Help understanding TX stage

General discussion and support for the Peaberry SDR V2.

Help understanding TX stage

Postby mosheen » Mon Apr 20, 2015 1:31 pm

I'm troubleshooting my TX stage (zero output) right now and would like to understand it better.

-Q9: Is this a 1:1 buffer? B and C have the same output on my scope.
-Is Q9 the carrier buffer, or is it the I/Q buffer?

-Q4: Is this the buffer for signal modulation?
-Q10 and Q6: Preamp? Is this a push pull setup? How does Q5 play into this and how does the feedback through the transformer play into the amp stage?
-Q7 and 8: Final power amplification correct?

Sorry if this is a bunch of complicated questions, or if I'm way out in left field. I'm just trying to understand the radio better. I'm just going point to point until I see were the fault is. Thanks for looking.
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Re: Help understanding TX stage

Postby W4MMP » Mon Apr 20, 2015 2:38 pm

-Q9: Is this a 1:1 buffer? B and C have the same output on my scope.
-Is Q9 the carrier buffer, or is it the I/Q buffer?


Q9 is a preamp. You should see lots of gain from base to collector. The output of U8 is the carrier which is to be amplified. It is not I/Q data.

-Q4: Is this the buffer for signal modulation?
-Q10 and Q6: Preamp? Is this a push pull setup? How does Q5 play into this and how does the feedback through the transformer play into the amp stage?


All these components are for DC biasing of the final amplifier. They have nothing to do directly with the carrier. The TX input to Q4 is the signal that turns the biasing circuit on and off. It is not an analog signal. It is an ON/OFF 5V toggle.


-Q7 and 8: Final power amplification correct?

Yes, that is correct.

Let us know if you need more assistance.
73
Ron / W4MMP
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Re: Help understanding TX stage

Postby mosheen » Mon Apr 20, 2015 4:28 pm

Thanks. That helps a lot. I had it stuck in my head that everything was signal in that area.

So the output of Q9 going into T4 on leg 1 is the complete signal going out to the Amp / antenna. I assume everything below T4 from leg 2 around to C43 is biasing, and the TX switch?

I guess I should check Q4 as well seeing how I have 0.00v across 50 ohm dummy load. Seems like I would have something there even with a fault. I've already pulled T4 and rewound it. It is curious that Q9 has 1:1 output, but I need to get T4 back in.

I'll keep plowing through it one solder joint at a time.
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Re: Help understanding TX stage

Postby W4MMP » Mon Apr 20, 2015 7:35 pm

So the output of Q9 going into T4 on leg 1 is the complete signal going out to the Amp / antenna. I assume everything below T4 from leg 2 around to C43 is biasing, and the TX switch?


Yes, that pretty much sums it up. A couple notes: 12V (or supply voltage, I run my Peaberry at 13.8V which is the design spec) is only applied to Q9 when the Peaberry is in transmit mode. TX goes high which in turn turns on Q4 and Q10. If you don't have 12V on the collector of Q9 in transmit mode, there obviously is a problem. Restore T4 and check this area first. On the other hand 12V is always applied to the PA Mosfet finals. The when the bias circuit is off (No TX) the Mosfets are biased into the "off" condition.


Get the preamp working first then move on further into the PA if necessary.
73
Ron / W4MMP
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Re: Help understanding TX stage

Postby mosheen » Mon Apr 20, 2015 9:21 pm

RIP Q10. I don't have any voltage on the drain during TX and if I jump it the amp lights up. A huge thank you for the help!

EDIT: never mind. Rewetted everything in that area and BAM! I'm talking.
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Re: Help understanding TX stage

Postby W4MMP » Mon Apr 20, 2015 10:48 pm

Great that you have it working ! Good show :)
73
Ron / W4MMP
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Re: Help understanding TX stage

Postby mosheen » Fri Apr 24, 2015 5:54 pm

Just to follow up, I made a 5700 mile QSQ to Russia last night on JT65! I'm loving this little radio. I was using a 20 meter (poorly tuned) wire dipole strung inside my apartment. It runs through 3 rooms to hooks on the walls. It's crazy but it works! :D
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Re: Help understanding TX stage

Postby Sanath » Mon Apr 27, 2015 2:37 pm

One novice question on the same topic! should we expect a modulated carrier @ C44 ?

I completed the build for the TX driver and Mixer and am having issues testing it .

I hear (heard on an AM radio after i touched C44 with finger) a carrier at the Tune frequency, however did not hear my voice when i tried to modulate it (have a mic connected to my sound card). I do see my voice spectrum HDSDR scope though but it is not heard on the radio. The manual (p23) says expect a carrier on c44 which is seen in my build but i dont see a modulated carrier there.

I have not played with any HDSDR settings as of yet , anything i may be missing ?
Cheers
Sanath
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Re: Help understanding TX stage

Postby W4MMP » Mon Apr 27, 2015 2:56 pm

I hear (heard on an AM radio after i touched C44 with finger) a carrier at the Tune frequency, however did not hear my voice when i tried to modulate it (have a mic connected to my sound card). I do see my voice spectrum HDSDR scope though but it is not heard on the radio. The manual (p23) says expect a carrier on c44 which is seen in my build but i dont see a modulated carrier there.


Hi,
First, to answer your question, yes you should see a modulated carrier. Do you see any carrier at all?

But I need to ask a couple of questions before I can go any further.
1) Do you have transmit I/Q reversed in HDSDR? They should be.
2) Is the rig making power? Set the HDSDR mode to FM and click the TX button. The rig should output full power. IF you have volume sliders for the Peaberry Radio sound device, make sure they are set to full volume. Make sure the HDSDR Power slider is set to maximum.
73
Ron / W4MMP
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Re: Help understanding TX stage

Postby Sanath » Mon Apr 27, 2015 4:29 pm

Thanks Ron you nailed it.
The TX I/Q swap was not set. I could now hear myself Oh boy what a joy !.PS I am running winxp and dont have any volume sliders for peaberry Radio sound device.
Thanks and hope to meet you all in the air soon !
Cheers
Sanath
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