"Alfred Lorona" wrote in message
...
Most mixers I've seen are not matched to 50+j0. They are simply terminated
in 50+j0. Is this the best or optimum termination or does it matter?

A commutating (or reversing switch) type mixer, such as a diode
ring mixer (DBM) will, ideally, present the same impedance at its input
as it is terminated in. This is because, in theory, the output is
always connected to the input, but the phase of the connection
reverses with time. Thus in a downconverting DBM, if the IF is
terminated in 50 ohms, the RF input will have a 50 ohm input
impedance. If the IF is terminated in 75 ohms, then the RF input
will have an impedance of 75 ohms.

In practical DBMs, transformers are used. These are optimized for
a particular impedance, so you will get best performance at the
specified impedance level.

The LO port input impedance has no particular value, being dependent
on drive level, diode turn-on voltage, etc. Typically, it is considerably
less than 50 ohms in a 50 ohm mixer. The manufacturer still advises
driving with a 50 ohm source so that the transformer will be optimally
terminated on at least the primary. I have driven DBM's with voltage
sources and current sources and they will often work OK that way.

Rick N6RK

"Josh" wrote in message
...


John R. Strohm wrote:
"John Moriarity" wrote in message
...

"John R. Strohm" wrote in message
...

"Alfred Lorona" wrote in message
...

Most mixers I've seen are not matched to 50+j0. They are simply

terminated

in 50+j0. Is this the best or optimum termination or does it matter?

There is no "best" or "optimum" termination, beyond the fact that you

want

a

purely resistive termination. (DUH!). 50 ohms is just the standard

value.

That *may* be true, but most mixers are characterized
with 50 ohm (resistive) terminations at all ports.

Use other impedances at your own risk!


In the immortal words of Natasha the Sex Goddess: Well, DUH!



Where do I find this goddess?

http://www.amore.com/ Normally, I'd say "Google is your FRIEND", but the
site Google has for her is gone. However, the other site, which the Google
entry reminded me of, is still around. This site is actually for real, by
the way.

"Josh" wrote in message
...


John R. Strohm wrote:
"John Moriarity" wrote in message
...

"John R. Strohm" wrote in message
...

"Alfred Lorona" wrote in message
...

Most mixers I've seen are not matched to 50+j0. They are simply

terminated

in 50+j0. Is this the best or optimum termination or does it matter?

There is no "best" or "optimum" termination, beyond the fact that you

want

a

purely resistive termination. (DUH!). 50 ohms is just the standard

value.

That *may* be true, but most mixers are characterized
with 50 ohm (resistive) terminations at all ports.

Use other impedances at your own risk!


In the immortal words of Natasha the Sex Goddess: Well, DUH!



Where do I find this goddess?

http://www.amore.com/ Normally, I'd say "Google is your FRIEND", but the
site Google has for her is gone. However, the other site, which the Google
entry reminded me of, is still around. This site is actually for real, by
the way.

"Alfred Lorona" wrote in message ...
Most mixers I've seen are not matched to 50+j0. They are simply terminated
in 50+j0. Is this the best or optimum termination or does it matter?

Hi Alfred,
a mixer is a three-port device. If I remember the theory, a three-port
device can never be matched to the same characteristic impedance on
all three ports. If two ports are matched, the third one is isolated,
so that the network degenerates to a two-port device. Therefore a
mixer with 50+0j ohm RF input cannot have 50+0j ohms neither at the LO
input nor at the output. Some mismatch losses seem to be necessary
with mixers, but they can be easily overcome by increasing the LO
power and more amplification at the output.

BR from Ivan

"Alfred Lorona" wrote in message ...
Most mixers I've seen are not matched to 50+j0. They are simply terminated
in 50+j0. Is this the best or optimum termination or does it matter?

Hi Alfred,
a mixer is a three-port device. If I remember the theory, a three-port
device can never be matched to the same characteristic impedance on
all three ports. If two ports are matched, the third one is isolated,
so that the network degenerates to a two-port device. Therefore a
mixer with 50+0j ohm RF input cannot have 50+0j ohms neither at the LO
input nor at the output. Some mismatch losses seem to be necessary
with mixers, but they can be easily overcome by increasing the LO
power and more amplification at the output.

BR from Ivan

hello
all about theory of mixer on web site of minicircuits.com
serge

hello
all about theory of mixer on web site of minicircuits.com
serge

That might be true for lossless, linear networks (I honestly don't
know), but it's a simple manner to create a lossy three-port network
that presents 50 ohms at all three ports. For example, a 16.67 ohm
resistor from each port to a common point, or simply a 50 ohm resistor
at each port with no internal connections between ports. A mixer is
nonlinear besides having loss, so there's no good reason to believe that
it conforms to rules that apply to lossless, linear networks.

Roy Lewallen, W7EL

OK1SIP wrote:
"Alfred Lorona" wrote in message ...

Most mixers I've seen are not matched to 50+j0. They are simply terminated
in 50+j0. Is this the best or optimum termination or does it matter?


Hi Alfred,
a mixer is a three-port device. If I remember the theory, a three-port
device can never be matched to the same characteristic impedance on
all three ports. If two ports are matched, the third one is isolated,
so that the network degenerates to a two-port device. Therefore a
mixer with 50+0j ohm RF input cannot have 50+0j ohms neither at the LO
input nor at the output. Some mismatch losses seem to be necessary
with mixers, but they can be easily overcome by increasing the LO
power and more amplification at the output.

BR from Ivan

That might be true for lossless, linear networks (I honestly don't
know), but it's a simple manner to create a lossy three-port network
that presents 50 ohms at all three ports. For example, a 16.67 ohm
resistor from each port to a common point, or simply a 50 ohm resistor
at each port with no internal connections between ports. A mixer is
nonlinear besides having loss, so there's no good reason to believe that
it conforms to rules that apply to lossless, linear networks.

Roy Lewallen, W7EL

OK1SIP wrote:
"Alfred Lorona" wrote in message ...

Most mixers I've seen are not matched to 50+j0. They are simply terminated
in 50+j0. Is this the best or optimum termination or does it matter?


Hi Alfred,
a mixer is a three-port device. If I remember the theory, a three-port
device can never be matched to the same characteristic impedance on
all three ports. If two ports are matched, the third one is isolated,
so that the network degenerates to a two-port device. Therefore a
mixer with 50+0j ohm RF input cannot have 50+0j ohms neither at the LO
input nor at the output. Some mismatch losses seem to be necessary
with mixers, but they can be easily overcome by increasing the LO
power and more amplification at the output.

BR from Ivan

The post appears to have generated some interest. Specifically, how does the
match/mis-match, as the case may be, affect one of the (perhaps the) most
important paramater which is the input 3rd order intermodulation distortion
level? And what about loss?

In other words, will a mixer (not limited to diode mixers) operate equally
well into any termination as long as it is R+j0?

AL