Receiving RF signals with an SA602

[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]

...and adjusted the input capacitor and coil until I get the largest signal across them using a spectrum analyser.

How did you connect SA to the input circuit?

[joelby]

How did you connect SA to the input circuit?

My spectrum analyser has an RG58 cable connected to its 50 ohm input with bare wires at the 'probe' end. I've put the one conductor from the cable on each side of the input inductor/variable capacitor while the circuit was powered off and a signal generator transmitting from a short distance away.

I understand that this is probably also an impedance mismatch (as the SA602's balanced input is 3K) but figured that I only care about finding the highest input level by adjusting the component values rather than the actual input power at this early stage of the investigation.

[flolic]

No, you can NOT connect SA that way and expect to correctly tune input circuit, especially at that high frequency. Cable and SA input capacitance affect resonating frequency too much. In fact, you may tune it but when you disconnect SA (cable) it will not be tuned any more.

[joelby]

Good point! I don't really know what I'm doing I'll try tuning the input while looking at the buffered output. Thanks!

Nope, that didn't seem to work either. I tried a simpler output stage of a resistor and two capacitors designed for audio (which I've seen in many example circuits online) and set the frequency offset to about 150 KHz (my SA goes down to 100 KHz) and couldn't see anything there either

[joelby]

Well, some good news at last! It turns out that it *does* work, using only a coupling capacitor and DSO or spectrum analyser on output pin 5. The problem is that my input signal was incredibly weak - maybe due to a very poorly tuned front end, or because my signal generator is pathetic. Putting the signal generator's radiating antenna a couple of centimetres away finally elicited a response. Time to add some RF amplifiers? Thanks everyone.

[joelby]

Hi all,

A bit of an addendum if anyone is interested - the way I ended up tuning the (re-designed) front-end was to connect both sides of the output to ground via 1.5K in parallel with 3pF (the SA602A is a double-balanced mixer). Then connect a vector network analyser to the antenna port and fiddle with your component values until you get as close to 50 + j0 ohms at the centre frequency as you can.

Testing filters for it is a bit tricky with standard test equipment because of the 3K input and output impedance!