On Dec 16, 8:46*am, frank wrote:
Hi all,

I'm converting an old 6m transverter for a friend to work on the 4m
band. You can see the schematic at the following URL:

http://www.radioamateur.eu/schemi/AC...ronicSystem_Tr...

This schematic shows in my opinion a general poor project, but I think I
can make it behave in a satisfactory manner.

As you can see from the schematic, there're no component values, so I
had to "reverse engineer" almost all parts. Fortunately I have some test
equipment so I could redo filters and such pretty easily.

My biggest concern so far is the LO oscillator.
First of all, I found a 22 MHz TTL "can oscillator" in place of the
single BJT oscillator. Even if this oscillator appeared to do its job,
I can't play the same "game" because 42 MHz oscillators are very hard
to find as it's not a standard value.
No, I can't change the IF frequency (28 MHz) as the *owner has a
dedicated equipment for this band.

Now, would a single BJT oscillator put enough LO power for the single
diode mixer? There's hardly enough room to fit a second tuned stage,
otherwise I'd have already done this.
The current diode mixer is made with two "ferrite bead" hand wound
transformers and 4 x 1N4148 diodes. I'd substitute them with 1N5711, but
I'm not sure it would help at this stage.
Does anyone have experience with a similar design?
Any hint is really welcome!
I wouldn't want to experiment too much and risk to damage the PCB (a few
substitutions are ok, but if the maker used a pre-made oscillator maybe
the design in the schematic was not very usefull).

Best regards
Frank IZ8DWF

Hi Frank...

Wow, the mixer in that circuit looks really weird. No, make it: that
mixer looks WRONG. I would not expect single diodes to be directly
across transformer windings like that.

I'd expect you can get plenty of power out of a single transistor (3rd-
overtone) oscillator to drive a double-balanced mixer with Schottky
diodes, certainly. +7dBm is normal for that. That's only half a volt
into 50 ohms, or 1V rms into 200 ohms. I'd probably pull a
Minicircuits mixer out of my junque box, and not try to build one from
diodes and transformers; a Minicircuits ADE-1 (or any of several
others) in a package less than 6x8mm should do the job fine. Also
small: active mixers. I've been playing with a Linear Technology
LT5560, which works fine with 0dBm LO power.

Cheers,
Tom