5627	2011-01-01 08:38:15	Chris Howard	a question about O'scope waveform
Message	Date	From	Subject
My "First Transmitter" has been a lot of fun. I have put my O'scope on the output and the waveform looks shapely. But it has a bit of a smear at the peaks, not crisp like some of my other transmitters. I did a bit of "customization" on my transmitter, so no criticism intended of the original design. I changed the final transistor to one that was in my junkbox which I believe is more of a VHF transistor. I also am using a different low pass filter design. My question is: How do I interpret the waveform I am seeing? How do I track down the source or which parts should I focus on?
5628	2011-01-01 08:47:16	Chris Howard	Re: a question about O'scope waveform
additional information: It looks cleaner transmitting into the dummy load and "smeared" when transmitting through my tuner to the antenna. I am not in an enclosure. Maybe getting RF back into the rig?
5629	2011-01-01 08:50:29	Chris Trask	Re: a question about O'scope waveform
> >My "First Transmitter" has been a lot of fun. > >I have put my O'scope on the output and the waveform looks shapely. >But it has a bit of a smear at the peaks, not crisp like some of my >other transmitters. > >I did a bit of "customization" on my transmitter, so no criticism intended >of the original design. > >I changed the final transistor to one that was in my junkbox which >I believe is more of a VHF transistor. I also am using a different >low pass filter design. > >My question is: How do I interpret the waveform I am seeing? How >do I track down the source or which parts should I focus on? > Sounds like a parasitic oscillation, especially since you're using a VHF transistor. Place a ferrite bead on the base or collector lead and see if it goes away. Chris Trask N7ZWY / WDX3HLB Senior Member IEEE http://www.home.earthlink.net/~christrask/
5630	2011-01-01 08:56:13	Chris Trask	Re: a question about O'scope waveform
> >additional information: > >It looks cleaner transmitting into the dummy load and "smeared" >when transmitting through my tuner to the antenna. > Parasitic oscillation for certain. Chris Trask N7ZWY / WDX3HLB Senior Member IEEE http://www.home.earthlink.net/~christrask/
5631	2011-01-01 09:48:33	Dean Blake	Re: a question about O'scope waveform
Agree... also remember that using VHF / UHF transistors in HF circuits can lead to more problems with stability. Sounds like they should work better (thats what I used to think) but the facts are they actually have low internal capacitances, much higher gain in the HF region (which can lead to spurious oscillations and even distortion of the HF envelope if the circuit doesn't use fixed gain design or feedback circuitry) the ferrite beads can help... K4DSB Dean To: emrfd@yahoogroups.com; emrfd@yahoogroups.com From: christrask@earthlink.net Date: Sat, 1 Jan 2011 09:50:22 -0700 Subject: Re: [emrfd] a question about O'scope waveform > >My "First Transmitter" has been a lot of fun. > >I have put my O'scope on the output and the waveform looks shapely. >But it has a bit of a smear at the peaks, not crisp like some of my >other transmitters. > >I did a bit of "customization" on my transmitter, so no criticism intended >of the original design. > >I changed the final transistor to one that was in my junkbox which >I believe is more of a VHF transistor. I also am using a different >low pass filter design. > >My question is: How do I interpret the waveform I am seeing? How >do I track down the source or which parts should I focus on? > Sounds like a parasitic oscillation, especially since you're using a VHF transistor. Place a ferrite bead on the base or collector lead and see if it goes away. Chris Trask N7ZWY / WDX3HLB Senior Member IEEE http://www.home.earthlink.net/~christrask/ [Non-text portions of this message have been removed]
5664	2011-01-04 10:49:50	Chris Howard	Re: a question about O'scope waveform
Recap: This is the "First Transmitter" from emrfd fig 1.39. I get a smearing of the waveform when transmitting into my antenna but not into the dummy load. I applied the beads to collector and base leads of my PA transistor. The base bead caused a large decrease in output power so I removed it. But in any case neither solved the problem. I increased the value of the RF choke on the collector, which also did not solve the problem. I think the RF choke did smooth out a bit of a kink that was getting into the power rail so I have left the larger value in. Then I probed more with my scope. The PA output goes through a pi network then there is a .01 cap to the LPF. I am not using the LPF from EMRFD but a different one that I built from the recent QEX articles. Probing that .01 cap I see a clean signal on the PA side but a smeared signal on the LPF side. This is while transmitting into my antenna. With a dummy load it is clean all the way through. So, I'm wondering if I have some kind of matching problem with my LPF. I had made a second LPF with the same values to measure return loss and it looked good at 50 ohms input and output, or so I thought. I have not attempted to measure the output impedance from the PA stage. I don't think I have the right capacitor values to build the LPF from EMRFD but maybe that is the way to go forward. But it seems like a good learning experience to try to figure out what is going on.