HF beacon harmonics - Page 1
Forum section: electronics / rf-microwave

author: james_s
author_rank: Super Contributor
timestamp: March 02, 2020, 06:27:32 am
content: I wanted to do some RF tinkering so I built a couple of different HF beacon circuits. One based on a single transistor oscillatorhttps://aa7ee.wordpress.com/2018/05/07/the-boris-beacon-a-1mw-solar-powered-hifer-beacon/and another using a 74HC02https://blackcatsystems.com/rf-products/pics/beacon/beacon_schematic.pngI didn't buy the kit, I just deadbugged the circuit following the same schematic.I built the transistor based one first and it seemed to work fine however I eventually took a look at the output on a spectrum analyzer and noticed that the second harmonic is down around 30dB from the fundamental which seems a bit worse than I'd typically expect for a CW transmitter, a friend of mine suggested that 40dB would be a better target to aim for. Then today I built the 74HC02 based beacon and was surprised to find it's very similar, actually it's uncanny how similar it is, with the plots from both units overlaid they're almost identical, blue is the transistor beacon, magenta is the IC.So now I'm wondering if the results I'm getting are typical for a circuit like this or whether mine are not working as well as they ought to be. I'm tempted to buy the kit for the IC based one and see if that performs any differently than the one I built, I did make one change in that I used the same T37-6 toroids I used for the other one. I suspect that starting out with a nice clean sine wave would result in a cleaner output so I could also try a different kind of oscillator.

author: mark03
author_rank: Frequent Contributor
timestamp: March 03, 2020, 01:07:47 am
content: I don't know, but I'm curious too. What about the third harmonics? Do they also match? I assume you tried changing the attenuation to verify that the harmonics are real and not spectrum-analyzer front-end distortion.Maybe it is just a coincidence.

author: james_s
author_rank: Super Contributor
timestamp: March 03, 2020, 01:20:39 am
content: Yes I did, and I built the filter first and tried driving it with a square wave from a 33120A and that resulted in a sine with the second harmonic down a bit over 50dB. The waveform coming out of the oscillator is not nearly as nice of a square as that from the signal generator. The transistor oscillator produces something vaguely sine-like but it too is a bit ugly, especially once loaded by the filter.I'm surprised there have not been more responses from people who have built low power CW transmitters, I'd have thought they would be a rather common thing given the simplicity. Perhaps most of the people building them lack the means to test the performance though.

author: mark03
author_rank: Frequent Contributor
timestamp: March 03, 2020, 03:11:50 am
content: I am not an RF or analog guy (although I too am trying to learn). But in the transistor oscillator, I believe the output impedance is approximately the impedance of the collector load, i.e. the tank circuit, which is "large" at resonance. Perhaps there would be less distortion and a better match, taking the output from a secondary winding of one or two turns on the tank inductor, instead of the capacitively coupled arrangement in the schematic?