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...v50MHz_sch.pdf

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

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...v50MHz_sch.pdf

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
If a TTL "Can" oscillator would work, then you could use a TTL or CMOS
quad gate wired up as an oscillator with a 42 MHZ 'rock' and use that.
There are schematics on the web for building the equivalent of a can
oscillator using a 74x00 quad gate, some resistors, caps, and a crystal.
If you need a buffer amp between the oscillator and the mixer, here is
an idea: http://www.kitsandparts.com/rfamp1.1.php

Hi Kenneth,

Kenneth Scharf wrote:

If a TTL "Can" oscillator would work, then you could use a TTL or CMOS
quad gate wired up as an oscillator with a 42 MHZ 'rock' and use that.
There are schematics on the web for building the equivalent of a can
oscillator using a 74x00 quad gate, some resistors, caps, and a crystal.

I think that 74xx00 oscillators are difficult to make with 3rd overtone
xtals (like the 42 MHz one available), I might be wrong and probably
I'll have a try.

If you need a buffer amp between the oscillator and the mixer, here is
an idea: http://www.kitsandparts.com/rfamp1.1.php


if I had space, I'd put a similar buffer after a well proven good xtal
oscillator. I have many successful designs handy, but not much space
inside this transverter :/

Best regards
Frank IZ8DWF

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

Hi Tom,


K7ITM wrote:

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 believe they made the schematic wrong!! I checked the actual circuit
and seems just right :-)
After all it was working on the 6m band.
Though they did really many mistakes that I tried to correct.
Q6 is a transistor with Ft=150 MHz, just for a little example, but many
mistakes can be spotted directly on the schematic.


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.


I thougth so also, but this beast has only one mixer for RX and TX and
RF/IF ports are exchanged from RX to TX. This migth be a problem or not,
but if it worked at 50 MHz with 1N4148s it will work on 70 MHz with
1N5711 (maybe even reasonably well with 1N4148, but diodes are cheap
anyway).

Of course my own transverters are much better, I started from well known
and working schematics. I've been asked to modify this one just because
the owner isn't going to get active on the new band if he has to spend
much more than a bunch of parts. Having as many new station QRV as
possible is good, the nearest 4m station to my QTH currently is at a bit
less than 500 Km and that's after two years that we have had temporary
permits.

Thanks for suggestions, I probably will try with a simple BJT
oscillator, I was just a bit worried by the unknown collector load, I
don't have any info on T4. I would need to remove and test separately
with a signal generator.

best regards
Frank IZ8DWF

In message
,
K7ITM writes



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.


The mixer is indeed wrong. Only one diode is 'correct'.
To correct the circuit, you could take D8 as the 'correct' diode.
D6 - reverse.
D5 - connect anode to D6 cathode.
D7 - connect cathode to D6 anode.
--
Ian