I have a question about RF measurement techniques.
The partial schematic above is the front end of an AM radio receiver kit. I have assembled the kit. The kit manual states,
Transistor Q7 not only amplifies the RF signal, but also simultaneously oscillates at a frequency 455kHz above the desired radio station frequency. Positive feedback from the collector to the emitter of Q7 is provided by coil L5 and capacitor C31. During the heterodyning process the following four frequencies are present at the collector of Q7.
- The local oscillator frequency, OF.
- The RF carrier or radio station frequency.
- The sum of these two frequencies, OF + RF.
- The difference of these two frequencies, OF – RF.
When I measure the ground referenced voltage at Q7, with the radio playing a local station, all I see on the scope's (Tektronix TDS-380, 1996 vintage) FFT display is the OF signal, which is very distinct. That OF signal changes with the movement of the tuning dial and is about 495 kHz, not 455 kHz, above the received radio station frequency of 990 kHz. The scope probe is set to 10X. When I set it to 1X the station disappears, which I think means that the 1X probe is loading the signal(s) at the collector of Q7 with the scope probe.
Should I be able to see the RF carrier as well as the sum and difference frequencies when measuring with an oscilloscope in this manner?