http://groups.yahoo.com/group/UV-3R/message/310 From: cctbcn Subject: Re: Serious harmonics problem on VHF! Hi, Actually, I didn't really check what the high end (160 MHz) did with the 33= pF capacitor since I don't have much of an interest in transmitting there -= I only checked the 47pF over the range after I was done. * * * One thing that I'd realized that I'd forgotten to mention after the last me= ssage (and I'm not sure why) was the "problem" with low power. On my UV-3R, Low power was on the order of 50-100mW - plenty of power to go= almost any "line-of-sight" distance. The problem is that, as was noted previously, the 2nd harmonic is "seriousl= y present" - in my case, it as about -10 to -15 dBc at 146 MHz - reduced to= -30dBc or so after the modification. (In other words, the absolute power = levels of the 2nd harmonic after modification on low and high power are *ro= ughly* equal on my radio - that is, a few 10's of microwatts.) If other radios are like mine, however, I would *NOT* run it on low power o= n VHF unless it were modified! On UHF, the power level goes from about 1.6 watts or so on high to 1 watt o= n "low" power and the 2nd harmonic is about the same (e.g. -55dBc or so) in= each case. * * * Now, exactly how LOW/HIGH power is selected is unknown for certain, but I s= uspect that the RD1846's "PABIAS" pin is used: This is a voltage-programma= ble pin (1-3 volts, additionally scaled by resistors) that is routed to the= appropriate PA via Q11. Now, twiddling the bias on FETs adjusts their gain - but it also affects th= eir operating point, so I what I *suspect* is happening is that at very low= bias for low power on VHF, the output device's impedance is seriously skew= ed and it happens to work as a surprisingly efficient frequency doubler! (= I also suspect that, at least with my radio, the wrong bias is used - perio= d!) Now, it is *possible* that in the radio's EEPROM is stored a value for the = "PA Bias" pin and with it, one could adjust that parameter, tweaking both t= he output power level as well as adjusting the PA bias to push it into a mo= re linear range and reducing the harmonic content somewhat. This sort of t= hing may also change somewhat with battery voltage. Whether or not this parameter is available via serial interface or the save= d memory data (along with a number of other things) is unknown to me - but = it might be worth poking around. Remember that this *is* a bias setting and it's possible that one *could* p= op a final if set too high!!! * * * As for the added capacitor in the picture, my guess is that it is installed= at the position of "C89" (marked "NA" on the diagram) but I'd have to seri= ously scrutinize the board to be absolutely sure of this as I've not seen a= board layout. * * *=20 Finally, it's worth mentioning that the "full-sized" picture of the picture= s that I took of the board can be seen if one click on the "Original" tab a= bove and to the left of the pictures near word "View" - but this shows up o= nly when you have selected and displayed a single picture. 73, Clint KA7OEI --- In UV-3R@yahoogroups.com, "andyfoad@..." wrote: > > Well done Clint. >=20 > Just out of curiousity, when using a 33pf capacitor, how did the=20 > power output compare on the high end of VHF ? >=20 > 73 de Andy >=20 >=20 - Previous messages snipped to keep the message length reasonable! -