Them mixers create a whole career's worth of complications to figure out.
My understanding is that a diode ring mixer has no specific impedance,
it simply passes the impedance it sees at the incoming port on to the output port.
I'd try adding a 6dB pad between the front end low pass filter and the mixer.
Also a 6dB pad between the local oscillator and the LO port of the mixer.
Once both of those are done, I'd expect the SAW filter to be getting driven
correctly from a clean 50 ohms resistive.
And once the SAW filter is behaving correctly, perhaps we can figure out how to
pick up the lost gain.? Take a look at W7ZOI's design for a spectrum analyzer:
? ?
He's got 6dB pads pretty much everywhere to avoid impedance mismatches.
He's got an MMIC amp immediately after the first mixer to pick up the gain again.
Which I don't quite understand, but am confident W7ZOI did.
I assume it comes down to managing the gain distribution for maximum dynamic range.
If the tinySA works well enough on $10 worth of parts, maybe that's good enough.
But I am curious if an extra dollar could make it better.
And a flat passband does strike me as better.
Jerry, KE7ER
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On Fri, Jan 3, 2020 at 03:27 AM, <erik@...> wrote:
After some more testing I discovered my nanoVNA was not correctly calibrated when measuring the SI4432 input impedance.
The SI4432 input impedance turned out to be fairly ok at 434MHz, about 45ohm.
The culprit was the mixer. After adding some isolation between the mixer (10dB attenuator or an amplifier) the pass band became flat again.
Do not put too much amplification between mixer and 434MHz filter as this will ruin your 1dB compression point and make it impossible to measure low levels of distortion as you lose your dynamic range
I tried 15dB and that was way too much.?
But not having the flat passband really did not make a difference in tinySA measurements so for now I will leave it as is.
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HBTE Files section:?/g/HBTE/files
Erik, PD0EK
