[Elecraft] An idea for the K2

[Stewart Baker]

Probably as you say that a tin box around the whole assembly would be an 
overkill. I was thinking that the crystals/caps in the filter could be spaced 
out more, and the whole area ground-planed top and bottom. I agree that coupling 
to the main board maybe a route for blow-by, and if space could be found a 
screen fitted to the underside of the board may bring some improvement. It 
should be possible to estimate the degree of leakage caused by P2/P3, and the 
signal diodes. Maybe PIN diodes would give higher isolation.
With the excellent phase noise performance of the K2 it is a shame that some of 
this benefit may be being thrown away in the SSB filter. I am given to 
understand by people who know a lot more than me about ladder filters that there 
is more to be had.

73
Stewart G3RXQ

 On Wed, 2 Mar 2005 18:54:54 -0000, Geoffrey Mackenzie-Kennedy wrote:
> I believe that you would be proven correct about the RF feedback.
>
> My thought about a tin box totally enclosing the PCB stems from an
> observation that the skirts of the varicap "controlled" filter set for say
> 700Hz bandwidth are a little wider with the KSB2 installed than when it it
> is not, and noticeable from approx 40db below passband on down. I made no
> measurements because I was Birdy hunting/killing at the time. If this
> blow-by is due to leakage between P2 and P3 or through the switching diodes
> (off) on the KSB2, then the tin box would have little if any effect. But the
> KSB2 does come close to forming a bypass route just due to its proximity to
> the post mixer amp, RF board filter etc, and my gut feeling is that a box
> around the KSB2 or its replacement would be a good start. The box would also
> help to keep out the RF from nearby oscillators.
>
> 73,
> Geoff    GM4ESD
>
> On Wed.2nd March 2005, Stewart G3RXQ wrote:
>
> Any form of increasing the mic gain internally (eg rf speech clipper) needs
> to
> be carefully thought out. The microphone signal routing and earthing from
> the
> front panel socket thru the RF board and into the KSB2 is far from ideal.
> Common mode, series mode - the lot - RF feedback is the likely result.
>
> A groundplane board with the crystal filter laid out to reduce blow-by may
> be
> the way to go. A tin can over the whole assembly may help. The biggest
> hurdle to
> overcome in making changes to the KSB2 is interfacing with the existing
> firmware
> which is far from straightforward.
>
> 73
> Stewart G3RXQ
>
> On Wed, 2 Mar 2005 08:11:54 -0000, Geoffrey Mackenzie-Kennedy wrote:
>> Intersting ideas. If you did enlarge the SSB board to get all the bits in,
>>
>> in which directions would you enlarge the exixting board?
>> IMHO it would be essential to contain the "new" board in a 'tin' box in
> turn
>> mounted on metal, which raises the question of access to test points etc
> on
>> the RF board, but a solution might be available.