the RF output of your crystal oscillator using a simple diode & capacitor RF detector and a high impedance DC volt meter.? If there is RF present it should be in the range of 4 to 12 volts.

? (2)? If the transistor you are using for the Crystal oscillator is not capable of operating at 10 MHz or above, it may be not oscillating with the higher frequency crystals.? The solution for this is to use a better transistor.? In the US we have 2N2222 and 2N3904 available for very reasonable prices so this is what I use for most of my HF work.? These devices are rated to 150 MHZ, and will usually oscillate at even higher frequencies.

? (3)? In your crystal oscillator circuit it is possible that one or more of the capacitors you are using is not the capacitance value that you think it is.? Check each capacitor on a capacitor tester to determine its actual measured capacitance.? Then verify on the schematic and parts list that you are using the correct value for the design.? If you do not have a capacitance meter, you can use the simple AC transformer and Digital VoltMeter (DVM) method outlined in one of my earlier emails to the forum.? A schematic and picture of this is also available in the PHOTOS section of the forum archives (look under my call sign, " K7HKL " )

? (4)? The DC bias may not be correct in your crystal oscillator circuit.? Check the voltages on the oscillator transistor using a high impedance voltmeter.? The difference between base and emitter voltages should be approximately 0.7 volts for silicon transistors (it will be about 0.3 volt for germanium transistors, but I have not seen one of those antiques for several years).? The collector voltage should be about 1/2 the supply voltage.? If your oscillator design has an emitter resistor you can measure the voltage drop across that device and with a little math determine the collector current of your oscillator transistor.

? (5) It is possible that one or more of the resistors in your crystal oscillator circuit may not be close enough to the design value.? A quick ohm meter check on each resistor should detect this situation.? The resistor values are not particularly critical, but they should be within 10 to 20 percent of those indicated in Farhan's design.?

Now, the "possibilities" for your counter being the problem:

? (1)? Many years ago I built a number of frequency counters using 7490 dividers and 7447 LED drivers.? One of the major problems experienced with those circuits was a lack of HF sensitivity in the input amplifier and count gating circuit.? Designing a frequency counter to have a very high input impedance and flat frequency response over a wide bandwidth is nearly impossible.? I finally resorted to engineering for a lower input impedance and using a very broadband input amplifier to drive my 7400 count gating circuit.?

? (2)? One mistake that is prevalent is to use a low impedance coaxial lead as the input to your frequency counter but configure the counter for high impedance input.? Obviously this is a contradiction in impedances.? If you are doing this you can try "terminating either end of your counter input coax with a low impedance (non-inductive) resistor.? Start with about 470 ohms and work up or down from there.? An alternative might be to use a very small capacitor (about 10 pf) in series between the counter input cable and the circuit under test.? This lets the counter see a higher impedance, and does not disturb the circuit-under-test as much by loading it with whatever input cable impedance you are using on your counter.

? (2)? If you have a signal generator that will go higher than 10 or 12 MHz you could verify that your counter is capable of reliably counting to those higher frequencies.? At the same time you might want to "calibrate" the sensitivity of your counter so that you will know how much RF voltage it takes at 5, 10, & 20 MHz to result in a reliable count of that frequency.? Do this by using a carbon potentiometer to attenuate the input to your counter and use your diode detector and high impedance voltmeter to measure the RF voltage level at which counting becomes unreliable.? Be sure to use the same test leads on your counter for both sensitivity calibration and for measurements on your BITX20 circuits.

Regarding the use of 3.579 MHz crystals as IF Filters:

Using 3.579 MHz crystals as IF Filters is an interesting experiment.? The older and larger sized crystals that I have available exhibit much different characteristics than those of smaller 10 MHz crystals.? I was unable to obtain consistent motional parameters from my test setup when using 3.579 MHz units, but the 10 MHz crystals worked first time.? For an experiment I selected a pair of 3.579 crystals that were 1.4 KHz different in frequency and used these to build up a half-lattice filter.? The half-lattice design requires use of a toroid transformer, and here I did use a ferrite core for that purpose.? If you don't have a suitable ferrite core available you can try the Tap Washer cores that Farhan suggests, but at this time I have no data on what might be the proper number of turns.?? I found the half-lattice filter using 3.579 MHz crystals to be adequate for SSB work.? However, the use of a half-lattice filter does not readily adapt to the bi-directional design used in the BITX20, so this may not be a good solution.

It is also possible to use a single 3.579 MHz crystal in series with the signal path as the IF filter in simple superhetrodyne receivers.? Again this becomes problematic if one tries to use this arrangement in a bi-directional design like the BITX20, because the terminating resistor is on the wrong end of the crystal in transmit mode.? I suppose one could do some sort of diode switching of either the whole crystal filter or just the terminating resistor, but that is rapidly getting beyond the simple nature of the original BITX design objective.

Here is a thought that I have been playing with for several days...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...?

OK, I have probably covered much more than you wanted to know.

I hope this helps.

Arv - K7HKL
_._?

On Fri, 2004-07-16 at 04:30, MILAREPA wrote:

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--- In BITX20@..., "Mike W" <mike@c...> wrote:

I have found a 'source' of minature binocular balun cores in old UHF
TV tuners and computer UHF modulators. there are usually, it
appears, one or two per tuner. Not tried them yet though.
atb Mike W, G8NXD

Mike

Some time ago (actually, many years ago) I remember reading somewhere
that you can make up your own "binocular cores" by stacking two rows
of conventional toroid cores. That's about all I remember of the
original technical note, so you are on your own from there.

Arv
_._

Hi

Sadly it appears that a spammer has joined our group and sent a spam email
which I received here at 1809 UTC.

I have deleted the offending message from the messages archive, but those of
you who have elected to receive individual emails will already have received
it.

I have removed the spammer from the group and modified the membership
setting to "restricted". This means that from now on, new members must be
approved by one of the group moderators (myself or Farhan) before they are
permitted to join the group. I suggest that new applications be approved
only if the email address appears to be genuine, for example if it contains
a valid callsign or if a google search on the name identifies a real person
- if there is any doubt I will reply to the applicant asking some question
about why they want to apply and what their interest in the group is. I
would imagine that this will deter spammers and if they do attempt a reply,
will enable easy identification.

I don't want to make it difficult to join the group, and didn't want to have
to restrict the membership like this. But apparently it is necessary in
order to maintain the quality of the group and protect the privacy of the
membership.

Any comments or suggestions are welcome.

73 de Hans G0UPL

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Hi

Hans' comments (see below) are very appropriate to the measurement of Crystal frequencies.? I too found that if you solder the crystals in place, it may be necessary to wait several minutes for things to cool down to where frequencies are stable.? This applies to crystals in the 10 mhz filter as well as those in oscillator positions.? When soldering the crystals in place, it helps to use a heat sink on the leads between the solder joint and the crystal.? Just gripping the lead with a pair of pliers while soldering is usually adequate.? Wrap a rubber-band around the pliers handles to keep them on the lead if you need three hands to hold the pliers, hold the solder, and hold the soldering iron.

If you build the AF section, the IF-to-AF mixer, and the BFO before you build the IF Filter it is possible to use these as part of your test equipment in alignment of the 10 mhz filter.? Build the VFO but temporarily replace the VFO resonant circuit with a spare 10 mhz crystal and a series variable capacitor.? Use this oscillator as the signal source to "test" your IF Filter.? When the "VFO Crystal" and the BFO are tuned you will find a zero-beat if they are both at the same frequency.? Tuning either of these oscillators off-frequency will result in an audio tone that represents the difference in frequency.?

Using a simple diode detector and high impedance voltmeter you should be able to plot the bandpass characteristics of your IF Filter (turn the BFO "OFF" for these measurements).? The ideal bandwidth of the filter would be about 2.1 KHZ wide at the -6 db points and 4.2 khz wide at -60 db.? You may not be able to achieve quite that good a response with the simple ladder filter used in the BITX series, but it is the goal to strive for.? The slope at the sides of your filter should be as steep as possible.?

Once you have a picture of your filter bandwidth you can adjust the BFO frequency to position it at the -20 db point on one side or the other of that pass-band.? Which side of the passband you use is determined by which sideband (USB or LSB) you wish to transmit.? Why put the carrier at the -20 db point...because that helps to attenuate some of the lower audio frequencies in the desired sideband.? These lower audio frequencies are not really necessary for intelligibility, but they do use up a significant amount of power if they are transmitted.? Filtering out the lower audio frequencies ( 0 hz to about 350 hz ) allows more of your transmitted power to represent only the audio spectrum that is necessary for communication.? This same characteristic helps improve the apparent quality of received signals by reducing received audio power that might be devoted to these lower and unnecessary frequencies.

ALTERNATIVE FILTER PLOTTING METHOD:
If you do not have a frequency counter for monitoring the 10 mhz signal source, there is an alternative method for plotting the bandwidth of your filter.? With the BFO "OFF", use the RF voltmeter arrangement and your tunable VFO Crystal oscillator to determine the center of your filter pass band.? Set the 10 mhz VFO crystal oscillator to the center of that bandwidth.? Now turn your BFO "ON" and adjust it for a zero-beat with the VFO crystal frequency.? Now comes the hard part!? Slowly adjust your VFO Crystal frequency to find the estimated 1000 hz tone location on both sides of this zero-beat.? Carefully mark those frequencies on the dial of your VFO Crystal tuning knob.? Now you can turn the BFO "OFF" and proceed to use your "calibrated" signal source (the VFO Crystal oscillator) to make an approximate plot of your IF Filter bandwidth.? Note, that this is only an approximate way to tune your filter, but it will get you close enough for most operations.? The signal level through the filter should decrease very rapidly as you tune beyond the 1 khz points ( +1 khz and -1khz represent a total of 2 khz and should be the approximate edges of your filter bandwidth).

The IF Filter is probably the most confusing part of the BITX design, but it is also the easiest to duplicate if you use exactly the same components that Farhan specified.? Departing from Farhan's design specifications is what seems to cause the most confusion and doubt.? My recommendation would to build at least one BITX20 that is exactly like Farhan's original design.? That exercise provides the necessary experience to support deviating from his design, and it will provide you with a reference transceiver that may be used for comparison type troubleshooting.

73's
Arv
_._

From:? Hans Summers <>
Date:? Wed?Jul?14,?2004? 2:38 am
Subject:? RE: [BITX20] CRYSTAL MEASUREMENTS


?
> Frequency reading is still moving / drifting,
> although circuit has been warming up about 5
> minutes.
> Q: How do you "note/catch" the frequency in this situation ?
?
Your circuit and method look Ok to me... how bad is the drift, and what happens if you wait longer than 5 minutes? The drift could also be in your frequency counter (or PC if you are using that as a frequency counter). The crystal itself will also drift, we tend to think of crystals as rock stable but they aren't at all. Relative to an LC oscillator maybe, but not in absolute terms. Just read about the problems I had getting a stable frequency in my 30m QRSS beacon (?and ).
?
In my BITX20 evaluation, I managed to find a socket which fit the crystals. If you are soldering the crystals in, you might find that the initial drift is rather high due to the crystal being so hot from the soldering iron and needing to cool back down to room temperature. A socket is better for testing. My socket was a proper HC49 socket temporarily "borrowed" out of a different unfinished project. But you can hunt around the junk box and another type of socket which will work Ok. The wires of the crystal are long enough that you can bend them slightly to fit in whatever socket you find to be suitable. Even with a socket, there was some initial drift due to the temperature stabilisation after me holding the crystal in my fingers.
?
73 Hans G0UPL
http://www.HansSummers.com

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Hi Arv,

Thanks a lot for the very detailed reply to my query and for offering to advise me on any problems I might face with building the trx.

Knowing that I can bank on the group's knowledge and experience sure encourages me to experiment and learn and to build the project.

Very warm regards.
73s
-----
Bharat D Balsavar
VU2BDX





----Original Message Follows----
From: Arv Evans <arvevans@...>
Reply-To: BITX20@...
To: BITX20@...
Subject: Re: [BITX20] Re: BITX20 & BITX40
Date: Tue, 13 Jul 2004 12:22:44 -0600

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
<arvevans@...> 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.

-----------------------------------------------------------------------------------------

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.

-----------------------------------------------------------------------------------------

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
_._

On Mon, 2004-07-12 at 23:46, Bharat D Balsavar wrote:

Hi Arv,

I'm thinking of building a 40m version of the BITX20. As Ashhar
mentions in one of his earlier mails, that is the band more popular
here in India.

Could you please guide me on the different stages of the circuit you
modified and the nature of changes you made?

Also, may I please have a copy of the gif file of the discreet
components AF Amplifier?

Thanks in advance.
73s,
Bharat, VU2BDX


--- In BITX20@..., arvevans@e... wrote:

Hello to the BITX20 group (this is my first posting on this forum).

Farhan and myself have exchanged a couple of off-forum emails

regarding mods to his excellent design. One interesting possibility
is replacement of the LM-386 with a discrete component AF Amplifier.
He said that he had not had the time to work up a schematic...so I
have provided one (see attached .gif file). This can allow those
without access to an LM-386 to build the unit.

Also, my layout for the BITX20 uses 10.7 MHZ IF transformers

salvaged from dead AM/FM radios ( I dislike winding toroids! ). My
construction is a BITX40 (40 Meters) and requires 55 pf across the
10.7 IFs to resonate on 7.2 MHZ. These 10.7 IF transformers might be
made to resonate on 20M if the internal capacitors were removed (
break them with a screwdriver point ) and a small variable ( 5-25pf )
used to bring them to resonance.

73's
Arv - K7HKL

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Paolo

In the US, plastic knitting needles are available for an attractive price.? These range from 1/8 inch to 1/2 inch diameter.? The smaller ones are easily ground to a screwdriver point using sandpaper taped to a flat surface.
If one is careful you can even shape them to hex or octagon shapes for adjusting slug tuned coils.

TIP:
For trial testing of coils to see if they need increased or decreased inductance you can build a simple tool to temporarily shift the inductance up or down.? Use a plastic drinking straw and insert a small brass rod (about 1/4 inch long) into one end.? Insert a small rod of ferrite material (a chunk of broken ferrite ring works fine) in the other end.? Now you can temporarily insert either end into your coil-under-test and observe the results.? The brass will cause an increase in frequency (decreased inductance) and the ferrite will cause a decrease in frequency (increased inductance).
No, I have not tried this with the "Tap Washer Toroids" used in the BITX20, but I will later today because I am curious to see what the results might be.

Arv
_._
On Tue, 2004-07-13 at 01:52, Paolo Cravero as2594 wrote:

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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.

-----------------------------------------------------------------------------------------

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.

-----------------------------------------------------------------------------------------

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
_._

On Mon, 2004-07-12 at 23:46, Bharat D Balsavar wrote:

I have found a 'source' of minature binocular balun cores in old UHF
TV tuners and computer UHF modulators. there are usually, it
appears, one or two per tuner. Not tried them yet though.
atb Mike W, G8NXD

开云体育

Hi Paolo,

I cut/filed a piece of PCB to the right size and
sanded of the copper. The slight advantage over a
toothbrush is that the material is less soft, so it
can handle screws that are tight or stuck.

72 de Hubert

--- Paolo Cravero as2594 <pcravero@...> wrote:

Mike W wrote:

following Arv Evans's useful tips I would like to share with you how I
built a non-conductive screwdriver to regulate capacitor trimmers.

Whats wrong with the old plastic knitting needle ?. When playing with HV circuits they are a lot longer and therefore safer.

WOW! Got to ask my mother if she has some spare plastic ones!

For those non-native English speakers that might wonder what a "knitting needle" is, see the description here or just the picture here

Now I wonder if knitting needles are more widespread than toothbrushes! :-)

Paolo

PS: if someone ever makes a component bag for BITX20 s/he should consider adding a knitting needle or a toothbrush to the complete set of tap washers! :-)

--
QRPp-I #707 + www.paolocravero.tk + I QRP #476
Beacon @ 28.3219 MHz + QRPp + QRSS3 + JN35TC

On 13 Jul 04, at 9:52, Paolo Cravero as2594 wrote:

Hi,
following Arv Evans's useful tips I would like to share with you how I
built a non-conductive screwdriver to regulate capacitor trimmers.

Whats wrong with the old plastic knitting needle ?. When playing
with HV circuits they are a lot longer and therefore safer.

Another useful tool is a tuning wand, brass ferrule on one end of a
knitting needle and a ferrite ferrule on 'tother. When placed into the
flux field of a coil the ferrite end raises the inductance and the
brass end reduces it. Enabling a relativly easy way to determine if
the coil is too big or too small. Sadly, AFIK, it does'nt work with
toroids though.
atb Mike W
--

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Hi,
following Arv Evans's useful tips I would like to share with you how I
built a non-conductive screwdriver to regulate capacitor trimmers.

A quick glance at the BITX20 schematic did not reveal any "floating"
trimcap, all of them have at least one terminal to ground.
In these situations it is important to know that the trimmer screw is
electrically connected to one of the trimcap terminals: solder that
terminal to ground! This way you can use any screwdriver without
detuning the circuit with your hand.
Use an ohm-meter between the screw and terminals to identify which
one(s) are to be grounded.
(This might be obvious, but I didn't know until a few monhts ago!)

If the trimcap is floating instead, you need a plastic screwdriver.
Since it might be rather expensive to buy (at least in Italy) I built
one out of a toothbrush. Toothbrush should be changed often, ;-), and it
is usually made of some hard plastic (+ rubber for a better grip).

So, cut off the brush and shape the stick with a knife until you come up
with something that looks like a screwdriver (flat) head. I found the
plastic to be quite resistant, so you need to work it in small steps as
if you were creating a wood sculpture.

Apply the usual disclaimer about personal safety while handling the
knife and you're on the way to have a simple and ergonomical all-plastic
screwdriver.

Has anyone identified other materials that could be used for such a purpose?

72,
Paolo IK1ZYW

--
QRPp-I #707? + www.paolocravero.tk +? I QRP #476
?? Beacon @ 28.3219 MHz + QRPp + QRSS3 + JN35TC

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