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