A few minor clarifications and addendums to my previous post:
>? So the bare B3550 SAW filter without a matching network
>? should have a resistive impedance of 31.59 ohms.
>? In series with that resistance is 65.87 ohms of capacitive reactance.
Looking into one of the two ports of a B3550 SAW filter element,
we would encounter an impedance of 31.59 - 65.87j
We might say that's the impedance from one of the B3550 pins to the center of the B3550.
And it is only true if the far side is properly terminated, perhaps by a 24.2 nH coil
(which is +65.87j ohms at 434mhz) and a 31.59 ohm resistor in series to ground.
If the far side is not properly terminated, all bets are off about the impedance we will see.
Eric's tinySA features a 250mhz LPF driven from a 50 ohm source, going into a mixer,
going into the B3550 SAW filters, going into the Si4432.
The only points where impedance is well defined are (hopefully) the signal source
and the Si4432 input port, which should both be close to 50 ohms.
If one of those two impedances is off (or the parts between aren't exactly right)
all the impedances from front to back are wrong, and the filters and mixers
don't work as expected.
Another issue is that the impedance of the 250mhz LC LPF and the SAW filters will vary
by frequency.? For example, a 500 mhz signal into the LPF will?see a very high impedance
instead of the expected 50 ohms, and the voltages of this undesired signal at the input
might be relatively high.? High enough to sneak through the filter, or perhaps capacitively
couple?into nearby circuits
For both of the above reasons, a good design often has a bunch of 6dB resistive pads
to keep things at 50 ohms.? And then some gain stages to make up for the loss.
But if the tinySA can get good performance while staying simple, I'm all for it.
When writing a complex number, python insists that the "j" follows the numeric digits
so it can easily figure out it is still a number:? ? 31.59 - 65.87j
More typical of electrical engineering texts is to put the "j" in front:? 31.59 - j65.87
The math majors use "i" instead of "j":? 31.59 - i65.87
since they don't have "i" reserved as a variable to represent current.
Jerry, KE7ER
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