PCS Electronics TX - Sprogs problems

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nrgkits.nz
Neckmin
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Re: PCS Electronics TX - Sprogs problems

Post by nrgkits.nz » Tue Sep 04, 2018 8:22 pm

I've found that transistors have a far greater chance of putting out rubbish when not matched correctly or under driven. I haven't yet had this problem using mosfet/ldmos devices. Transistors are current driven, mosfet/ldmos devices are voltage driven. You don't need to reduce the RF drive, just need to reduce the bias gate voltage to change the power output - although doing it this way affects its efficiency. Really the best way to vary the power output is to vary the collector/drain supply's all at the same time so the interstage and output impedance matching remains the same. On my new PA design I'm using a PWM signal to control the PSU output voltage from the PIC micro - the PA efficiency is much better than using just the bias to control the drive.

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radionortheast
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Re: PCS Electronics TX - Sprogs problems

Post by radionortheast » Tue Sep 18, 2018 1:10 pm

I got the 8015, i’ve been dieing to get an unmodified transmitter to test my aerial, its really good, I used the little fan out of my Chinese transmitter and the 12 v power supply, the interesting thing is the fan from cz transmitter is noisey, but its fairly quiet at lower power levels, but the actual air flow is better than on my quiter fan.

i’m not going to mess with the output stages on this, took out 3 capacitors in the 3000 + put in ic sockets, theres noway to measure capacitors in a circuit, with them problaly been low even harder. It can still match on certain frequencies, i’ve never beable to get more than 7w with it the way it is..like nrg is saying the match changes with power. I had problems when I first got the 3000 +, trying to use a folded dipole that had a ferrite balun in it, mostly why I changed the output section, better matching meant it would stay at a lower tempreture, it was going up to 70 and tripping it. If I tried to push the power, it would draw excessive current

with the 8015 using the aerial i’ve made, I get 13 w on a high frequency, 0 relected, 15 w on a low frequency 0.3 reflected down my aerial, the aerial isn’t quite tuned for a low frequency at the moment, i’m not sure I believe the readings. I have my 3000 + set to 5w max, when I set it to full power, I get 6.2 w with 0.8 reflected, was the best I could get it on that frequency.
The thing i’ve found out is you match a transmitter better by using the correct lenght of co-ax, cutting an extra 50 cms of co-ax, using different lenghts seems to do a simliar thing to messing with the output stage. I wish i’d of know sooner, i’ve done the samething with my pci max 3000 3w, but unlike the more powerful 15w version, I can still have ruffly the same values of capacitors, works over the band. Theres no circuit diagrams so its a stab in dark to know what the original values where.

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