RS DX-394B: why 2nd LO buffer added?
 

Apart from the new pcb, the main electrical difference between the Radio
Shack DX-394B and its immediate predecessor, the DX-394A, is the addition of
an emitter follower buffer amp between the output of the 2nd local
Oscillator (the emitter of the one transistor 44.5MHz VCXO) and the 2nd
mixer. The latter is a 2SK210-Y/YY VHF N-channel JFET; 1st IF is injected at
the gate and the 2nd LO at the drain.

Why would the designers add the buffer? What performance comparison should I
make between the A and the B model to assess whether it would be worthwhile
to retrofit a similar buffer to the A model? I can say that I have not
noticed any dramatic difference with my usual listening habits.

Tom

--
Carl
WA1KPD
Visit My Boatanchor Collection at http://pages.cthome.net/nord/wa1kpd.html
"Tom Holden" wrote in message
.. .
Apart from the new pcb, the main electrical difference between the Radio
Shack DX-394B and its immediate predecessor, the DX-394A, is the addition
of
an emitter follower buffer amp between the output of the 2nd local
Oscillator (the emitter of the one transistor 44.5MHz VCXO) and the 2nd
mixer. The latter is a 2SK210-Y/YY VHF N-channel JFET; 1st IF is injected
at
the gate and the 2nd LO at the drain.

Why would the designers add the buffer? What performance comparison should
I
make between the A and the B model to assess whether it would be
worthwhile
to retrofit a similar buffer to the A model? I can say that I have not
noticed any dramatic difference with my usual listening habits.

Tom

Now that was informative.

"Carl WA1KPD" wrote in message
...


--
Carl

"Tom Holden" wrote in message ...
Apart from the new pcb, the main electrical difference between the Radio
Shack DX-394B and its immediate predecessor, the DX-394A, is the addition of
an emitter follower buffer amp between the output of the 2nd local
Oscillator (the emitter of the one transistor 44.5MHz VCXO) and the 2nd
mixer. The latter is a 2SK210-Y/YY VHF N-channel JFET; 1st IF is injected at
the gate and the 2nd LO at the drain.

Why would the designers add the buffer? What performance comparison should I
make between the A and the B model to assess whether it would be worthwhile
to retrofit a similar buffer to the A model? I can say that I have not
noticed any dramatic difference with my usual listening habits.

They obviously wanted to provide some isolation and/or impedance
matching between the oscillator and the mixer. It's a good engineering
practice, but may not have much practical effect. If you don't notice
a difference, then don't bother retrofitting.

Art Harris N2AH

Art Harris wrote:
"Tom Holden" wrote in
message
[snip]
Why would the designers add the buffer? What performance
comparison should I make between the A and the B model
to assess whether it would be worthwhile to retrofit a
similar buffer to the A model? I can say that I have not
noticed any dramatic difference with my usual listening
habits.

They obviously wanted to provide some isolation and/or
impedance
matching between the oscillator and the mixer. It's a
good engineering
practice, but may not have much practical effect. If you
don't notice
a difference, then don't bother retrofitting.

The oscillator is an emitter follower, too, so I doubt that impedance
matching is the objective. What would the increased isolation achieve? Would
it preclude mixing of 1st IF energy in the oscillator? Should this be of
some benefit for intermodulation products?

Tom