I have an RF Amplifier kit that is not working. The main problem is that the input transformer presents the radio with a high, unexpected SWR. It does amplify, so something works.

The transformer is wound as a T2:T.5 turns radio. The below pictures display it. I suspect that it might have a blob of solder shorting out part of it inside or a wire is cut.

I really do not know, the circuit is a simple class ab amplifier with mosfets and feedback.

I plan to unsolder various resisters and capacitors and feed an input signal into the transformer directly and see what come out the other side.

I have an oscilloscope and cheap ebay signal generator. The signal generator I have can produce up to a 25 Mhz output into an open. Meaning, not into a 50 ohm load, like the usual signal generator. If I attach a load, it should reduce the signal generator output. Is the transformer a load, how much?

I am planning to operate this around 7.2 or 14 Mhz.

If I hook the signal generator to this transformer, will it be able to produce an output that I can measure? Should I use am op amplifier / follower circuit to increase the power/buffer between the signal generator and the primary?

The input transformer is usually directly connected to the 50 ohm output of a transceiver.

I would expect to see outputs that are 25% of the input signal 180 degree phase reversed.

I would like to do experiments to see what the swr is and how the later mosfet , class ab amplifier (with the feedback) affects it. I suppose there is a way to do this by way of loadng the secondary?

Ideally, I can gather enough information to model this in LTSPice. What this transformer is doing and its impedance is key.

Can I send my signal generator input into this transformer and get a useful measurement/learn something for ltspice? Any other advice? Thanks

schmadic Amplifer

input transformer

  • \$\begingroup\$ How are you going to measure, do you have an oscilloscope? Find out the output impedance of your sig-gen, and add resistors if needed to get it to 50 ohms. Then measure its output into a 50 ohm load. There's no need for high power to make a gain measurement, so even if your sig-gen won't drive 50 ohms at full output, that's not a problem. Then use it to measure the transformer. Do not use an opamp, one more thing to control and be suspicious of, you can't beat resistors for simplicity. \$\endgroup\$
    – Neil_UK
    Aug 1 at 15:30
  • \$\begingroup\$ That doesn't look like a transformer to me; there only appears to be one winding AND, if that's all there is it'll be a really crappy input impedance for circa 10 MHz. Post a schematic. \$\endgroup\$
    – Andy aka
    Aug 1 at 15:34
  • \$\begingroup\$ @Andyaka The secondary a single turn through tubes terminating to the tiny PCBs at the ends of the binocular core. The ref des is T1. Or rather it's two half turns, with the mid point going through C3 to ground. \$\endgroup\$
    – Neil_UK
    Aug 1 at 15:45
  • \$\begingroup\$ @Neil_UK I have an oscilloscope. I can see what Pk-Pk goes in, by frequency. My signal generator, as reviewed/tested by someone else states its output is described as going into an open load, not the usual 50 k ohms. I plan to connect one channel of the o scope to the input and one channel on the output and use it like that. \$\endgroup\$ Aug 1 at 15:46
  • \$\begingroup\$ @Neil_UK OK I see it now! All the same, the input impedance will be shabby at circa 10 MHz. \$\endgroup\$
    – Andy aka
    Aug 1 at 15:48

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