[Elecraft] TX IMD

[Joe Subich, W4TV]

Unfortunately neither the Toshiba or ASI datasheets for the 
2SC2782 used in the KPA3 provide any IMD specifications.  
Unlike the information available for the 2SC2879.  

The 2SC2879 has a distinct "sweet spot" at 60 Watts output 
with 12.5V and 100 mA idle current.  A push pull pair of 
2SC2879 (e.g., FT-990) would be expected to produce -38dB 
IMD at 120 watts at the collector (100 W from the LPF) with 
8 - 10 W of drive and Vcc 12.5V at 28 MHz.  Note that the 
2SC2879 is also rated for -24dB IMD at 100 Watts per device 
(or 200 W PEP from a pair) ... there is no such specification 
for the 2SC2782. 

I haven't been able to find IMD data on the low level devices 
for either the FT-990 (2SC2166 predriver, 2SC3133 drivers) or 
K3 (RD06HHF1 predriver, RD15HVF1 drivers).  However, I am a 
bit concerned that the K3 predriver is only rated for 30 MHz 
operation! 

73, 

   ... Joe, W4TV 
 
 


> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Dave 
> Hachadorian
> Sent: Monday, January 26, 2009 11:42 AM
> To: Elecraft reflector
> Subject: Re: [Elecraft] TX IMD
> 
> 
> There is something that has been puzzling me for the last 17 years.
> 
> In November of 1991, QST reviewed the Yaesu FT-990, which
> used a 13.8 vdc PA. Worst case measured 3rd order IMD
> was -38 dB, 5th order was -47 dB. This measurement was at
> 100 watts output. The review is available on ARRL's 
> members-only web site.
> 
> ARRL had this comment, "It's refreshing to see that it's 
> possible to make a 100-W MF/HF radio with low voltage finals 
> AND such low-distortion transmitter performance".
> 
> As far as I know, no ham 13.8 volt radio has even come close
> to this TX IMD performance ever since.  Looking at the 990
> PA, there's a 2sc2166 predriver, a pair of 2sc3133
> drivers, and a pair of 2sc2879 finals, all npn transistors, 
> nothing exotic.
> 
> So what was Yaesu's secret, and why hasn't anyone else even 
> come close?
> 
> Dave Hachadorian, K6LL
> Yuma, AZ
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