RF CKT Matching Question

[Jack J]

I have to connect the output of a SAW filter to an amplifier input.
The SAW filter output is not 50 Ohms and the amplifier input is not
either. But the manufacturer of the SAW and the amplifier provide
matching network schematics assuming 50 Ohm termination.

My question is do I need to put an explicit 50 Ohm resistor between
the two matching networks or can I put the two matching networks
together directly?

SAW -> match1 -> 50 Ohms -> match2 -> amp

or

SAW -> match1 -> match2 -> amp

Thanks,

 

[Larry Brasfield]

I have to connect the output of a SAW filter to an amplifier input.
The SAW filter output is not 50 Ohms and the amplifier input is not
either. But the manufacturer of the SAW and the amplifier provide
matching network schematics assuming 50 Ohm termination.

My question is do I need to put an explicit 50 Ohm resistor between
the two matching networks or can I put the two matching networks
together directly?

SAW -> match1 -> 50 Ohms -> match2 -> amp

or

SAW -> match1 -> match2 -> amp

You can just put them together, without
an extra resistor. The 50 Ohm resistor
would actually cause a mismatch.

However, if both devices without the
matching network are on the same side
of 50 Ohms, it might be better to make
a single matching network to match the
actual impedances of the adjoining
devices to each other.

 

[Fred Bloggs]

You can just put them together, without
an extra resistor. The 50 Ohm resistor
would actually cause a mismatch.

However, if both devices without the
matching network are on the same side
of 50 Ohms, it might be better to make
a single matching network to match the
actual impedances of the adjoining
devices to each other.

How can you say that, pretentious retard? All the device
characteristics, especially the filter, are defined in terms of
broadband resistive termination. Looks like RF is another area you know
damned little about.

 

[Larry Brasfield]

(Responding to the question:
I have to connect the output of a SAW filter to an amplifier input.
The SAW filter output is not 50 Ohms and the amplifier input is not
either. But the manufacturer of the SAW and the amplifier provide
matching network schematics assuming 50 Ohm termination.

My question is do I need to put an explicit 50 Ohm resistor between
the two matching networks or can I put the two matching networks
together directly?
)

You can just put them together, without
an extra resistor. The 50 Ohm resistor
would actually cause a mismatch.

However, if both devices without the
matching network are on the same side
of 50 Ohms, it might be better to make
a single matching network to match the
actual impedances of the adjoining
devices to each other.

How can you say that, [pointless name-calling cut]
? All the device characteristics, especially the filter, are defined in terms of broadband resistive termination.

The SAW filter does not depend on broadband
termination for its selective frequency response.
The amplifier with an input matching network will
have broad enough real input range to permit the
filter plus its matching network to see the load
for which it was designed, in the vicinity of its
passband. At frequencies where the filter load
becomes significantly complex, well outside that
passband, the response will be different than if
a resistor was the load, but it should still reject.

If the OP wants stop-band response that looks
like the SAW spec sheet, he will have to insert
either a broadband matching network or a 50
Ohm attenuator. But if he cares that much, he
would more likely take my suggestion about
devising a custom matching network.

So, Fred, what is your constructive suggestion?

 

[Fred Bloggs]

The SAW filter does not depend on broadband
termination for its selective frequency response.
The amplifier with an input matching network will
have broad enough real input range to permit the
filter plus its matching network to see the load
for which it was designed, in the vicinity of its
passband. At frequencies where the filter load
becomes significantly complex, well outside that
passband, the response will be different than if
a resistor was the load, but it should still reject.

If the OP wants stop-band response that looks
like the SAW spec sheet, he will have to insert
either a broadband matching network or a 50
Ohm attenuator. But if he cares that much, he
would more likely take my suggestion about
devising a custom matching network.

So, Fred, what is your constructive suggestion?

My constructive suggestion to the OP is to not waste time listening to
you, you obviously have no clue what you're talking about.

 

[Mark]

Most amplifers don't care much about what they "see" at their input.
However most SAw devices do care about what they see at their output.
You need to design a matching ckt that converts the amplifers input
impedance to the provide the load impaedance that the SAW wants to see.
Don't worry as much about what the amplifier sees. Depending upon
what these values actually are, i.e. uif the amplifer input Z is very
high, it may be helpful to add a resistor somplace in the network to
help to define the Z.

Mark