Allison,
> Some of you speculation is about how the si5351 works and much is not given away.??
Yup, the si5351 is poorly documented.
It takes a little bit of reading between the lines and experimentation to figure out what it really does.
On the other hand, it is a $1 part meant mostly to replace a few fixed frequency crystal oscillators.
Using their ClockBuilderPro software to cook up the magic numbers, doing that is well documented.
Building an EER type radio with it is not well documented.
> All you need to test the signal generation is a raduino.?
At which point you get a somewhat intelligible SSB signal with a lot of artifacts.
Those artifacts due to lack of modulation will hide any artifacts due to i2c write delays.?
So hardly seems a conclusive test of i2c writes to the si5351.
> Then you have the overriding thing that moving 8bits via SPI even at 800khz takes about 11us.
Indeed.? Hence my thoughts of using some other method than i2c writes to vary the frequency.
> Putting a varactor on the crystal would create FM, you don't want that.?
The si5351 output clock will vary in frequency, just as it does using your i2c writes.?
I want that.? Even if you choose to call it FM.
> You would have to process the mic input to?a D/A output and then your guessing on frequency as its in the analog realm.?
It the response in frequency to voltage on the varactor diode is non-linear.?
If I were to pursue this, it would be using a lookup table mapping control voltage to the measured deviation.
(Or piece=wise linear interpolation.)
> then the output of the 5351 will multiply that (crystal to VCO then divide to output frequency).?
I'm well aware that if I will get 100x the deviation at 700mhz on the VCO (expressed in Hz), as I will get at 7mhz.
> On a good day that would be band limited.
Yes, I did acknowledge that deviation won't be sufficient using the varactor diode.
> The use of a mixer, really?
> Mix the 5351 and I presume audio, then a crystal filter as you have both sidebands.
You totally missed the point.
With +/- 240ppm deviation using the Si5351b VCXO pin, we don't get enough deviation for direct synthesis on 80m.
(However it should be sufficient for synthesis on 40m and above.)
So for 80m, we would need to create the desired square wave out of the si5351b at 7mhz or higher,?
then translate that square wave down to 80m, apply it to the class C,D,E amp, and modulate the amp.
If the Si5351b were not available so cheaply, I'd consider using the varactor diode to create a square wave
at VHF (where I can get a large deviation, expressed in Hz), then down convert that square wave to HF.
But much easier to just use the VCXO pin on the si5351b.
Given the lack of previous success with this, my impression is that i2c writes and PWM modulation
is not going to get us to a clean signal.?
Better to throw $10 worth of parts at this (16 bit DAC's, Si5351b, buck mode switcher),
and work on algorithms without dealing with the serious handicap of marginal hardware.
Best to start out at 0dBm or so and a nice linear modulator (as you are doing) till it works properly.
Then go to a buck mode switcher for the modulator, it should probably be switching at 1mhz or more
to minimize response time.? Hacking the ebay cheapies to make them programmable might not cut it.
Jerry, KE7ER
