Total posts : 45366
Carl and other interested observers,
I got around to doing my experiment and here’s what I found. I had previously adjusted R23 to give a good undistorted waveform so I started with this. The test frequency is 1670 kHz, power supply voltage is 13.77 V, audio test frequency 800 Hz sine wave and the antenna load resistance is 80 ohms dummy load.
The voltage at the R34/C23 test point = 4.77 VDC.
DC input power to Q3 = 201 mW.
RF output power to the load = 32 mW.
The audio input voltage was increased until the maximum undistorted modulated RF waveform was observed. Distortion began on the zero level excursion but the maximum excursion appeared undistorted. The maximum modulation percentage undistorted = 86%.
It was noted that the distortion on the waveform at the R34/c23 TP exactly matched the distortion seen on the modulated RF envelop and that both occurred at the same audio level.
I reset the R23 bias to 7.1VDC as instructed in the manual. With this setting I measured:
Pin = 452 mW
Pout = 66 mW
Maximum undistorted modulation = 51%.
The distortion under these conditions started with the top of the modulated RF waveform being flattened and with further increases in audio it clipped at zero. It was noted that the waveform at R34/C23 distorted with exactly the same pattern and at the same audio level as did the output. The peak voltage at R34/C23 when this started was 10.5 volts.
Since the peak voltage into the final stage was 10.5 volts with no distortion I conclude that further attempts to increase the voltage peak distorted the waveform because with the 13.77 V power supply the maximum output voltage of the modulator stage was reached.
My data do not indicate any deficiency in L4. According to my test, the limit on % modulation is due to limitations in the audio/bias circuit and not in the RF circuit. There is probably an optimum setting of R23 between the values I used where the distortion appears at both the top and bottom of the modulated waveform at the same time but I have not pursued this (yet).
Since this unit runs well above the part 15.219 limits for DC final input power under my test conditions, my recommendation is to set R23 to achieve the maximum undistorted modulation percentage which will be below the recommended 7.1 V setting at the test point. If one is concerned with the input power (and one should be) then another operating setting needs to be used.
I have left out many details regarding my measurement techniques but I assure all that they were done with accepted engineering practice. If anyone has specific questions about the methods I will respond.