|
John,
toggle quoted message
Show quoted text
Why offline? Perhaps we can all learn something. I have no experience with this particular design. But I don't think you have a layout issue. Most CW transmitters are broadband all the way out through the final. Being class C, the final creates plenty of harmonics, but the output lowpass filter (L5,L6 in this case) should be sufficient to clean that up. The writeup for the Rose-80 says: >??Transformers T3 and T5 (Mouser 42IF123) are used to filter unwanted harmonics and spurs. T3 should be adjusted for >? maximum output at the higher end of the band while T5 should be adjusted at the lower end of the band. The writeup also says that the VFO operates between 4.3 and 4.4 mhz.? The gets mixed with 8mhz in the NE602 at IC5, creating an output signal between 8-4.3=3.7mhz and 8-4.4=3.6mhz. Unfortunately, the NE602 also has some of the 4.3mhz VFO coming through and heading out to the final. That 4.3mhz is too close to the transmit frequency for the output lowpass filter at L5,L6 to knock out sufficiently. So the designer added tuned circuits at T3 and T5 around the transmit pre-driver to knock out the VFO. At least, that's my best guess. Tuning T5 for the low part of the band and the T3 for the high part would spread the peak out, but perhaps not evenly enough to meet your expectations. Are you trying to hit more than a 100khz slice of the 80 meter band with this transmitter? You may be best off to narrow down the slice you are trying to hit until performance is sufficiently uniform after tweaking T3 and T5. If trying to hit 3.5 to 3.6mhz, for example, I'd try peaking T5 for 3.525mhz and T3 for 3.575mhz. So peak at 25% in from the desired band slice edges, not exactly on them. Remember, if power varies between 5 and 3 watts out, that's less than half of an S unit.? The guy at the other end may not even notice. If I am right and it is just a matter of the VFO sneaking through, one fix would be to move the VFO above the 8mhz intermediate frequency instead of below. To tune from 3.5 to 3.6mhz, the vfo would be operating between 11.5 and 11.6mhz. Doing this would require defeating the tuned circuits at T3 and T5 somehow. An analog VFO becomes increasingly unstable as you go up in frequency like that, but one of the many si5351 based VFO's should work quite well.? Hans (who has checked in here) sells a very nice full featured VFO that could? be used in lots of different projects, and also works as a general purpose signal generator: ? ?? Me, I'd just live with the loss of 2 Watts at the band slice edges. Good Luck! Jerry, KE7ER? ? Flora OR? (hey, not just too far!) On Wed, Dec 11, 2019 at 05:56 PM, John Roland wrote:
To make a long story short, I am have a problem with the output power varying a couple watts over the tuning range depending on where I peak the filter (T3 T5).? I built the rig using ugly construction and have tried to improve the layout of the transmitter once but no joy.?? Now that the crummy weather has returned to the PNW, its down off the shelf again and I'm looking at the filter as the culprit.?? I'm thinking I have probably made an error in the assembly of that section but can't see my problem.? If you have any experience with this circuit and could elmer me through this, I'd be very grateful.?? |
