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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.

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.

Even the common emitter/common collector two transistor amplifier with series voltage feedback shows signs that it would benefit from the first transistor being a microwave BJT in my simulations.

So it looks to me like using microwave transistors for all the bidirectional amplifiers is a good idea if you can afford it. I may have to go looking in my junk box some more for some more microwave transistors.

73,

Rob, KL7NA
--
Rob Frohne, Ph.D., P.E.
E.F. Cross School of Engineering
Walla Walla College

Rob

It will be interesting to see what specific microwave transistors you
recommend for the BITX design. Hopefully, they will be generally
available and at a relatively low cost. Since the BITX philosophy has
tended toward this being the Volkswagen of transceivers (i.e. made from
inexpensive parts that are universally available worldwide) migrating to
microwave transistors may complicate parts procurement for some
builders.

The transistors used in television tuners might be candidates for better
Cmu performance, and they might be generally available as salvage or
maybe replacement parts. I wonder if these might be acceptable?
Problem is though that it would take a number of junk TVs to get enough
transistors for a BITX transceiver.

I find the analysis and results from what you are doing to be
fascinating. The findings could make a significant difference in a
number of different QRP equipment designs.

Arv K7HKL
_._

I use 2SC1815 for my BITX, which i could get for $2/100pc. My first built was using 2SC945 salvaged from IC-R71 receiver, both are multipurpose NPN, easy to obtain.

2N3904 specs is higher from 2SC1815 and 2SC945, cost about $7.50/100pc, its ft min. 300MHz, compared to 2SC1815 80MHz, 2SC945 150MHz.

2N2222 is almost similar to 2N3904, different package, cost a bit cheaper than 2N3904.

Maybe someone should do a test on how these transistor perform on BITX, i didnt hurt to have better performance on our trx.

i wonder if anybody had discuss about BITX antenna? since this also had contribution on how BITX perform.

rgds
julius

* REPLY SEPARATOR *

On 6/22/2005 at 8:48 PM Arv Evans wrote:

Rob

It will be interesting to see what specific microwave transistors you
recommend for the BITX design. Hopefully, they will be generally
available and at a relatively low cost. Since the BITX philosophy has
tended toward this being the Volkswagen of transceivers (i.e. made from
inexpensive parts that are universally available worldwide) migrating to
microwave transistors may complicate parts procurement for some
builders.

The transistors used in television tuners might be candidates for better
Cmu performance, and they might be generally available as salvage or
maybe replacement parts. I wonder if these might be acceptable?
Problem is though that it would take a number of junk TVs to get enough
transistors for a BITX transceiver.

I find the analysis and results from what you are doing to be
fascinating. The findings could make a significant difference in a
number of different QRP equipment designs.

Arv K7HKL
_._


On Wed, 2005-06-22 at 17:43, Rob Frohne 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.

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.

Even the common emitter/common collector two transistor amplifier
with series voltage feedback shows signs that it would benefit from
the first transistor being a microwave BJT in my simulations.

So it looks to me like using microwave transistors for all the
bidirectional amplifiers is a good idea if you can afford it. I may
have to go looking in my junk box some more for some more microwave
transistors.

73,

Rob, KL7NA
--
Rob Frohne, Ph.D., P.E.
E.F. Cross School of Engineering
Walla Walla College




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Rob

It will be interesting to see what specific microwave transistors you
recommend for the BITX design.? Hopefully, they will be generally
available and at a relatively low cost.? Since the BITX philosophy has
tended toward this being the Volkswagen of transceivers (i.e. made from
inexpensive parts that are universally available worldwide) migrating to
microwave transistors may complicate parts procurement for some
builders.?

The transistors used in television tuners might be candidates for better
Cmu performance, and they might be generally available as salvage or
maybe replacement parts.? I wonder if these might be acceptable?
Problem is though that it would take a number of junk TVs to get enough
transistors for a BITX transceiver.

I find the analysis and results from what you are doing to be
fascinating. The findings could make a significant difference in a
number of different QRP equipment designs.?

Arv K7HKL
_._


On Wed, 2005-06-22 at 17:43, Rob Frohne 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.
>
> 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.
>
> Even the common emitter/common collector two transistor amplifier?
> with series voltage feedback shows signs that it would benefit from?
> the first transistor being a microwave BJT in my simulations.
>
> So it looks to me like using microwave transistors for all the?
> bidirectional amplifiers is a good idea if you can afford it.? I may?
> have to go looking in my junk box some more for some more microwave?
> transistors.
>
> 73,
>
> Rob, KL7NA
> --
> Rob Frohne, Ph.D., P.E.
> E.F. Cross School of Engineering
> Walla Walla College
>
>
>
>
> ______________________________________________________________________
> Yahoo! Groups Links
>?????? * To visit your group on the web, go to:
>????????
>??????????
>?????? * To unsubscribe from this group, send an email to:
>???????? BITX20-unsubscribe@...
>??????????
>?????? * Your use of Yahoo! Groups is subject to the Yahoo! Terms of
>???????? Service.
>

---

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Hi Rob,
the termination of the filter is not in accordance with Fahrans design
50 ohm. Between the mixer and the filter is a bi-amp. The mixer is
likely 50 ohm so the other side of the bi-amp is about 200 ohm.
I have a 4.9 MHz filter my self, you can make the impedance higher by
adding two small transformers. See BITX17 in the photos directory.

About the transistors: I hear everyone complaining about BC547. I made
the BITX17 with BC107 and it works fine. A frind of me made it with
BC547 and he had also rather poor results. It should not make much
difference between these two types? How to explain?
Best regards,
Chris.

--- 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.}

--- 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