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| Receiving RF signals with an SA602 |
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| joelby:
Hi all, I'm trying to build a 150MHz radio receiver using a SA602 oscillator/mixer chip. I've built the VCO part of the circuit detailed at http://hem.passagen.se/communication/pic/mix6rec.gif (but without the PLL for now, and without the MC3371 and transformer L3) and have manually tuned it to 160MHz. I've set up a 150 MHz CW transmitter nearby and adjusted the input capacitor and coil until I get the largest signal across them using a spectrum analyser. Now, the problem is that I get nothing at all across the output! I've connected my SA across pins 4 and 5 and don't see anything at about 10MHz as I'd expect. I thought that this might be an impedance mismatch issue - the SA602 has a 1.5K output and my SA is 50 ohm, so I added a capacitive transformer - 1nF and 9nF, with no resonant inductor, to give a 10MHz centre and 2MHz bandwidth. This didn't help at all. I'd have thought I'd see something even with a mismatch. Has anyone had much experience with this chip, or can anyone provide general advice? Thanks a lot! |
| jahonen:
I'm no expert on RF or SA602, but I suspect that the output inductor/transformer L3 might be necessary for internal DC-biasing. Without it, the chip may not work correctly. Perhaps you could test it by putting some inductor between pins 4 and 5. Then try connecting the spectrum analyzer to the output. Regards, Janne |
| joelby:
Thanks Janne! I gave it a go and unfortunately it doesn't seem to have helped. The datasheet shows a few different output configurations, including single-ended ones that go straight into filters. I'll try a few other things, like connecting pin 5 to a 1.5K load. Nope, that didn't work either! |
| joelby:
I added on a JFET buffer: * Gate connected to mixer output via 22pF and biased to 1/2 Vss using two 2.7K resistors (approximating a 1.5K input impedance). * Drain grounded via 47R (to approximate a 50 ohm impedance). * Source connected to Vss. Spectrum analyser connected to drain via 22pF This also had no effect ??? |
| flolic:
--- Quote from: joelby on January 13, 2010, 06:58:55 am --- ...and adjusted the input capacitor and coil until I get the largest signal across them using a spectrum analyser. --- End quote --- How did you connect SA to the input circuit? |
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