[Elecraft] Re: Elecraft Digest, Vol 37, Issue 19

[Geoffrey Mackenzie-Kennedy]

Hi Gian,

Your messages have arrived!!

Giancarlo Moda i7swx at yahoo.com wrote:
on Wednesday, May 09, 2007 2:48 PM

>> Geoff you are right. Certainly the H-Mode Mixer has
> higher IMD performances then TUF-1 and NE602.
> Some friends in Italy had problems with K2 (and also
> K1) on 40meters during contests using good antennas.
> This is how the idea of trying to improve the K2
> started.

I experience the same problem here with the K2 on 40m all the time when the 
BC stations above 7100 kHz are being received at S9 ++, and have to use 
another receiver which uses H-Mode mixers etc.

> Certainly the worst performance is in the post mixer
> amplifier and this is the reason why the idea of the
> simple 2 xtal roofing filter was implemented. THe
> following bandpass xtal filter is protecting the IF
> and demodulator stages.

Understood. Without running IMD tests on my K2/100's receiver and its IF 
cascade I can only guess that this might not be enough protection when the 
BC station carrier levels are up in the -10dbm region, and you are trying to 
copy a signal that is in between them. This is the situation that I have 
when using SSB between 7150 and 7197 kHz. The situation improves of course 
if the beam is pointing away from the BC stations. There is also the 
question of roofing filter linearity and Rx phase noise. There is not much 
that we can do about BC station Tx phase noise!

>> Although you may have not "enough improvements"
> because the stahge is limited by the phase noise ... I
> believe it is better than having both the mixer and
> the PLL limitations.

Understood!

>> NO. The H-Mode Squarer has a balance adjustment and
> you do not need to put your hands in the PLL. Also the
> HMM require 0dBm drive or less, so youmay have less
> spurs. The BFO 50-50 balance is not necessary as the
> LO frequency is low and the squarer gives quite a good
> 50/5o outputs.

I understand, but I was looking at the Squarer as part of the circuitry to 
be added to the K2's LO (VCO) *output* which provides injection to the TUF-1 
mixer.

--------------------------------------------
>From your second message:

> The H-Mode Mixer is not sensitive to impedances
> terminations like a classic diodes d b mixer,
> particularly to the LO. Even the IF output when loaded
> with a variable load like a xtal filter does not
> reduce too much the IP3 (tests give between 5 to 10dBm
> changes.. i.e from +40dBm to +35 or +30dBm as second
> of type of filter.)

Agree about terminations, but there is a limit!! The SD5000 FET array (now 
obsolete) with three transformers yields an Input IP3 of +50dbm or better at 
the worst case input levels below compression, but my test equipment is 
becoming stretched. Perhaps the newer arrays will provide the same 
performance.

> Certainly the filter is a good filter but if IMD is
> reduced in the TUF-1 or more in the 2N5109 post mixer
> amplifier, the IF filter cannot do much.

Have you measured the filter's input IP3 with the varicaps' back bias at the 
minimum value used?

Good luck and many thanks.

73,
Geoff
GM4ESD