Chirp on PTT down

#1764

Hi again

My homebrew (scratch built) G6LBQ BiTx txcvr is assembled and in debug stage. Overall, I'm pleased with the results, the Rx has lots of gain and the AGC works well.? I routinely enjoy listening to US amateurs on 7.150 to 7.170 in my evenings here (VK3), despite the quite poor band conditions being experienced around the world right now.??

My build is:

- Arduino Nano/LCD/si5351 with a single broadbanded 2n2222 buffer/amp for both VFO and BFO
- TDA2003 replaces LM386
- IF is 9MHz
- 5 (not 6) crystals in the filter, capacitors are 68p and 82p, couplings are the original 100p, the filter is a little bassy so I will experiment with some smaller values to move the passband a bit higher
- PA uses one RD16HHF1 (not two in push pull) in a standard QRP 5W PA circuit
- For T/R switching, one digital input on the Nano senses the PTT switch going low, then raises another digital output, which pulls in the two relays on the PA board; one relay has a set of contacts that switch 12v between the low level stages of the Rx and Tx.??
??
Otherwise it is exactly as per the MkII circuit diagram.? The build is on a hand drawn PCB that is mostly ground plane, and with surface mount parts.? The Exciter board is bolted tightly to an aluminium case lid. The MC1350 took a few tries to get aligned, it was distorting until I got the tuned circuit on its output pins close to resonant, but now the receiver sounds clean and shows no signs of instability.??

Back on the AGC, mine is not quite adjusted right yet, the no-signal voltage is too low, so when you turn on the AGC the receiver noise level drops too far. The AGC action is good on strong signals.??

The curious thing is that when I press PTT there is a sharp chirp of audio generated.? It is as loud as the subsequent spoken voice, starts off about 1khz and drops quickly, lasting about 100-200mS.? It is prominent enough that it will draw comments from other stations in a QSO.? It? suggests an electrolytic capacitor charging/discharging.? I removed the 50uF supply filter cap on the mic amp -- no difference.? I reduced the 10u in series with the mic amp hot input to 2u, no change.?
?
If I manually switch the rig to Tx it still happens, so it might have something to do with power switching between Rx and Tx.?
?
Just wondering if anyone has seen this.?

The Tx is working too, but becomes unstable with the PA bias above about 400mA, so it needs better shielding, I'll spend some quality workshop time with hacksaw file and PCB stock to create some compartments inside the box.? ?

73 Paul VK3HN.? ?

#1765

Hi Paul

Nice job on the HB G6LBQ rig.

Over on the Bitx forum, the same problem exists in the change over between TX/RX.

There is solution. K5BCQ has available a kit to stop the change over problem. Have a look on the forum.

If you're a bitx member, look in the files section under K5BCQ.

73

Ken VA3ABN

Show quoted text

On Wed, Jun 6, 2018 at 7:16 PM, Paul VK3HN <prt459@...> wrote:

Hi again

My homebrew (scratch built) G6LBQ BiTx txcvr is assembled and in debug stage. Overall, I'm pleased with the results, the Rx has lots of gain and the AGC works well.? I routinely enjoy listening to US amateurs on 7.150 to 7.170 in my evenings here (VK3), despite the quite poor band conditions being experienced around the world right now.??

My build is:

- Arduino Nano/LCD/si5351 with a single broadbanded 2n2222 buffer/amp for both VFO and BFO
- TDA2003 replaces LM386
- IF is 9MHz
- 5 (not 6) crystals in the filter, capacitors are 68p and 82p, couplings are the original 100p, the filter is a little bassy so I will experiment with some smaller values to move the passband a bit higher
- PA uses one RD16HHF1 (not two in push pull) in a standard QRP 5W PA circuit
- For T/R switching, one digital input on the Nano senses the PTT switch going low, then raises another digital output, which pulls in the two relays on the PA board; one relay has a set of contacts that switch 12v between the low level stages of the Rx and Tx.??
??
Otherwise it is exactly as per the MkII circuit diagram.? The build is on a hand drawn PCB that is mostly ground plane, and with surface mount parts.? The Exciter board is bolted tightly to an aluminium case lid. The MC1350 took a few tries to get aligned, it was distorting until I got the tuned circuit on its output pins close to resonant, but now the receiver sounds clean and shows no signs of instability.??

Back on the AGC, mine is not quite adjusted right yet, the no-signal voltage is too low, so when you turn on the AGC the receiver noise level drops too far. The AGC action is good on strong signals.??

The curious thing is that when I press PTT there is a sharp chirp of audio generated.? It is as loud as the subsequent spoken voice, starts off about 1khz and drops quickly, lasting about 100-200mS.? It is prominent enough that it will draw comments from other stations in a QSO.? It? suggests an electrolytic capacitor charging/discharging.? I removed the 50uF supply filter cap on the mic amp -- no difference.? I reduced the 10u in series with the mic amp hot input to 2u, no change.?
?
If I manually switch the rig to Tx it still happens, so it might have something to do with power switching between Rx and Tx.?
?
Just wondering if anyone has seen this.?

The Tx is working too, but becomes unstable with the PA bias above about 400mA, so it needs better shielding, I'll spend some quality workshop time with hacksaw file and PCB stock to create some compartments inside the box.? ?

73 Paul VK3HN.? ?

#1766

Thanks for replying Ken. I'm on [email protected] and have seen the series FET switch and kit. I'll look into it.? I have a hunch that it might be the big electrolytics in the TDA2003 audio amp, I will test out this theory first.??

When this nice little rig is working sufficiently well I will write up a blog post and post a link here. The G6LBQ design is a fine variant on the Bitx pattern that makes it just right for my interests and purposes.?

73 de VK3HN.??

#1767

Hi Paul

The kit that K5BCQ sells is actually a design by ND6T. In the files section under K5BCQ is the schematic. Duplicate it.

Many have tried to solve this problem, but it seems that the one by ND6T works 100%. Have a look.

73

Ken VA3ABN

Show quoted text

On Thu, Jun 7, 2018, 5:22 AM Paul VK3HN <prt459@...> wrote:

Thanks for replying Ken. I'm on [email protected] and have seen the series FET switch and kit. I'll look into it.? I have a hunch that it might be the big electrolytics in the TDA2003 audio amp, I will test out this theory first.??

When this nice little rig is working sufficiently well I will write up a blog post and post a link here. The G6LBQ design is a fine variant on the Bitx pattern that makes it just right for my interests and purposes.?

73 de VK3HN.??

#1769

Hi Paul

Great to hear that you have completed a scratch build of my MKll BitX multi-band transceiver.

I have not seen or experienced the chirp you mentioned when going from RX to TX so I suspect the problem is associated with the TDA2003 amplifier stage.

As standard the MKll multi-band exciter has an LM380-8 as the final audio output stage and I chose this as my experiences with the LM386 have not been great!

Both the original BitX and my multi-band effort do suffer with speaker "thud" when going from RX to TX, in my circuit Q11 in the audio stage does help reduce the thud to an acceptable level but there is still room for improvement and one day I may look at this again.

When working correctly with everything aligned the receive performance is excellent considering its simplicity. The AGC makes a huge difference over the standard BitX without an AGC and the transmit audio is greatly improved with the op-amp and its associated filtering.

Keep us posted on your progress and it is great to hear that you are enjoying the project.

Regards & 73

Andy de G6LBQ

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Chirp on PTT down