Re: Si5351 audio pops and PLL Lockups


Hoo boy, the joys of programming up a synthesizer for your rig!

I've been doing a lot of programming on a Si5351a with I & Q outputs recently but haven't had it actually connected to a receiver much as yet.

However, doing similar things with a Si570 I have heard that noise while tuning. It can be pretty pronounced and distracting but I'm grateful it's gone when I stop turning the knob.

In the early days of the KX3, Elecraft was fielding a lot of questions about this. The noise was referred to as "zipper noise".  Theirs was again an Si570 and the noise wasn't loud but was noticeable. I think they mainly used careful routing of certain wires to mitigate it. This may have been the I2C signals, I don't remember for sure.

Since it might come from one or more of three areas, you might try software experiments to see which makes the noise, or makes the most noise.  

First, have your encoder signals active but your MCU not sending any Si5351a control signals or LCD updates.

Next, have your MCU continuously updating the Si5351a with no encoder input and no LCD updates.

Third, have the MCU continuously updating the LCD with no encoder or data to the Si5351a happening.

As far as the chip losing PLL synch ... I'm not sure I've seen that on my Si5351a project but I used to see it some with the Si570 and I concluded it was RF interference. If my output from the chip was unterminated I'd have the problem, but if it were properly terminated, it did not occur. I think this happened mostly at VHF though.

In my current I/Q Si5351a project, I'm adding the bells and whistles to make it into a full featured VFO with things like band selection and having a keyed line input to tell it when to do the offset shift, plus RIT. 

As far as shifting between TX and RX goes ... it works fine but I realized a bit late I could have used the 3rd clock to go to the TX while the other two provide I & Q for the RX and there would be no need for switching when going from RX to TX and back.

I'm not using a library for the Si5351a control, which makes it easier to examine the code. I do have libraries for LCD and Wire (I2C).


Nick, WA5BDU

On Tue, Mar 17, 2020 at 8:09 AM WB9YZU via Groups.Io <> wrote:
Hi Folks, I'm sure you folks have fielded this question before, but search results haven't been very fruitful.
My rig is a Forty-9er that I have modified to accept a VFO instead of a XTAL. I based the idea from a 2016 article in QST where the author used a I2C DDS and a parallel display.

I decided to take on the design and programming of the Arduino backbone myself and went with a Si5351a and put the display on the I2C bus via an adapter card that just piggybacks on the display. Nothing fancy. I'm using an inexpensive digital encoder to change frequencies.

The frequency is constantly being updated to the Si5351a, this is because keying requires that I change from a receive to a transmit frequency on the fly, and to avoid transmitting while the Si5351 is changing frequencies, I am keying the Forty-9er with the Arduino.

Everything seems to work pretty well, the radio does what it's supposed to do, and I have made a number of nice QSOs.
However the enjoyment is somewhat muted by this annoying popping and PLL dropout when dialing the rig.
When the PLL goes to lunch, I can't recover and I need to reset the radio. Though the symptom can appear most anywhere, it is much more prevalent around 7.055.

Steps I've taken do date:
Scoured the code and internet for clues. I am on my NthX2 attempt to write a decent code, the last attempt actually made it worse!
I verified that I am using the latest Etherkit Si5351a Library; version 2.1.4.
I tried to debounce the encoder by using an interrupt call and a debounce routine on the interrupt.; I also added .01uf caps across the encoder CLK and DT Lines to GND.

I read that the LCD is a noise source, so I installed 100uh coils on the power feed (+ & -)  to the display board. That cleaned up some audio noise.

From the number of Si5351a based QRP rigs, I assume a solution(s) has been found? What has worked for you folks?
, Ron WB9YZU

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