--- In BITX20@..., "ajparent1" <kb1gmx@a...> wrote:

Two problems:

The board pattern is on the wrong side (pattern
is on top not bottom.) To use it (I may not)
the parts must be placed on foil side!

You are sure? I don't know, but if it is, what a problem!

No information on parts location on the board.
A picture of the assembled board or an overlay
would be helpful.

You can try my "manual overlay". It is located in the group files
section, here:

pPrbXMZgXY2_vH6pqJs-e9G2bWPeL7M7n38dFFLCLpj1CpS-lP_N/BITX%20PCBs%20%
28Printed%20Circuit%20Boards%29/bitx_component_placing.jpg

Good bitxing...
73s
Manuel
EB7HPM/EC7ALV

--- In BITX20@..., Rob Frohne <frohro@w...> wrote:

Hi All,

I had some surplus 6 MHz crystals here, and so I'm using that as an
IF. If I use the 50 ohm termination impedance that Fahran has set
up, the filter ripple is more than 10 dB in the passband. (It should
be better with a 10 MHz IF.) If I can raise the termination
impedance it helps a lot on the ripple. It is harder to keep the
high frequency response with the higher impedances I'm trying to use
as well.

One possible solution is terminate the filter at 200ohms and use
a 4:1 balun at each end. That yeilds a 50ohm interface to the
amplifiers and allows a filter at a more reasonable impedence.

FYI: you may consider a 5:1 or even as high as 9:1 ferrite loaded
transformer as at 6mhz my experiments with filters for SSB passband
indicated a 400-500ohm termination was more redially achievable
and gave better shape factors with less ripple. The losses in the
transformers are small and can result in lower losses in the filter
at a more workable terminating impedence.

Allison
KB1GMX

--- In BITX20@..., Rob Frohne <frohro@w...> wrote:

Hi All,

I have been doing some Spice simulations, and after playing around
with it a bit, it is evident that using microwave transistors will
improve things a lot over the HF general purpose transistors. The
culprit is C sub mu, the capacitance between the collector and the
base due to the reverse biased collector base junction. The problem
is made worse by having both the transmit and receive transistors
capacitance in parallel. This provides more parallel current
feedback than we want at higher frequencies. Simulating using
microwave transistors shows that the 50 ohm input impedance goes up
to almost 100 MHz, but with the general purpose HF transistors, you
only get to a very maximum of 10 MHz (often less), and then the
C sub mu messes things up.

That is indeed true but in practice transistors like the 2n3904
and 2n/pn2222A easily perfom to 20+ mhz. These devices have a
FT or 300-350mhz. A rule of thumb is to stay below FT/10 for
predictable feedback amplifier operation at medium gains. For
example 2n3904 has Ft of 300mhz (min) and nominal HFE of 100
at low frequencies or DC. At 30mhz the effective AC hfe is
only 10 so gains of greater than 10 (20db voltage) are not
likely with broadband circuits. This is due not only to C-B
feedback but also C-E capacitance. The bias currents, working
impedences and feedback amounts do interact. At lower impedences
(50 input and 50 to 200ohm output) the devices perform well.
Even with small degradation the cost of these devices are often
un the less than 10 cents(US) range make them attractive. I usually
source 2n3904, 2n3906, PN2222 types in quantities of 20 for 6
cents(US) each.

So for this design based on experience devices in the 2n3904 and
2222 are good for the IF, LO and maybe the RF up to 20m (14mhz).
A device like the 2n5179 would be advised for the RF amp(Q1)as
is both lower noise and has an FT in excess of 1ghz. Others
that work well are 2N3866, 2n5109, 2sc2570. Devices characterized
for CATV use are excellent.

Things to look for in an IF transistor for this circuit, good HFE at
10-20ma (over 100 is advised). An FT of not less that 300mhz is
adaquate and there are many 2n, and 2SC devices that easily meet
this.

For the RF amp (Q1) the noise figure of the transistor is important.
most good VHF and uhf devices are low noise. However the 2n3904
is surprizingly good (typical 4-5db) for low (below 20mhz) HF use.

I've used 2n3904, 2n2222A with excellent results as high
as 40mhz for circuits such as IF amp, buffer amplifiers and
general use wide band amplifier to 30mhz. Another device the
2n4124 has been used for 6m as amplifier(wideband) and oscillator.
The trick if there is any is recognizing that trying to achieve more
gain at working frequncy than the device will resonably yeild is
moving toward instability and requires a different topology than a
wideband feeback amplifier, or a better device.

An aside: if you going to analyse the amplifiers also anaylyse their
interaction with the crystal filter. You will find that there are
other first order and second order effects as the filter band edges
are approaced and the balanced mixers are not overly fond of
mistermination. These are as significant as C sub mu interactions.
One of the ugly facts of feedback amplifiers as used is that output
impedence interacts with input impedence (and also reverse) and as a
result gain also changes. Crystal filters as you approach band edges
and out of band undergo significant changes in termination phase and
resistance.


Allison,
KB1GMX {in any project there is: Good, Fast, Cheap, pick any two.}

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(sometimes called nail varnish) of the clear color works well.


Allison
KB1GMX


--- In BITX20@..., Rahul Srivastava <vu3wjm@y...> wrote:

Hi!

To be safe, a layer of plumbers PTFE tape on balun before winding

the coils is helpful.

73

Rahul VU3WJM



Steve Hartley <g0fuw@t...> wrote:
Ashhar and freinds
I have solved my balun core problem. As you guessed, it was caused

by sharpe

edges inside the core. I small file and emery paper removed them and
windings now well isolated from core.
For UK builders, Robin at Sycom (sycomcomp.co.uk) stocks the cores -

part

number FX2754 about 50p each, I think.
Now, on with the Tx.
73, Steve, G0FUW
----- Original Message -----
From: "Ashhar Farhan" <farhan@p...>
To: <BITX20@...>
Sent: Friday, July 08, 2005 6:14 AM
Subject: Re: [BITX20] BALUN Core Winding

the problem with balun is that it has a sharp edges. these edges

nick the

enamelled wire being passed around. and the wire gets scrapped and

touches

the core. if more than two wires get nicked, they short, showing high
resistance like you said.

you need to be careful in not pulling the wire too tighly around the
edges of the balun. a better way might be to coat the corners with

a bit

of paint.

- farhan

On Thu, 7 Jul 2005, g0fuw wrote:

Guys
I am building a BITX and have the rx working very nice.
However, there is a problem of microphics around the detector/af

amp.

Ceramics replaced, as per past postings. No change.
I have now traced this to the balun core and find I have a

'short' of

about 60k between one of the windings and the core.
I have tried different wire and smoothing the holes before

winding but

every time I get a 'short'.
I have tried a toroid core and that works without the problem.
Interested to know if anyone else has experienced this and if they
found a fix!
73, Steve, G0FUW





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I build for VHF so most of the PA problems have not been noted.

The first model I built was fairly close to basic BITX-20 save for
no power amp. I used Ferrite (FT37-43) torrids as I have them.
this version was built as testbed using dead bug (ugly construction)
with output power sufficient to drive a 20m input for a 6m
transverter. Only a few milliwatts are needed for that. Works very
well in the role of an IF for VHF use. The first significant
modification was a Diode based attenuator and audio derived agc.
Works well enough to preserve my ears without causing undesired
effects. The diode Attenuator is placed after the (board level) RF amp
in the recieve path and before the 1st mixer. The attenuator used is
page 6.18 in EMRFD, it's a proven design that works fairly well with
1n4007 rectifier diodes (at 14mhz). It uses a lot of parts and
works well but as an engineer I'm also interested in fewer parts
so I am looking at alternate circuits. One note: using toroids the
stability of the reciever is better than using plastic washers, the
magnetic field is restrained better on the toroid and fewer turns of
wire are required.

I am now building a second model for 6m use. Instead of the VFO
I am using a tested PLL of my own design at ~40mhz and the IF will be
9.6mhz. The use of 9.6mhz is to avoid WWV bleeding into the IF on
10mhz and also the crystals were available in quantity for matching.
A PLL avoids the problems of trying to build a stable VFO for 40mhz!
I've elected to use a available PC board from Far Circuits but
from looking at the board the parts placement is a mystery (maybe a
contest?). Also I will have to clean up the etch as there are wiskers
and other undesired tracks on the board. A picture of a complete
board (with part in place) or a drawn layout would help. The RF power
chain will not use a IRF powerfet as their perfomance (both IMD and
gain) at 12v is poor at 50mhz. I will use more common bipolar such
as 2sc1969, 2sc1970 or 2sc1971 for power. I have used FETs for power
before but at VHF you need better devices.

Allison
KB1GMX

--- In BITX20@..., Ashhar Farhan <farhan@p...> wrote:

Far Circuits is offering the BITX20 PCBs.

- farhan

Ordered one and got it.

Two problems:

The board pattern is on the wrong side (pattern
is on top not bottom.) To use it (I may not)
the parts must be placed on foil side!

No information on parts location on the board.
A picture of the assembled board or an overlay
would be helpful.

Allison
KB1GMX

Hi! All

Recent Comments by Farhan on a modified BITX20 got me thinking. One of
the suggestions was increasing the Rx gain. I too had encountred low
gain due to poor quality devices used by me.

I made a modified (Ver3 under files) that used shunt feedback amp in
IF this solved the problem to an extent, however a SPICE by Prof Rob
indicated that noise was slightly high for application. It was back to
EMRFD, where it all began.

A study of various ckts of negative feedback amplifier revealed that
that 1K-2K C-B feeback/ base bias resistor value was more in
consistence in design using very High ft trasistors and mostly of
power rating of 2N5109 class. This low value also probably resulted in
higher feedback that caused the gain of the amp to roll off at a much
lower freq when gen purpose devices were used. 3K3 was more in
acacordance with devices og 2N5179 class.

Chap-2 EMRFD- Feedback amplifier section 2.27 gives detailed analysis
of this type of amplifier. Table 2.2 gives various values on 14mhz for
a 2N3904 biased at 20mA. The combination used in BITX 20 of load at
200 ohms with R-degen 10ohms, R-feedback 910 (1K in BITX) the gain is
16.8db. These are the conditions in our present design.

The table also gives: ( For 200 ohm load)
R-dgen 4.7ohms, R-feedback 2.7K Gain 23.9db
R-dgen 3.9ohms, R-feedback 3K Gain 24.8db

I feel like changing a few resistor and re-checking the performance.
We will need to reduce the emitter resistor to bring back the current
upto 20mA. In BITX config we can try 5k6 or 6k8 in feedback for higher
gain. This increase in value will also reduce the early gain fall
off.

I invite your comments and suggestions before proceeding with changes
on my existing BITX.
While my friends like Sunil VU3SUA are using their BITX on air mine
seems to be more on the workbench Hi! Hi!.

73

Rahul VU3WJM

Ashhar and freinds
I have solved my balun core problem. As you guessed, it was caused by sharpe
edges inside the core. I small file and emery paper removed them and
windings now well isolated from core.
For UK builders, Robin at Sycom (sycomcomp.co.uk) stocks the cores - part
number FX2754 about 50p each, I think.
Now, on with the Tx.
73, Steve, G0FUW
----- Original Message -----
From: "Ashhar Farhan"
To:
Sent: Friday, July 08, 2005 6:14 AM
Subject: Re: [BITX20] BALUN Core Winding


>
> the problem with balun is that it has a sharp edges. these edges nick the
> enamelled wire being passed around. and the wire gets scrapped and touches
> the core. if more than two wires get nicked, they short, showing high
> resistance like you said.
>
> you need to be careful in not pulling the wire too tighly around the
> edges of the balun. a better way might be to coat the corners with a bit
> of paint.
>
> - farhan
>
> On Thu, 7 Jul 2005, g0fuw wrote:
>
> > Guys
> > I am building a BITX and have the rx working very nice.
> > However, there is a problem of microphics around the detector/af amp.
> > Ceramics replaced, as per past postings. No change.
> > I have now traced this to the balun core and find I have a 'short' of
> > about 60k between one of the windings and the core.
> > I have tried different wire and smoothing the holes before winding but
> > every time I get a 'short'.
> > I have tried a toroid core and that works without the problem.
> > Interested to know if anyone else has experienced this and if they
> > found a fix!
> > 73, Steve, G0FUW
> >
> >
> >
> >
> >
> > Yahoo! Groups Links
> >
> >
> >
> >
> >
> >
>
>
>
> Yahoo! Groups Links
>
>
>
>
>
>
>

---

NEW - crystal clear PC to PC

Ashhar and freinds
I have solved my balun core problem. As you guessed, it was caused by sharpe
edges inside the core. I small file and emery paper removed them and
windings now well isolated from core.
For UK builders, Robin at Sycom (sycomcomp.co.uk) stocks the cores - part
number FX2754 about 50p each, I think.
Now, on with the Tx.
73, Steve, G0FUW

Hi friends,
This is my first post on the group, my compliments to farhan for
bringing out the bitx, and thanks to vu3wjm for working and bringing
out the pcb`s for the tcvr, with some changes in the ckt.
I have been using the bitx for some months now. My bitx is driven by
a 20 watt linear amp, and I have worked many vu and dx stations with
very good reports. I am keenly looking forward to operate my bitx
with any ham having a bitx. A sked can be worked out for a two way
qso with bitx
Best wishes and 73`s
Sunil
VU3SUA

Hello Javed,

This is discribed in the article "a simple ssb tranceiver" by Asshar
Farhan. You can find that article in the Files box of this group.

73

Ruud (PE2BS)

--- In BITX20@..., "javed4004" <javed4004@y...> wrote:

hello all friends,

Please tell me how to select the crystals (10mhz)for ssb ladder

filter.

Thanks,

Javed

hello all friends,

Please tell me how to select the crystals (10mhz)for ssb ladder filter.

Thanks,

Javed

digikey has them for about 5 box and sends all around the world.



--- In BITX20@..., "eb7hpm" <msraya@t...> wrote:

I want to reply to Farhan and others about the PLL issue although
this maybe a little off-topic.

Two years ago I were attempting to make a "PLL in a PIC" as Farhan
state starting from the Huff-Puff design appearing in Elektor
magazine: ei9gq/stab.html .

This design used an external 8 bits counter, internal 8 bit
preescaler and the internal 8 bits main counter, so that the

reading

of frequency was in 24 bits. However the process of reading was
tedious upon having to insert bits in order to read the two

counters

to which access was not had, the reading could slow down to

(256+256)

* 2 cycles of 400ns = 0.5 mS.

Once the reading was carried out it is necesary to make a

comparison

of 24 bits and to generate a pulse with a size(width) proportional
to that difference. All this process is very slow, it can be used
only for small differences from original frequency. So, the dynamic
range of control is very small in my opinion.

I think that it is impossible to carry out this type of control

with

a PIC fast or slow. In my opinion, if it is not possible to find

the

145170 one could use any standard PLL like those of National
Semiconductor of the LMX series, or the MB1502 of Fujitsu, the
Phillips SAA1057 or any another equivalent. In last case a DDS

could

be used. The main advantage of the MC145170 is their operation up

to

180MHz.

Greetings
Manuel
EB7HPM/ EC7ALV

I want to reply to Farhan and others about the PLL issue although
this maybe a little off-topic.

Two years ago I were attempting to make a "PLL in a PIC" as Farhan
state starting from the Huff-Puff design appearing in Elektor
magazine: ei9gq/stab.html .

This design used an external 8 bits counter, internal 8 bit
preescaler and the internal 8 bits main counter, so that the reading
of frequency was in 24 bits. However the process of reading was
tedious upon having to insert bits in order to read the two counters
to which access was not had, the reading could slow down to (256+256)
* 2 cycles of 400ns = 0.5 mS.

Once the reading was carried out it is necesary to make a comparison
of 24 bits and to generate a pulse with a size(width) proportional
to that difference. All this process is very slow, it can be used
only for small differences from original frequency. So, the dynamic
range of control is very small in my opinion.

I think that it is impossible to carry out this type of control with
a PIC fast or slow. In my opinion, if it is not possible to find the
145170 one could use any standard PLL like those of National
Semiconductor of the LMX series, or the MB1502 of Fujitsu, the
Phillips SAA1057 or any another equivalent. In last case a DDS could
be used. The main advantage of the MC145170 is their operation up to
180MHz.

Greetings
Manuel
EB7HPM/ EC7ALV

this is going off-topic, so i hope to make this a last post on PICs.
the frequency counting code for PICs is pretty nifty and it does the timing based on execution times of its own code and reads off the number of cycles-ticks tha accumulate in the timer register.

you can go through david tait's excellent archives on the net to get you started. i have used microchip's own IDE to simulate, debug and assemble the pic programs (written in assembly ofcourse).

the frequency counters based on PIC invariably use the code given in an application note that you can download from their website on www.microchip.com.

- farhan

the PIC timer can be used to accurately measure the oscillator frequency
(as normal PIC based frequency counters do) instead of displaying the
frequency on a display, it can (in addition) also pump a capacitor that
in turn is used to bias a varactor across the VFO.

As a latecomer to pics and picaxes I have managed to stumble through
some of the basics, but causing a number of LEDs to come on and off is
hardly exciting.
I have assembled a kitset frequency counter without having a clue as
to how it worked, because the chip was preprogrammed.

Can anyone point me towards an internet source where I might get an
explanation of how a pic ( or picaxe ) might be used for this purpose,
i.e what the program has to do, and why.

Regards, Sam C. ZL1SAM

On Jul 10, 2005, at 7:04 AM, Ashhar Farhan wrote:

while PLL chips are difficult to find, the 16F84 or the 16F628 are
plentiful. why can't we program the PIC to behave as a PLL on it's own?

the PIC timer can be used to accurately measure the oscillator frequency
(as normal PIC based frequency counters do) instead of displaying the
frequency on a display, it can (in addition) also pump a capacitor that
in turn is used to bias a varactor across the VFO.

if chris and his merry band can do a single chip HnP, there is no need to
assume that they can't turn out a single mirochip PLL too.

Given how cheap a PIC is, I think it's just a matter of time before we stop using standard TTL/CMOS logic, and start programming PICs as replacements. They won't be pin-for-pin replacements, but they'll be easier to locate than some of the more esoteric logic chips.

A PIC PLL sounds like a fabulous project.

Jim N6OTQ

while PLL chips are difficult to find, the 16F84 or the 16F628 are plentiful. why can't we program the PIC to behave as a PLL on it's own?

the PIC timer can be used to accurately measure the oscillator frequency (as normal PIC based frequency counters do) instead of displaying the frequency on a display, it can (in addition) also pump a capacitor that in turn is used to bias a varactor across the VFO.

if chris and his merry band can do a single chip HnP, there is no need to assume that they can't turn out a single mirochip PLL too.

the rumors about having to program the pics are exaggerated. probably an audio amplifier is more of a challenge than wiring togther a PIC programmer. i build one whenever i need to program a pic, it is usually easier to solder a couple of diodes than search out the old programmer again.

- farhan

Hello fellow BITX'ers,

Because I want do do some experiments with a ladderfilter on 9 MHz, I
studied the amplifier stage Asshar discribed and used in his BITX20.
I have some questions about it.

- I understand that the input and output impedance depends on the
resistors Rf and Re: Rf*Re=Rout*Rin. In the exemple with Rf=1000,
Re=10 and Rin=50, Rout becomes 200. But what happens if the input
is 100? There will be a mismatch but where will it be? Is the output
impedance fixed because of Rload (220 in the schema) and will be the
mismatch at the input of the amplifierstage?

- I studied the ladderfilter theory and different C's means different
input and output impedance of the filter. How to match the amplifier
to these impedances? Witch resistors do I have to change for the
exemple of 100? How to calculate them?

- The last question is about the two amplifier stages around the
ladderfilter. Left of the filter the emittor resistance is 220. Right
of the filter the emittor resistance is 470. The other resistors
around Q2 and Q3 are the same.Why is this? Does this has anything to
do with matching the impedance?

I hope someone will spend some time to answer my questions.

73

Ruud,
PE2BS