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Hello group,

After looking at the very nice schema of the uBITX () i have a question about the K1 relay.
The K1 relay is used for the RX and TX power switching and also for connecting the antenna in RX to the LP filter.

On RX (and TX) the LP filter is also a connection to the TX power amp by C80, that will give a (very) small load on the antenna signal at RX and a mixer (Q90 diode BE) for strong received signals?
It can be changed by by switching the pins 12 and 14 of K1 and connection pin 16 of K1 to C80 (remove that one from the LP filter)?

I do not own a board (yet), so i cannot test it.
Maybe there are very well reasons for using it as it is, I did not read the total history.

73, Fred
pe0fko

Good idea to make use of K1-16 to disconnect Q90 during receive.
This would also protect Q90 from local QRO transmissions into a local antenna,
The similar Q13 on the Bitx40v3 is often blown in such situations, even when the rig is powered down.

Jerry, KE7ER

Yes.
Jerry

Yes perfect, that is the same idee. Nice picture also.

NIce idea, How to impliment it?
Easy to cut PCB tracks and use some jumpers?
I think pin 16 to GND may be difficult to get at...
Maybe remove the relay and extend the legs, give some space under for the new jumpers?
73, Nick

Maybe-- By leaving that trace connected to the RX during TX, it might
pick up enough RF to make annoying sounds. The grounding of the line
(and by implication the whole of the RX section may be a good idea.

Try it and see what happens...

john
AD5YE

Hi John, there is no RX signal path on the relay when in TX mode.?
I can't test it, I also search for the PCB layout but was not able to find it. Is there a PCB (schematic) layout in eagle available?

73, Fred,
pe0fko.

I use the schematic on the hfsignals website. As far as
I know, there is no pcb layout picture available.

That's true; there is no RX path in TX mode now, because
pin 16 is grounded. However, the mod would take away that
ground and allow the trace to float. I am not so sure that is a
good idea...but as I said, try it and see what happens. Hi.

Another thing to consider is what effect the Raduino signals would
have with that pin ungrounded....

john
AD5YE

Hello again.

I attach two images with the signal paths in the RX and TX modes.

On paper I do not see any problem, but you have to prove it.

How to do it is something for everyone.

We accept all kinds of ideas.

RX Mode



TX Mode

The only thing I see is that in TX mode the output signal can be coupled, over the two open relay contacts, to the input of the amplifier (it is less then 1pf I think) and there is some gain, resonance?
It will be nice to pull the wire between K1 and K3 to ground, maybe the second contact of K3 can do that? I don't know what the M1 and M2 connections are, in top right the M1 and M2 are grounded.

73, Fred,
pe0fko

I agree Fred, it adds a feedback path around the TX amplifier which has quite a high gain. There is a potential for parasitic oscillation at some frequency and that will be affected by the load. It might be a good thing that the IRF510 does not give a high gain at high frequencies. It needs to be carefully checked out with measurements. Add-on linear amplifiers avoid the problem by only having 10 to 20dB gain.
73 Brian VK4BAP

Hi All,?
Dredging up this old one as I'm starting to plan out my scratch build uBitx.
Did anyone try this and confirm the results?
73 Nick VK4PP

Looking at another QRR radio, they use separate relays for RX/TX signal and PWR(TX/RX).
Then during TX, they mute the RX section with a transistor to GND on the input side of the RX chain(relay at ANT).
Audio AMP and SSB Filter only turns on at RX too.

Thoughts?

Alf Baranda,

That is desirable.? The problems is the wire from the K3 relay.? That wire runs the full width of
the ubitx board and picks up rf. but if it were coax it would likely not.


Allison

Brian, Fred, all,

THe use of K1 and 3 as in the schematic may work but the physical layout also interacts.
They are on opposing edes of the board and that interconnect for RX runs the width of
the board. It does pick up RF!

FYI the isolation of the contacts from set to set about 50db and from open contact to
wiper/other contact about 20-25db.? Contrast to the gain of the amp at about 60db.

Its a unsatisfactory arrangement and during amplifier improvement leads to oscillation.

Allison

simple mini coax run in place of the traces will take care of all of this if what u are saying is true. posts some pics on this oscillation.
scope pics,freq counter readings spetrum analyzer display.just cut the fricking board traces and run shielded mini coax.put on external filters,and 45 mhz filter fix and wala DONE.no more months speculation. if you have looked at other well made transceivers ,to prevent the oscillations
engineers run shielded mini coax to eliminate this problem.

or you can just remove the relays put them on a board and interconnect with shielded mini coax and you dont have to mess with the coils.just like our older rigs that are still on the air, even in tech savy companies that pride themselves in clean signals. sometimes a well placed capacitor works? too but not all the time.

i always run external filters to make sure my signal is clean. any cross the board runs ,if thats whats causing this needs to be cut, isolated from source to destination and have mini coax connected and grounded properly.do that plus ashars 45 mhz filter fix and you have a pretty cleaned up rig. $2. 00 for filter thats what i paid off ebay and about 2 to 5 dollars worth of mini coax,.done.

On Wed, Sep 19, 2018 at 07:10 PM, david todd wrote:
DONE.no more months speculation. if you have looked at other well made transceivers ,to prevent the oscillations
engineers run shielded mini coax to eliminate this problem.

Short answer to that.? I'm not HFsignals so its up to them to fix.?

Commercially I did all that and more. I know better as part of that is not getting
into that situation. However I spent a month trying to wake up the crowd and
when I did it became educating each new pop up.??

As to wala done... you must have missed the guys that will only do it someday if someone makes
a kit with a board or comments "I'm not carving up my board, I'll use it as is.".

I be tired.

Allison

On Wed, Sep 19, 2018 at 07:31 PM, david todd wrote:
i always run external filters to make sure my signal is clean

I don't.? All my HB gear and many kits I have are good no extra crap outside the box.
Of have hit the junk box as seriously broke.

So far the 45mhz fix is more wishful thinking.

Allison