[Elecraft] Re: band-passfilter location in SSB transmit chain

[wayne burdick]

pc5m, Carel wrote:

> I did perform extensive (spectrum) measurements on the TX spectrum at 
> 28MHz...

> Example:
>
> TX = 29.0 MHz....

Hi Carel,

Thanks for the data.

While the K2 was never intended to cover the entire 10-meter band (we 
spec'ed it in the manual at 28.8), it may be possible to attenuate the 
indicated product at 28.51 with a modification to the buffer amp. Or, 
it may be that the amp itself is not the cause, but is reflecting a 
mismatch back to the TX mixer. A better match may be called for in this 
case.

The way to check these possibilities is to temporarily terminate the 
mixer and/or the buffer with a 51-ohm resistor, then check the spectrum 
again. In either case, a 3 to 5 dB pad at the buffer's input or output 
may substantially reduce undesired products. Of course the gain would 
have to be made up after the pad (or after the band-pass filter). 
Alternatively, you could replace the present buffer with a dual amp 
(LT1253, I believe) and configure it for differential output drive 
using a broadband transformer.

The K2 has been around for quite awhile, and it's nice to see operators 
pushing the envelope with it  :)  Our policy has always been to keep 
track of customer modifications (as well as our own) for possible 
future inclusion in the kit. A recent example is the keying 
modification, which resulted in the K2 having among the cleanest keying 
envelopes of any commercial transceiver.

73,
Wayne
N6KR