> But no longer do you need to run a dozen "pills" (man I hate that > term). Here is all you need for 1250w 1.8-600 MHz:
Unfortunately, the LDMOS device data sheets provide *no* IMD spec's for linear operation. The only data is for CW and pulse service. If one is to extrapolate from similar LDMOS devices (single FET vs. two FETs on a common die), one would need to derate to 800-900 W PEP in order to achieve reasonable IMD levels. That conclusion is further corroborated by the compression spec's (actual vs. ideal output power) which show the onset of output compression above 59 to 60 dBm (59 dBm => ~800W). The devices work at 1200 W CW (or JT65) because those modes are single tone and work with saturated operation (class C amplifiers) where IMD performance is not "tested". At full output these devices would be as dirty in SSB operation as the old FM "brick" amplifiers were when run in SSB service - perhaps like the RMA Italia solid state amps <G>. BTW, since these are 50V parts they are not suited for mobile use at 12V (13.8 V nominal) service. > $241.50 compared with the new price of a 8877 is pretty cheap! To even approach the IMD performance and reliability of an 8877 you would need two devices and the cooling problems (cost of heat sinks, heat spreaders, etc.) are much more difficult with two of these devices than with a single 8877. Any cost advantage for even two of the LDMOS devices over an 8877 will be more than offset by cooling system (in addition to splitter/combiner and protection system) costs. 73, ... Joe, W4TV On 2014-12-13 8:03 PM, Edward R Cole wrote:
FWIW I am running the AN-762 at 140w using 12v with either the K3 or KX3 with no complaints. It takes only 3.5w drive on 20m. But no longer do you need to run a dozen "pills" (man I hate that term). Here is all you need for 1250w 1.8-600 MHz: The MRFE6VP61K25HR6 1.25KW LDMOS from Freescale at $241.50 from: http://www.communication-concepts.com/mrfe6vp61k25hr6-1-25kw-rf-mosfet/ Or several others. I haven't seen the article in QST but suspect they are using this device. It runs on 50v at under 50A and only requires 3-4w drive for 1200w output. KX3, FT-817, K3/10 all capable of driving it directly. 73, Ed - KL7UW $241.50 compared with the new price of a 8877 is pretty cheap! 73, Ed - KL7UW They don't need to be dirty ... the problem is that it takes so many "pills" to make the power that designers tend to use the 60, 80 or 100W rated transistors right up to their saturated output rather than limit each pair of devices to 50 - 60% of PEP output. For example ... the MRF-421 is rated as a 100W transistor but third order IMD is minimized around 50-55W. An "eight pill" design could make 900W in saturated mode but in order to keep it clean, the output would need to kept to 500 W PEP. This, in spite of the fact than Communications Concepts sells a 2 x MRF-421 board set as a "180 Watt HF amplifier" (Motorola AN-762). 73, ... Joe, W4TV 73, Ed - KL7UW http://www.kl7uw.com "Kits made by KL7UW" Dubus Mag business: dubus...@gmail.com ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to li...@subich.com
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