Re: Harmonic on Homebrew RF Transmitter

Hello dgc,


I'm the guy who posted the Bias on NPN Grounded Emitter query a few days ago. Well thanks to some suggestions here I ended up installing some collector to base feedback (3000 ohm resistor), ...


IIRC Atlas did that w/o, just using a diode that was thermally mounted to the heat sink of the transistors.


...and now have a clean sin wave at 7.1245 MHz on my output at 1.25 watts. Trying to get 2 watts rms out of the 12v supply into a 50 ohm load just wasn't realistic while trying to stay linear. Anyway here is the puzzling part. At 21.372 MHz I hear my transmitter on a nearby receiver (maybe 3 feet away) yet I see no sign of a 21 MHz waveform on my oscilloscope which is monitoring the transmitter output. On the receiver S meter the 21.372 Mhz signal appears as strong, if not stronger, than my 7.1245 Mhz signal. Could this be due to the proximity of the receiver to the little transmitter? Could my output be clean while I still pick up a harmonic close by? ...


Could be. Where are you on the S-meter scale? Make sure you don't run into protective diodes that would generate harmonics from a clean signal.


...Is the oscilloscope output a definite indication of the non presence of harmonics?


Definitely not. Even a harmonic content of 5% (which is a whole lot) is hardly discernible on a scope. You'd need a spectrum analyzer. A good receiver can be used as well. The trick is to establish a reliable path from dummy load via various attenuators into the receiver. I have those in the lab but you might have to make some. Don't go more than 10dB per attenuator stage. I think the ARRL handbook shows how to build them (and mind the power levels).

Don't try this with some cheap SW-receiver in a plastic enclosure. It's not going to work.


... I want to know this before I try the transmitter to an antenna. Right now I'm working into a 50 ohm dummy load.


Good. Some folks would just fire it up and then wonder why the FCC guys showed up. I always rely on a good lowpass such as pi-filters.

Regards, Joerg

http://www.analogconsultants.com
.

Relevant Pages

  • Re: Harmonic on Homebrew RF Transmitter
    ... > of the 12v supply into a 50 ohm load just wasn't realistic while trying to ... > 21 MHz waveform on my oscilloscope which is monitoring the transmitter ... On the receiver S meter the 21.372 Mhz signal appears as strong, ... > indication of the non presence of harmonics? ...
    (sci.electronics.design)
  • Re: Harmonic on Homebrew RF Transmitter
    ... > of the 12v supply into a 50 ohm load just wasn't realistic while trying to ... > 21 MHz waveform on my oscilloscope which is monitoring the transmitter ... On the receiver S meter the 21.372 Mhz signal appears as strong, ... amplitude of any harmonics present in the tx output. ...
    (sci.electronics.design)
  • Re: Harmonic on Homebrew RF Transmitter
    ... Well thanks to some suggestions here I ended up installing some collector to base feedback (3000 ohm resistor), and now have a clean sin wave at 7.1245 MHz on my output at 1.25 watts. ... At 21.372 MHz I hear my transmitter on a nearby receiver yet I see no sign of a 21 MHz waveform on my oscilloscope which is monitoring the transmitter output. ...
    (sci.electronics.design)
  • Harmonic on Homebrew RF Transmitter
    ... of the 12v supply into a 50 ohm load just wasn't realistic while trying to ... 21 MHz waveform on my oscilloscope which is monitoring the transmitter ... On the receiver S meter the 21.372 Mhz signal appears as strong, ... indication of the non presence of harmonics? ...
    (sci.electronics.design)
  • Re: Harmonic on Homebrew RF Transmitter
    ... Well thanks to some suggestions here I ended up installing some collector to base feedback (3000 ohm resistor), and now have a clean sin wave at 7.1245 MHz on my output at 1.25 watts. ... At 21.372 MHz I hear my transmitter on a nearby receiver yet I see no sign of a 21 MHz waveform on my oscilloscope which is monitoring the transmitter output. ... As others have mentioned it can be hard to see harmonics on your scope, and you should have your 3rd harmonic at least 40dB down for amateur radio. ...
    (sci.electronics.design)
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