Actually, the thing you call a detector IS a mixer. You can probably
find some references for "H-mode mixer." It's good, to be sure, but
it's inaccurate to say that it's _perfectly_ linear. (I'd LOVE to find
a practical sampler which had zero distortion...though then I'd need
amplifiers with zero distortion, too...)

As someone else pointed out, any practical antenna you have for LF is
very unlikely to be a good match to 50 ohms, and is very likely to be
quite reactive so that by the time you add components to tune it, the
bandwidth will be pretty narrow. So if you have a tuned LF antenna,
which is quite usual, the response will be quite narrow, and why would
you care about a bandpass filter? I'd recommend a loop with a tuning
arrangement at its feedpoint (variable capacitance), and an appropriate
preamp to drive a feedline back to the receiver. With that, you won't
need any filter, just a transformer going into the mixer
(converter-detector-whatever). A while back, I did some work to modify
a design you can find at
http://www.cpinternet.com/~lyle/bal-pre/bal-pre.htm, so that the
control was done as a DC current , which also fed the power to the
preamp on the same line that signals come back on. It worked out well.
But check out other antenna options from Lyle's website
(http://www.cpinternet.com/~lyle/) or others devoted to LF, too.

Cheers,
Tom

If you read Gerald Youngblood's first QEX article on the SDR-1000
software-defined radio, you will see on page 7 another embodiment of this
type of mixer/detector. It was originally popularized and patented by Dan
Tayloe, but has recently been reconfigured (for patent purposes as much as
anything else I suspect), and renamed 'quadrature sampling detector' (QSD).
The original embodiment shown in the QEX article has no transformers.

As a passing comment, one of the writers here said a mixer IS a detector.
He's absolutely correct. A detector is just a special case of frequency
mixing where the RF is mixed directly down to DC baseband. As another writer
said, ALL mixers/detectors, regardless of whether they're the modern
switching type or the antique-type based on square-law nonlinearity, have an
overload point beyond which they make unacceptable distortion. Switching
mixers, which include most double-balanced diode mixer (DBM) modules and
most Gilbert-cell IC mixers, just happen to be more linear than many
square-law devices up to the overload point, then they go to hell in a
handbasket. The QSD is a just special case of switching mixer that can
produce quadrature baseband outputs very conveniently, but it is a bit
better in the distortion department than many diode DBM's.

Joe
W3JDR




TRABEM wrote in message ...


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

TRABEM wrote in message ...
[...]
Thanks,

T

Heresy maybe but I'd be inclined to just dump the transformer and bandpass
coils and caps and 10ohms etc. Use opamps for a low pass and a high pass
filter. Then add another inverting opamp to provide the antiphase for the
mixer chip. High 'Atmospherics' at these low frequencies mean pretty much
any opamp will be OK.
Use a resistor to present any input Z to the Ant'.
The FST3126 (or 74HC4066) works well with opamp drive.
regards
john

I'd hardly call it heresy, John. In my search for really good op amps
to use up to 50 and 100MHz (very low distortion and low noise), I've
come across some that would be really outstanding up to a couple
hundred kHz.

In fact, since you can get 24bit ADCs that cover up to that range
(e.g., AD7760, AD7762) with very good linearity and low noise, you
could make the whole LF receiver with just the tuned antenna, the
preamp out at the antenna, _maybe_ a bit of gain, and the ADC feeding
into a PC. Then the "quadrature mixing" would all be done digitally,
with much better accuracy than you'll get with an analog mixer. Yeah,
yeah, you have to write some software to get it to work...but a modern
PC should have no trouble keeping up with doing all the signal
processing. There may even be sound cards out there with response out
to 100kHz--that's an area I don't keep up with.

Cheers,
Tom

On Mon, 12 Sep 2005 17:39:22 +0000 (UTC), "Reg Edwards"
wrote:

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.


Not sure if you're serious or not. But, there are quite a few users
with SDR-1000 HF transceivers that use the technology....and all of
them aren't full of BS.

GL.

T

On 12 Sep 2005 14:01:59 -0700, "K7ITM" wrote:

Actually, the thing you call a detector IS a mixer. You can probably
find some references for "H-mode mixer." It's good, to be sure, but
it's inaccurate to say that it's _perfectly_ linear. (I'd LOVE to find
a practical sampler which had zero distortion...though then I'd need
amplifiers with zero distortion, too...)

As someone else pointed out, any practical antenna you have for LF is
very unlikely to be a good match to 50 ohms, and is very likely to be
quite reactive so that by the time you add components to tune it, the
bandwidth will be pretty narrow. So if you have a tuned LF antenna,
which is quite usual, the response will be quite narrow, and why would
you care about a bandpass filter? I'd recommend a loop with a tuning
arrangement at its feedpoint (variable capacitance), and an appropriate
preamp to drive a feedline back to the receiver. With that, you won't
need any filter, just a transformer going into the mixer
(converter-detector-whatever). A while back, I did some work to modify
a design you can find at
http://www.cpinternet.com/~lyle/bal-pre/bal-pre.htm, so that the
control was done as a DC current , which also fed the power to the
preamp on the same line that signals come back on. It worked out well.
But check out other antenna options from Lyle's website
(http://www.cpinternet.com/~lyle/) or others devoted to LF, too.


Thanks Tom,

I've followed lowfer technical discussions for some time now and
thanks to some recent input from a few of them, I have a much better
idea of what I need to do to get a decent antenna up.

My plan is to have a single turn or 2 turn centertapped loop, each
side being 10 to 12 feet long and the turns spaced 5 inches apart. The
conductor will be 200 A aluminum service entrance cable which I have
laying around.

The impedance of this antenna will be low and the Q should be quite
high, with lots of area, so it should drive the receiver well.

It has occurred to me that the antenna itself has a great deal of
selectivity, yet some loop users still report front end overload from
AM broadcast band and other megawatt LF rf sources.

The QSD is susceptible to harmonics also, so a very high attenuation
low pass filter is the minimum filter necessary to keep these signals
out.

Whether the tuned antenna by itself is adequate, I don't know.

But, I'm considering putting in a bandpass or low pass filter designed
to match the lower impedance loop antenna directly, so the filter
would have input and output impedances of 5 or 10 ohms.

Anyway, that's a topic for another day I suspect.

Regards and thanks for the input.

T

Hi Joe,

It appeared to me that the transformer was used as a convenient means
to introduce 1/2 of the Vcc to provide DC bias equally to each of the
4 switch inputs.

The 10 ohm series resistors look like a resistive impedance matching
scheme to me, with a built in 6+ db loss associated with them.

I'm thinking of redoing the entire input circuit to take out hte
reistors and to better match the lower impedance of most VLF loop
antnnas.

Thanks for your comments.

T





On Mon, 12 Sep 2005 23:08:13 GMT, "W3JDR" wrote:

If you read Gerald Youngblood's first QEX article on the SDR-1000
software-defined radio, you will see on page 7 another embodiment of this
type of mixer/detector. It was originally popularized and patented by Dan
Tayloe, but has recently been reconfigured (for patent purposes as much as
anything else I suspect), and renamed 'quadrature sampling detector' (QSD).
The original embodiment shown in the QEX article has no transformers.

As a passing comment, one of the writers here said a mixer IS a detector.
He's absolutely correct. A detector is just a special case of frequency
mixing where the RF is mixed directly down to DC baseband. As another writer
said, ALL mixers/detectors, regardless of whether they're the modern
switching type or the antique-type based on square-law nonlinearity, have an
overload point beyond which they make unacceptable distortion. Switching
mixers, which include most double-balanced diode mixer (DBM) modules and
most Gilbert-cell IC mixers, just happen to be more linear than many
square-law devices up to the overload point, then they go to hell in a
handbasket. The QSD is a just special case of switching mixer that can
produce quadrature baseband outputs very conveniently, but it is a bit
better in the distortion department than many diode DBM's.

Joe
W3JDR




TRABEM wrote in message ...


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

Hi Tom,

I think the loop antennas are pretty quiet compared to wire antennas
and you might need some gain.

But, since the antenna is high Q and tuned, it might have enough
voltage output to drive the soundcard more or less directly.

I don't know for sure.

But, if the switch is anywhere near linear, you would not want your
gain stage before the switch, would you?? I can't see using an rf amp
at the antenna that just creates non linearity when you could use a
nice quiet audio amp op amp on the far side of the analog switch.

An all software based receiver shoulnds like a neat idea until you
realize you run out of dynamic range by trying to sample such a wide
bandwidth directly.

I think the analog switch (hardware) is here to stay, at least for
ashile.

GL.

T

On 12 Sep 2005 17:13:12 -0700, "K7ITM" wrote:

I'd hardly call it heresy, John. In my search for really good op amps
to use up to 50 and 100MHz (very low distortion and low noise), I've
come across some that would be really outstanding up to a couple
hundred kHz.

In fact, since you can get 24bit ADCs that cover up to that range
(e.g., AD7760, AD7762) with very good linearity and low noise, you
could make the whole LF receiver with just the tuned antenna, the
preamp out at the antenna, _maybe_ a bit of gain, and the ADC feeding
into a PC. Then the "quadrature mixing" would all be done digitally,
with much better accuracy than you'll get with an analog mixer. Yeah,
yeah, you have to write some software to get it to work...but a modern
PC should have no trouble keeping up with doing all the signal
processing. There may even be sound cards out there with response out
to 100kHz--that's an area I don't keep up with.

Cheers,
Tom

There are easier ways to introduce the bias than to add transformers.
Personally, I hate transformers and go out of my way to avoid using them but
sometimes they're a necessary evil.

I'd venture to say that the transformer in this implementation of Tayloe's
detector was introduced to:
1) Improve second-order distortion performance
2) Eliminate the need for differential amplifiers at the output
3) Attempt to skirt some patent issues.

These are just my opinions...I'd like to hear different points of view.


As to your redesign goals, be aware that the input source resistance is an
important part of the inherent bandpass response that this detector has.
It's based on commutating filter principles.....you can read up on this by
doing a web search. The resistors were probably added to stabilize the
selectivity performance in the face of variable antenna impedances. The
resistors will add a little to the insertion loss and thus the system noise
figure, but this will probably not be an issue at LF frequencies where
atmospherics and man-made noise dominate.

Overall, I think this is an outstanding detector for an LF receiver. You
should be able to get very good quadrature LO phasing using the common &
simple Johnson counter approach. Once you get to quadrature detector
outputs, an audio frequency DSP should result in a very good receiver. As
you probably know, there are several public-domain DSP software packages
available for both Windows and Linux that will do a very good job for you.


Joe



TRABEM wrote in message ...
Hi Joe,

It appeared to me that the transformer was used as a convenient means
to introduce 1/2 of the Vcc to provide DC bias equally to each of the
4 switch inputs.

The 10 ohm series resistors look like a resistive impedance matching
scheme to me, with a built in 6+ db loss associated with them.

I'm thinking of redoing the entire input circuit to take out hte
reistors and to better match the lower impedance of most VLF loop
antnnas.

Thanks for your comments.

T





On Mon, 12 Sep 2005 23:08:13 GMT, "W3JDR" wrote:

If you read Gerald Youngblood's first QEX article on the SDR-1000
software-defined radio, you will see on page 7 another embodiment of this
type of mixer/detector. It was originally popularized and patented by Dan
Tayloe, but has recently been reconfigured (for patent purposes as much as
anything else I suspect), and renamed 'quadrature sampling detector'
(QSD).
The original embodiment shown in the QEX article has no transformers.

As a passing comment, one of the writers here said a mixer IS a detector.
He's absolutely correct. A detector is just a special case of frequency
mixing where the RF is mixed directly down to DC baseband. As another
writer
said, ALL mixers/detectors, regardless of whether they're the modern
switching type or the antique-type based on square-law nonlinearity, have
an
overload point beyond which they make unacceptable distortion. Switching
mixers, which include most double-balanced diode mixer (DBM) modules and
most Gilbert-cell IC mixers, just happen to be more linear than many
square-law devices up to the overload point, then they go to hell in a
handbasket. The QSD is a just special case of switching mixer that can
produce quadrature baseband outputs very conveniently, but it is a bit
better in the distortion department than many diode DBM's.

Joe
W3JDR




TRABEM wrote in message
. ..


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

An all software based receiver shoulnds like a neat idea until you
realize you run out of dynamic range by trying to sample such a wide
bandwidth directly.

In another reply to this thread, I mentioned commutating filters. At VLF,
one of these ahead of a good ADC would probably also yield very good
performance without downconverting to audio baseband first. As to speed of
the ADC, using undersampling you theoretically only need enough speed to
sample at twice the modulation (information) bandwidth. This is just a few
kilohertz sampling rate. I've heard of this technique, but don't recall ever
seeing it implemented in a real receiver design.

Can anyone comment and shed more light on this??

Joe
W3JDR