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Re: Photo Notifications
#photo-notice
Paul, ? Thanks for the feedback ¨C and to Dale, W4OP for posting his Si5351A spectrum result.? ? I¡¯ll have another look at the PLL VFO¡¯s output.? My memory may be mixed up with the result of a companion DDS VFO I built that uses an AD9850 which is known for its close-in, low-level spurs.? ?That VFO is shown below.? It¡¯s a more complex design since it uses two microcontrollers, one of which is responsible only for T/R sequencing.? Because a PIC microcontroller manages several VFO functions, it¡¯s output results in 1-3 ms of random timing jitter.? A tiny Adafruit microcontroller solves the timing problem.? An on-board RF amp boosts output to +20 dBm with a separate adjustable Hi-Z output of 30Vp-p.? ? ? ? Paul, W9AC? |
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Re: Photo Notifications
#photo-notice
Paul,
Thank you.? Very interesting.? I don't plan to ever get a 2NT but others will probably find that info really interesting. Someone could go to ebay and by an Si5351 board for $5 and an Arduino for $4, display for $5, encoder for $2, and have a very stable nice VFO for cheap. 73, Gary WB6OGD |
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Re: Another TR7 Journey begins
The PIN attenuator has no connection to the meter circuit. The forward wattmeter voltage is also fed to a Norton op-amp on the ALC board though diode CR1602 in parallel with a .01 capacitor. This is where the summing happens. There are no negative voltages there. Yes, the PIN attenuator is on the 2nd IF/Audio board but it is in the IF signal path.
Attached is a simplified meter circuit diagram. The meter is connected to the wattmeter sensor and an AGC buffer and referenced to ground via the Meter Zero pot. This is why the +10V adjustment and S-Meter/ACG alignment is done before the wattmeter calibration. Also worth noting is that there were some production changes done to very early IF/Audio boards. See: In particular check that your IF/Audio board has the R1112 and R1127 mods if it looks like a very early board. Later boards have these corrections in the board layout. I don't know if this could cause your problem. I have only seen one board that did not have these mods and it was years ago. As I think I already mentioned at some point, you can entirely eliminate the ALC board from the equation if you disconnect the FWD ALC feed from the wattmeter sensor. 73 -Jim NU0C On Tue, 16 Apr 2024 19:26:12 -0700 "Bill NZ0T via groups.io" <nz0tham@...> wrote: The meter works fine on RX and the S- meter adjustment worked perfectly. The problem is in TX where the meter reflects down below zero. In the circuit description in the SM on page 1-2 that there is a variable attenuator consisting of a PIN diode that is controlled by the ALC circuitry. ?The outputs of the forward and reflected wattmeter and the carrier control are summed to create a controlling signal for the attenuator. ?Is the PIN diode and attenuator located on the 2nd IF/audio board? ?I¡¯m too tired to be looking at schematics tonight lol. ?I¡¯ll look in the morning. ?I do think if I can get the ALC working properly the rig should be OK. -- 73 -Jim NU0C |
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Re: Another TR7 Journey begins
I don't think your problem is with the ALC board. You need to find out where the voltage that is causing the meter to deflect downward is coming from. The meter is driven by the wattmeter sensor in transmit and the ACG system of the 2nd IF/Audio board on RX. There is something crossed up on the 2nd IF/Audio board or the associated wiring.
73 -Jim NU0C On Tue, 16 Apr 2024 12:48:29 -0700 "Bill NZ0T via groups.io" <nz0tham@...> wrote: I've been busy with yard projects and a new deck railing but got back to the TR7 today.? I replaced the AA119 diodes CR2001 and CR2002 with new 1N60P which made no difference.? The? meter still deflects below zero and the two adjustments make no difference. The old diodes tested OK in circuit but it seemed worth a try.? So I guess I'll turn my attention back to the ALC board now. |
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Re: Photo Notifications
#photo-notice
Paul L.,
We are still curious about the 2NT VFO drive level.? From the manual it appears the Si5351 by itself will not be enough above 40meters. 73, Gary WB6OGD |
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Re: Photo Notifications
#photo-notice
Paul,
Too bad he won't give you the source code. Rather than a kit, it is pretty easy to build an Si5351 VFO with an Arduino and available source code. 73, Gary WB6OGD |
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Re: Photo Notifications
#photo-notice
Paul L, |
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Re: Photo Notifications
#photo-notice
Paul,
That looks like a really nice VFO! I am curious about amplitude too.? The Si5351 puts out 3.3vpp square wave.? Normally people say you need higher amplitude for a tube rig. DDS means Direct Digital Synthesis.? The signal is generated by building a sine wave via a DAC (Digital to Analog Converter).? Small aberrations to the sine wave create the close in spurs.? The better the chip, the less the spurs.? The low end AD9850/51 are pretty bad if used in a receiver (I tried) but are probably ok for a transmitter. The Si chips use a GHz PLL divided down.? They don't have close in spurs but do have phase noise but it not bad. 73, Gary WB6OGD |
Dick
Paul Christensen
Gary WB6OGD
Jim Shorney
