Seeking practical methods to estimate input and output LDMOS(MRF184)

Thread Starter

ubin10

Joined Aug 8, 2018
3
Hello folks,
I'm an amateuer and I have some some MRF184s and want to use it to build power amplifier (440 Mhz band).
Checking its datasheet, it doesn't have impedance information for the freq (they're available for 800-900 Mhz), however large signal S parameter is provided for 450 Mhz.
Is large signal S parameter are good guess to start ? (I need to design matching circuit for 50 ohm load and source)
note : Im not able to characterize the device on my own since i dont have access to analyzer, and i dont have ADS simulation tool either, if someone here can help to provide Zin and Zout figure for MRF184@450Mhz @60 watt that will be great help !

BR
 

Thread Starter

ubin10

Joined Aug 8, 2018
3
Hello,

Have a look at the pages 7 and 8 of the datasheet.

Bertus
Thanks Bertus,
Yes they're actually large signal S parameters, not a pull load data. In fact i made plan for matching circuit using this data (based on page 7-8). I hope this is a good guess, since i will implement microstrip L , and fixed value chip caps since using adjsutable trimmer may no longer suitable (due to parasitic problem)

regards
 

Janis59

Joined Aug 21, 2017
1,178
Why not to buy the corresponding 2kW cross-scale device for some 20-50 USD (ebay) and make a impedance matching by it, as well the accurate power measurements. For dummy load may use the cement resistor battery, some 50-100 pieces parallel between two plates, where from side to side the fans sery is blowing the wind. At least mine is well working until 3 kW and it inductance is below the 50 nH as shows my measurer. Have no knowledge, was it wise that I tried to compose all resistors let neighbours has inscription reverse, but at least its not wrong. The plates must be as small as possible, to diminish the fringent field. At 430-470 MHz it would be problematic with FR3 or FR4 as they has tg(fi) about 0,15. Better is to take the teflon base boards (https://www.pcbdirectory.com/manufacturers?material=Teflon) and some of them hgas tan(fi)=0,00003. Then the high power operation will not kill em all.
 

Thread Starter

ubin10

Joined Aug 8, 2018
3
For dummy load may use the cement resistor battery, some 50-100 pieces parallel between two plates, where from side to side the fans sery is blowing the wind.
Hi Janis,
yeah, many devices out there with stealing price in alibaba /ebay, but I have couple of mrf184 laying unused in my component box, beside I want gain my skill in RF design. But I'm interested with your cement battery dummy load construction. Any link/reference to look ?

Best regards
 

Janis59

Joined Aug 21, 2017
1,178
I am now 1200km off the home until the september thus I cannot make a picture right now. But You may ask me it at september privately. So, for a while I may only describe - as more the resistors You put in parallel, as much the inductances become smaller. And about semi-noninductive wires, the plate as geometric structure have few-fold better (smaller) inductance than bi-wire (sandwich) line, what has ca 10x less than ordinary wire having ca 10 nH per cm. Thus, the plates of about hand size must be drilled for resistor feet, and positioned let the distance is some 2 cm less than full length between leg ends. Insert one, move to one side so the other leg may be inserted, then bend sharply both legs for temporary fix it. Dont cut or solder anything yet. If I remember correct I used sth between 80 to 200 pieces parallel. And to get it in the holes was indeed a real nightmare. When its all ready, push both plates together how much tight the cement allows and only then start to solder them. The drillings pattern ought be - one raw shifted against next raw for better air rotating. Fans are mounted in two sides, and output of hot air in another two sides. Point of attach the cables is center of plates.
Link for cement resistors (I havent checked what is the inductance of it, just for aim to see how it looks https://allegro.pl/rezystor-opornik-cementowy-1r-1ohm-10w-prw10w-i6768317663.html. You need to find an offer with as small is possible L and range about the 5 kOhm (depending on count) and 10 or 20W each. I dont remember did I bought em in Farnell or at Digikey, but both are good places where to start seek. If no, try the Octopart, probably.
 
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