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This is about a very specific RF system on a chip -- NXP's SA636, "Low voltage, high-performance mixer FM/IF system...". Datasheet is here: https://www.nxp.com/docs/en/data-sheet/SA636.pdf. The datasheets and application notes differ in what looks like a critical way to me, and I'm wondering what route to take before I spend cash on PCBs.

I'm trying to build a radio for 162MHz, a frequency that should be well within limits for this IC. I'm injecting my local oscillator (LO) signal, but where, exactly, I do that isn't clear. The chip has four relevant pins, two for the RF input and two for the LO; the RF and LO signals are internally mixed via a Gilbert cell. On the LO side, the two pins look like this, with one pin connected to the base of an internal transistor and the other to its emitter: Internal circuitry for SA636 LO pins

The datasheet itself has the LO injected on the OSC_IN (base) pin: LO injection on the OSC_IN pin

But in the application notes, the LO is injected on the emitter side of that transistor: Oops, LO injected on the emitter? (note that the application notes are using the SSOP package, with different pin numbers compared to the HVQFN package. Pin 3 & 4 in SSOP == pin 1 & 2 in HVQFN, respectively)

So, I'm wondering where to connect my 151.3MHz LO -- should I connect to the base or to the emitter? My guess is the emitter, but that's going by feel and I'd like to think things through rather than just go with my gut.

Datasheet: https://www.nxp.com/docs/en/data-sheet/SA636.pdf AN1996: https://www.nxp.com/docs/en/application-note/AN1996.pdf

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  • \$\begingroup\$ I suppose a better question might be, what are the pros & cons of connecting to one or the other? \$\endgroup\$ – joemadeus Nov 16 '19 at 0:41
  • \$\begingroup\$ drive pin 4, IMHO. I used the NE602 with internal oscillator for Channel 2/3/4 down conversion. I also experimented, by injecting a 0dBm (0.632 volt PP) into the base of the oscillating bipolar, with the tank components removed. \$\endgroup\$ – analogsystemsrf Nov 16 '19 at 3:17
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The last image LO source to filter to Pin 4 looks correct in the App Note but the label in Box , OSC_Out is wrong and is reversed with Pin 3.

Pin 4 = OSC IN

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  • \$\begingroup\$ Just checked the datasheet and you're right, it is reversed. Don't know why I didn't notice that on my own. Many thanks!! \$\endgroup\$ – joemadeus Nov 16 '19 at 12:26

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