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JUMA PA1000 Linear Amplifier

PostPosted: 16 Nov 2011 21:23
by OH7SV
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Re: JUMA PA1000 Linear Amplifier

PostPosted: 17 Nov 2011 02:20
by 5B4AGN

Congratulations on your fine work so far with the PA1000. I am interested to understand your experience with harmonic energy levels from the MRFE6VP61K25. I have been experimenting with one of these devices for use in a low weight "airline friendly" travel amplifier for DX-pedition use. Initial encouragement by Freescale in the choice of the device was strong but experience has revealed problematic levels of harmonic energy at the output. Freescale now acknowledge this problem to be significant when using these devices at HF. They have quoted H3 at -7dBc and H5 at -13dBc being their experience. These figures are bad enough to require use of a power dumping diplexer at the output or development of some frequency adaptive class F filtering which seems rather impractical in a broadband amplifier. My own experience with the particular device I have on hand is H3 at 0dBc +/- 2dB and H5 at -2dB +/- 2dB throughout the frequency range where H3,5 are within the passband of T1/T2. Harmonic attenuation obviously increases outside the passband.

Extensive discussion with Freescale resulted in them saying, "Development of a wholly acceptable amplifier for this frequency range (1.8 - 54MHz) will be very difficult to achieve."

I am wondering whether you have had better experience with harmonic levels or found a way to address the issue satisfactorily?

Kind regards

Bob, 5B4AGN

Re: JUMA PA1000 Linear Amplifier

PostPosted: 17 Nov 2011 18:06
by OH7SV
Hello Bob,

Thanks for very informative reply. Nice to hear that you are also experimenting the Freescale MOSFET. Actually I don't know anybody else who is doing something in HF with the MOSFET.

Sorry to hear about your weak results. I think that the harmonics are related to incorrectly operating RF PA transformer. You probably know the problem related to class-AB operation. During the non conducting half cycle the MOSFET is in high impedance state. If you use a popular type of a transformer that end of transformer is not connected enywhere and the voltage bounces releasing high frequency energy to the output. I have also tried a popular type of a transformer and it resulted a breakdown of one LPF capacitor. This confirms that there were lot of harmonics with the bad transformer and the first LPF capacitor could not stand that high RF current.

We are using a very different type of RF PA transformer. Actually there are two transformers, one for each MOSFET and the ouputs are connected in parallel with opposite phase. We have measured the harmonics after the LPF filters and the harmonics are attenuated very well. We still need some improvement to the two-transformer construction to reach broadband performance. The intention is to replace the brass tube-PTFE wire combination with a low impedance coax.

I'll keep you informed when we know more.

73 Matti OH7SV

Re: JUMA PA1000 Linear Amplifier

PostPosted: 18 Nov 2011 14:49
by 4z4rb
I have been building amps using freescale devices and NXP
I am using a different LDMOS the MRF6VP11kh and the BLF578
I had problems toowith harmonic content and I have switgched to a different output matching and transformer.
Instead of using the traditional Faraday transformer concept I(ferrite and copper sleeves)
I made a transmission line transformer with a length of 25 ohm raychem coax wound on ferrites to form a 9:1 transformer.
the results were much better
I will upload pictures once I will find out how to hook my Anritsu color spectrum analyzer to my labs pc that has no parallel port...
Baruch 4z4rb

Re: JUMA PA1000 Linear Amplifier

PostPosted: 18 Nov 2011 20:14
by 5B4AGN
Hello Matti

Many thanks for your note. The output arrangement you have employed is an interesting approach to dealing with the high level odd harmonics generated by these devices. I know Baruch 4Z4RB has had success using output TLTs with other of the new Freescale series.

These devices hold great promise if they can be tamed. My holy grail is a sub 8kg (inc PSU) clean kW for DX-pedition use.

Great work.

73 Bob, 5B4AGN

Re: JUMA PA1000 Linear Amplifier

PostPosted: 19 Nov 2011 10:46
by 4z4rb

Re: JUMA PA1000 Linear Amplifier

PostPosted: 21 Nov 2011 20:39
by 4z4rb
Another version of TLT output transformer with a 4;1 ratio.
wound with a 25 ohm coax by Raychem .
central conductor is 16 gauge ,loaded with ferrite with Mu=850
note the big ATC caps 0.1 uF x3000V

Re: JUMA PA1000 Linear Amplifier

PostPosted: 29 Nov 2011 01:51
by wa1gfz
Do not waste your money on 25 ohm coax. #24 single shielded Teflon wire is very close to 26 ohms.
I've been converting Erbteck MRI amps to HF modifing their transformers. It is pretty easy to get 160 through 6 meters.

I just received a reply from Freescale today that the FET in question is rated at -28 dB IMD3 at full power and jumps to --41 dB at 600 watts. They have a 2 meter app note that shows the data. The bias was set for 2.5 amps drain current.
Frank WA1GFZ

Re: JUMA PA1000 Linear Amplifier

PostPosted: 29 Nov 2011 15:39
by 4z4rb
Hi Frank!
Indeed my experiments show it too.
great to see you

Re: JUMA PA1000 Linear Amplifier

PostPosted: 29 Nov 2011 19:52
by OH7SV
wa1gfz wrote:Do not waste your money on 25 ohm coax. #24 single shielded Teflon wire is very close to 26 ohms.

Frank, tnx for the good advice. What do you say about this Teflon cable? ... 0603278042

BTW, have you measured other gauges of shielded Teflon wire, like #22 and #20?

73 Matti OH7SV