I am trying to scale an existing front end receiver (butterworth
bandpass) filter to a different frequency range. Unfortunately, it has
a transformer in the original design, so I'm stuck. I also don't know
how to handle the load presented by the active front end component
other than it's probably not significantly reactive.

The existing filter is for a 7 Mhz receiver, I'd like to have a
similar filter design for 50 to 200 Khz.

The filter components and transformer winding details are in the
document at:

http://www.amqrp.org/kits/softrock40...0Assy%20v1.pdf

The input chip is an FST3126, spec sheet is at:

http://www.fairchildsemi.com/ds/FS/FST3126.pdf

The spec for the T30-2 transformer core is at:

http://partsandkits.com/T30-2.htm

I have aade filter design software, but it isn't allowing me to plug
in the transformer into the design page of the software....so, I need
to know it's equivalent circuit I think.

The transformer winding details are on page 4 of the document and the
schematic of the front end is page 9. Ultimately, I need new values
for L1, L2, C20, C21 and C22.

If someone can give me a reasonable guess as to the equivalent circuit
of the transformer and the IC (U5), I can do the rest of the job
myself using the aade filter software.

Thanks,

T

In article , TRABEM
says...
If someone can give me a reasonable guess as to the equivalent circuit
of the transformer and the IC (U5), I can do the rest of the job
myself using the aade filter software.

Thanks,

T


You might try FilDes:
ftp://ftp.lehigh.edu/pub/listserv/qr...ols/fds201.zip

It will let you specify different source and load impedances for the
filter being designed, so you can probably get away without the
transformer altogether. The AADE software has a nicer interface but
I've always liked the functionality in FilDes better.

-- jm

------------------------------------------------------
http://www.qsl.net/ke5fx
Note: My E-mail address has been altered to avoid spam
------------------------------------------------------

It looks like the filter is designed for 50 ohms in and out. The transformer
inductance is also a part of the filter. It's value should be about the same as
L1 so to use it at 50k to 200K, you'll need to rewind for a lot more turns on
the primary and both secondaries if you keep the same core material.

Scaling it all and then optimizing for Coil Q of 40 and regular cap and coil
values, I get new values like:

L1 680nH - 100uH
C1 470pF - .022uF
C21 180pF - .027uF
L2 1.8uH - 100uH
C22 470pF - .022uF
T1 680nH (13:6:6 Turns) - 100uH (1911:882:882 Turns)

I strongly recommend changing T1 to a much higher mU core material.

This should result in -3dB points at 50 and 200 kHz, flat pass-band and -30dB at
20 and 500 kHz.


Good luck finding a 50 Ohm antenna at these frequncies. An active antenna may be
the way to go, otherwise you'll need many acres for all the wire.



On Sun, 11 Sep 2005 12:09:16 -0400, TRABEM wrote:

I am trying to scale an existing front end receiver (butterworth
bandpass) filter to a different frequency range. Unfortunately, it has
a transformer in the original design, so I'm stuck. I also don't know
how to handle the load presented by the active front end component
other than it's probably not significantly reactive.

The existing filter is for a 7 Mhz receiver, I'd like to have a
similar filter design for 50 to 200 Khz.

The filter components and transformer winding details are in the
document at:

http://www.amqrp.org/kits/softrock40...0Assy%20v1.pdf

The input chip is an FST3126, spec sheet is at:

http://www.fairchildsemi.com/ds/FS/FST3126.pdf

The spec for the T30-2 transformer core is at:

http://partsandkits.com/T30-2.htm

I have aade filter design software, but it isn't allowing me to plug
in the transformer into the design page of the software....so, I need
to know it's equivalent circuit I think.

The transformer winding details are on page 4 of the document and the
schematic of the front end is page 9. Ultimately, I need new values
for L1, L2, C20, C21 and C22.

If someone can give me a reasonable guess as to the equivalent circuit
of the transformer and the IC (U5), I can do the rest of the job
myself using the aade filter software.

Thanks,

T

I am trying to scale an existing front end receiver (butterworth
bandpass) filter to a different frequency range. Unfortunately, it has
a transformer in the original design, so I'm stuck. I also don't know
how to handle the load presented by the active front end component
other than it's probably not significantly reactive.

The existing filter is for a 7 Mhz receiver, I'd like to have a
similar filter design for 50 to 200 Khz.
===========================
If you are happy with the HF receiver as is, would it not be easier and
more effective to build an LF (50 - 200 kHz) to HF converter. This should
be an easy project and you can select a quiet "HF band" for the conversion.

I have seen designs with a SBL-1 mixer but also a number with the NE612
osc/mixer. Perhaps others frequenting this NG have built such a
converter.

Frank GMØCSZ / KN6WH

On Sun, 11 Sep 2005 20:56:34 GMT,
wrote:

It looks like the filter is designed for 50 ohms in and out. The transformer
inductance is also a part of the filter. It's value should be about the same as
L1 so to use it at 50k to 200K, you'll need to rewind for a lot more turns on
the primary and both secondaries if you keep the same core material.

Scaling it all and then optimizing for Coil Q of 40 and regular cap and coil
values, I get new values like:

L1 680nH - 100uH
C1 470pF - .022uF
C21 180pF - .027uF
L2 1.8uH - 100uH
C22 470pF - .022uF
T1 680nH (13:6:6 Turns) - 100uH (1911:882:882 Turns)

I strongly recommend changing T1 to a much higher mU core material.

This should result in -3dB points at 50 and 200 kHz, flat pass-band and -30dB at
20 and 500 kHz.



OK, thank you very much.

I checked the amidon website, higher mu toroids are available but
looks like they're a little more expensive.

If I want to simulate the filter myself, can I just ignore the series
10 ohm resistors in each side of the transformer secondary? Do the 5
ohm ac series resistance of the switch and the .1 uf capacitor on the
far side of the switch enter into the calculations at all????

I was thinking all those R's and C's get reflected back through the
transformer and should impact the filter values.

If I can ignore the R's and C's on the secondary side of the
transformer, can I simply pretend the primary is a coil with a 50 ohm
shunt (to represent the output impedance)??

When the series 10 ohm resistor, the 5 ohm series ac resistance of the
switch and the .01 cap to ground on the output side of the switch is
factored in, does the output impedance still look like 50 ohms?

Good luck finding a 50 Ohm antenna at these frequncies. An active antenna may be
the way to go, otherwise you'll need many acres for all the wire.



Yep, we can do it. Got 40 acres. But, wasn't planning on using that
many acres:: My plan was to use a single turn loop, 10 to 15 feet on
each side and step up the impedance with a toroid at the antenna. The
wire is 200 A service entrance insulated aluminum, so it should have a
decent Q if I use some polystyrene caps to resonate it.



Regards,

T

On Sun, 11 Sep 2005 12:09:16 -0400, TRABEM wrote:

I am trying to scale an existing front end receiver (butterworth
bandpass) filter to a different frequency range. Unfortunately, it has
a transformer in the original design, so I'm stuck. I also don't know
how to handle the load presented by the active front end component
other than it's probably not significantly reactive.

The existing filter is for a 7 Mhz receiver, I'd like to have a
similar filter design for 50 to 200 Khz.

The filter components and transformer winding details are in the
document at:

http://www.amqrp.org/kits/softrock40...0Assy%20v1.pdf

The input chip is an FST3126, spec sheet is at:

http://www.fairchildsemi.com/ds/FS/FST3126.pdf

The spec for the T30-2 transformer core is at:

http://partsandkits.com/T30-2.htm

I have aade filter design software, but it isn't allowing me to plug
in the transformer into the design page of the software....so, I need
to know it's equivalent circuit I think.

The transformer winding details are on page 4 of the document and the
schematic of the front end is page 9. Ultimately, I need new values
for L1, L2, C20, C21 and C22.

If someone can give me a reasonable guess as to the equivalent circuit
of the transformer and the IC (U5), I can do the rest of the job
myself using the aade filter software.

Thanks,

T

Hi,

Hi,

I am trying to scale an existing front end receiver (butterworth
bandpass) filter to a different frequency range. Unfortunately, it has
a transformer in the original design, so I'm stuck. I also don't know
how to handle the load presented by the active front end component
other than it's probably not significantly reactive.

The existing filter is for a 7 Mhz receiver, I'd like to have a
similar filter design for 50 to 200 Khz.

Notice that the primary to secondary (total) turns ratio is
almost 1:1, that both shunt capacitors in the band-pass filter are
of the same value and that the path resistance on either side of
the secondary, through the FST3126M to the low-pass filters is
around 20-ohms. So what you are looking at (to a crude
approximation) is a design impedance of 50-ohm all the way
through.

As for the toroid, for optimum Q you might want to look for a
mix more suitable for the frequency range you quoted (maybe 15?)
and increase the diameter to take the extra wire as you will need
to scale the impedances for use at 50kHz. The FT series, I think,
need less turns for the same inductance and so should be easier to
wind.

Finally, a simple low-pass filter would probably be all you
need here as the transformer primary impedance will be a limit the
low end. Filter response is dependant on source as well as load
impedance and most antennas that you would use at those
frequencies will be quite reactive and difficult to design a fixed
input network for. You may be aware that a lot of HF receivers use
a separate hi-Z antenna terminal for LF/MF.

Having said that, if you do want to use a simple low-pass
configuration, remember that a symmetrical 'Pi' filter with source
and load impedances of 1-ohm and a cut-off of 1-rad/sec would use
two 1-Farad capacitors and a 2-Henry inductor. To scale to new
impedances and frequency, divide C (and multiply L) by the design
impedance and then divide both C and L by the new cut-off
frequency in rads/sec.


Cheers - Joe

Notice that the primary to secondary (total) turns ratio is
almost 1:1, that both shunt capacitors in the band-pass filter are
of the same value and that the path resistance on either side of
the secondary, through the FST3126M to the low-pass filters is
around 20-ohms. So what you are looking at (to a crude
approximation) is a design impedance of 50-ohm all the way
through.


OK, that's about what I thought, but I was estimating closer to 15
ohms for ac resistance between the secondary winding and ground. So, I
was having a hard time believing it could be 50 ohms in and out. It
probably makes little difference though in the filter values or
passband response.


As for the toroid, for optimum Q you might want to look for a
mix more suitable for the frequency range you quoted (maybe 15?)
and increase the diameter to take the extra wire as you will need
to scale the impedances for use at 50kHz. The FT series, I think,
need less turns for the same inductance and so should be easier to
wind.


I was looking at the CT series, specifically a CT-50-57, which is a
little bigger, but probably can contain all the wire easily.

Finally, a simple low-pass filter would probably be all you
need here as the transformer primary impedance will be a limit the
low end. Filter response is dependant on source as well as load
impedance and most antennas that you would use at those
frequencies will be quite reactive and difficult to design a fixed
input network for. You may be aware that a lot of HF receivers use
a separate hi-Z antenna terminal for LF/MF.


I'm aware of the separate antenna inputs some LF radios have. The
lowfer group has quite a bit of information on antennas, and it seems
that making a 50 ohm antenna or something approaching 50 ohms from a
large single or multiturn loop isn't that difficult.

Making a 15 or 20 ohm antenna is even easier, perhaps I should think
about 15 ohm input impedance and a 15 ohm output impedance since it's
actually easier to handle the antenna step up.

Having said that, if you do want to use a simple low-pass
configuration, remember that a symmetrical 'Pi' filter with source
and load impedances of 1-ohm and a cut-off of 1-rad/sec would use
two 1-Farad capacitors and a 2-Henry inductor. To scale to new
impedances and frequency, divide C (and multiply L) by the design
impedance and then divide both C and L by the new cut-off
frequency in rads/sec.


Most likely it's probably best to keep it at 50 ohms and not to worry
about the slight mismatch in that frequency range.

Thanks to you and to all who commented, it was just he sort of
information I needed to put me back on track.

Regards,

T

On Sun, 11 Sep 2005 23:43:36 +0100, "Highland Ham"
wrote:

The existing filter is for a 7 Mhz receiver, I'd like to have a
similar filter design for 50 to 200 Khz.
===========================
If you are happy with the HF receiver as is, would it not be easier and
more effective to build an LF (50 - 200 kHz) to HF converter. This should
be an easy project and you can select a quiet "HF band" for the conversion.

What would this solve ? You still need some selectivity in front of
converter.

I would also question the need for a bandpass filter, but a good low
pass filter would definitively required in any case. I would suggest a
low pass filter below 150 kHz in Europe, Africa and Middle-East and
below 500 kHz in the rest of the world to get the very strong LW/MW
broadcast band signals out of the mixer. If 455 kHz IF is used, the
LPF would have to be below 400 kHz in the rest of the world.

I have seen designs with a SBL-1 mixer

SBL-1 is specified for 1-500 MHz on the RF and LO port, so not really
suitable for this band. However, the SBL-3 goes from 25 kHz to 200
MHz. The SRA-6H goes from 10 kHz to 50 MHz and should be able to
handle up to +10 dBm signals.

but also a number with the NE612
osc/mixer.

I have used the Datong LF converter, which uses the Siemens S042
mixer/osc IC similar to the NE602/612 and it definitively needs a
preselector in front of it to get away with spurious responses all
over the LF band from broadcast stations.

Paul OH3LWR

What would this solve ? You still need some selectivity in front of
converter.

I would also question the need for a bandpass filter, but a good low
pass filter would definitively required in any case. I would suggest a
low pass filter below 150 kHz in Europe, Africa and Middle-East and
below 500 kHz in the rest of the world to get the very strong LW/MW
broadcast band signals out of the mixer. If 455 kHz IF is used, the
LPF would have to be below 400 kHz in the rest of the world.

I have seen designs with a SBL-1 mixer

SBL-1 is specified for 1-500 MHz on the RF and LO port, so not really
suitable for this band. However, the SBL-3 goes from 25 kHz to 200
MHz. The SRA-6H goes from 10 kHz to 50 MHz and should be able to
handle up to +10 dBm signals.

but also a number with the NE612
osc/mixer.

I have used the Datong LF converter, which uses the Siemens S042
mixer/osc IC similar to the NE602/612 and it definitively needs a
preselector in front of it to get away with spurious responses all
over the LF band from broadcast stations.


Thanks Paul, and yes....you're correct. Building a conventional
converter would still require a passband filter, so little is to be
gained, except that perhaps someone else has already done the
design::

Also, this receiver design has no mixer, it is simply a detector and a
very linear one to boot. No mixing byproducts are present because
there is no non-linear mixer. In effect, this design is already a
converter....except that it converts to audio directly from the rf
frequency input.

The "spurious responses all over the LF band from broadcast stations"
probably don't exist in this type of receiver, which is one of the
attractions for VLF use of this technology.

Take a look at the link to the design in the original message and you
will learn how it operates without mixers and without non-linear
detectors.

It's WHY I so interested in this particular method of reception and
WHY I want to make a front end for vlf for it. The concept is
explained in a QEX article in greater detail, Im happy to send the url
to anyone who wants to learn more about these high performance direct
conversion receivers.

Regards and again, Thanks,

T

The concept is
explained in a QEX article in greater detail, Im happy to send the
url
to anyone who wants to learn more about these high performance
direct
conversion receivers.
===================================

How do you know the QEX article is not a load of of old-wives tales.