I'm designing a box that will use four SA605 receiver/mixer chips that
all need the same crystal reference. Even with all the good
application notes for the SA605, I'm not sure how to do this. Do I
build a Colpitts oscillator from transistors, buffer it, and then use
this output as an "external LO" for the mixers? Or do I connect up
the crystal to the SA605 oscillator pins as usual and then tap off of
this for the other chips? Using a function generator as an external
LO, I get the best results injecting a 300mVpp sine wave, if that is
useful.

Thank you!

Either approach will work

Joe
W3JDR


wrote in message
oups.com...
I'm designing a box that will use four SA605 receiver/mixer chips that
all need the same crystal reference. Even with all the good
application notes for the SA605, I'm not sure how to do this. Do I
build a Colpitts oscillator from transistors, buffer it, and then use
this output as an "external LO" for the mixers? Or do I connect up
the crystal to the SA605 oscillator pins as usual and then tap off of
this for the other chips? Using a function generator as an external
LO, I get the best results injecting a 300mVpp sine wave, if that is
useful.

Thank you!

Amongst other things it depends on the freq of the LO, the physical
distance between the SA605s.

What freq is the LO ?

How far apart are the SA605s ?

-Mark

On Mar 8, 8:18 am, wrote:
I'm designing a box that will use four SA605 receiver/mixer chips that
all need the same crystal reference. Even with all the good
application notes for the SA605, I'm not sure how to do this. Do I
build a Colpitts oscillator from transistors, buffer it, and then use
this output as an "external LO" for the mixers? Or do I connect up
the crystal to the SA605 oscillator pins as usual and then tap off of
this for the other chips? Using a function generator as an external
LO, I get the best results injecting a 300mVpp sine wave, if that is
useful.

Thank you!

I think one of the biggest problems you might encounter is intermodulation
between mixers.

If a strong signal is present on any of the mixers, it will intermodulate
with the oscillator due to imperfect balance in the mixer. If these
intermodulation components get into the other mixer LO ports, it might cause
spurious responses.

Try to build some port-to-port isolation into the LO splitter, either
resistively (if you have power to spare), with a magnetic or LC 'hybrid' of
some sort, or vith individual buffer amps for each stage. If you choose the
latter, common base amps would be a good choice as they have very high
reverse isolation. Avoid ampliifiers with feedback, as feedback usually
ruins reverse isolation.


Joe
W3JDR



"MarkAren" wrote in message
oups.com...
Amongst other things it depends on the freq of the LO, the physical
distance between the SA605s.

What freq is the LO ?

How far apart are the SA605s ?

-Mark

On Mar 8, 8:18 am, wrote:
I'm designing a box that will use four SA605 receiver/mixer chips that
all need the same crystal reference. Even with all the good
application notes for the SA605, I'm not sure how to do this. Do I
build a Colpitts oscillator from transistors, buffer it, and then use
this output as an "external LO" for the mixers? Or do I connect up
the crystal to the SA605 oscillator pins as usual and then tap off of
this for the other chips? Using a function generator as an external
LO, I get the best results injecting a 300mVpp sine wave, if that is
useful.

Thank you!

Thanks, Joe. Mark, The frequency of the LO is 13.105MHz. The mixers
are all closer together, on the same board.

Here is another response I got, for general information:

"That's what I'd recommend... a Colpitts or perhaps a Vacker, followed
by a robust buffering scheme, with some isolation (resistive 3 dB
pads, and/or separate second-stage buffers) to drive the individual
mixer chips. Less chance of interaction between the SA605s that way,
and (from what I've heard) you'll probably get a cleaner LO signal
that way.

Design and built the oscillator for best performance. That usually
means not driving the crystal too hard (don't heat it up or overstress
it), and not loading the oscillator itself too heavily (doing so will
lower the Q and reduce the stability). The designs I've seen with the
cleanest behavior have a fairly gentle drive, and then use a JFET or
MOSFET as a first-stage buffer so that the tank circuit has negligible
loading on it. Then, add a second-stage buffer (2N2222 or 2N3904 or
something like that) and at least one stage of low-pass filtering, so
that you get a nice clean sine-wave out. (or, if you're driving a
switch-mode mixer, drive a squaring circuit of some sort). Then,
figure out how much voltage and drive capability your signal has, and
set up resistive-divider networks which will attenuate it down to the
desired level to drive your mixers, and let the mixer "look back" into
a 50-ohm source at all frequencies - this helps reduce spurs. [This
is pretty important in diode-ring mixers - I'm not sure how critical
it is with Gilbert-cell mixers such as the SA602] Of course, you could
always use another SA602 just as an oscillator, and then tap off /
buffer / divide its output to feed the other SA602s. That would still
give you good isolation between the SA602s and might be simpler (but
perhaps more expensive)."

On Mar 9, 7:04 am, "W3JDR" wrote:
I think one of the biggest problems you might encounter is intermodulation
between mixers.

If a strong signal is present on any of the mixers, it will intermodulate
with the oscillator due to imperfect balance in the mixer. If these
intermodulation components get into the other mixer LO ports, it might cause
spurious responses.

Try to build some port-to-port isolation into the LO splitter, either
resistively (if you have power to spare), with a magnetic or LC 'hybrid' of
some sort, or vith individual buffer amps for each stage. If you choose the
latter, common base amps would be a good choice as they have very high
reverse isolation. Avoid ampliifiers with feedback, as feedback usually
ruins reverse isolation.

Joe
W3JDR

"MarkAren" wrote in message

oups.com...



Amongst other things it depends on the freq of the LO, the physical
distance between the SA605s.

What freq is the LO ?

How far apart are the SA605s ?

-Mark

On Mar 8, 8:18 am, wrote:
I'm designing a box that will use four SA605 receiver/mixer chips that
all need the same crystal reference. Even with all the good
application notes for the SA605, I'm not sure how to do this. Do I
build a Colpitts oscillator from transistors, buffer it, and then use
this output as an "external LO" for the mixers? Or do I connect up
the crystal to the SA605 oscillator pins as usual and then tap off of
this for the other chips? Using a function generator as an external
LO, I get the best results injecting a 300mVpp sine wave, if that is
useful.

Thank you!- Hide quoted text -

- Show quoted text -

I vote for the 4 way resistive splitter
http://www.microwaves101.com/encyclo..._splitters.cfm fed
with lots of ergs from a gently tickled Xtal osc and sutiable
buffering. Terminate each limb in 50 ohms near each 605 and feed with
50 ohm microstrip from the central distribution point (buffer).

-Mark

On Mar 8, 8:18 am, wrote:
I'm designing a box that will use four SA605 receiver/mixer chips that
all need the same crystal reference. Even with all the good
application notes for the SA605, I'm not sure how to do this. Do I
build a Colpitts oscillator from transistors, buffer it, and then use
this output as an "external LO" for the mixers? Or do I connect up
the crystal to the SA605 oscillator pins as usual and then tap off of
this for the other chips? Using a function generator as an external
LO, I get the best results injecting a 300mVpp sine wave, if that is
useful.

Thank you!

wrote in message
oups.com...
I'm designing a box that will use four SA605 receiver/mixer chips that
all need the same crystal reference. Even with all the good
application notes for the SA605, I'm not sure how to do this. Do I
build a Colpitts oscillator from transistors, buffer it, and then use
this output as an "external LO" for the mixers? Or do I connect up
the crystal to the SA605 oscillator pins as usual and then tap off of
this for the other chips? Using a function generator as an external
LO, I get the best results injecting a 300mVpp sine wave, if that is
useful.

Thank you!


1 osc driving several paralell buffer amps, the one osc may be one of the
SA605s

One thought we've had is to build a SA602/SA605-based fundamental mode
Colpitts oscillator (with two capacitors and the crystal) and then tap
into the mixer output, leaving the RF intput disconnected. If the
mixer does not mix with anything, we will get an amplified/buffered
13.56MHz signal. Would this not work?

Also, I am not clear how to chose the capacitor values for the
Colpitts oscillator - there is no guidance in the application notes. I
think these values are not critical? Our crystal frequency is
13.105MHz.

Ben