Re: Amplifier chain design - best practices
Howard,toggle quoted messageShow quoted text
Yes, there are problems up front around the first and second IF's that create harmonics and spurs.
Yes, replacing the 30mhz LPF with band specific filters could help with both spurs and
harmonics during transmit, though does complicate what is trying to be a simple rig.
Here's my take on a couple of your points.
Unfortunately, I have not found time to do any experimenting on this stuff myself.
> To date most everyone has blamed the poor output filters. We could also fix this problem
> by insuring we always drive the transmitter section with a sine wave to begin with.
The output LPF's are fine, it's just board layout and how the relays got wired up
that allows way too much blow-by. When the board is hacked to clean this up,
transmissions are fine with respect to harmonics. The power amp will generate
some harmonics (and IMD) on its own, especially when pushing for more power on the
upper bands. Many CW rigs operate class C (so a square wave at the final),
simple output LPF's work fine even then.
> This clock comes from the infamous SI5351 chip that is also causing the spurs.
The primary cause of spurs seems to be in the diode ring mixers,
too much signal and not enough local oscillator.
The fix is less IF gain and higher local oscillator levels into the mixers.
The Si5351 is having a hard time dealing with the 50 ohm 6dB pads it's trying to drive,
but otherwise doing pretty good. Crosstalk due to the heavy loading is causing
some carrier leakage and perhaps a few extra spurs.
Would be interesting to know if anyone's tried this: https://groups.io/g/BITX20/message/35206
discussed further here: https://groups.io/g/BITX20/message/59220
Unfortunately, I have not found the time (or the logic analyzer) to evaluate it.
Do dig in if you can and let us know if you find some new tricks to help clean the rig up.
On Fri, Sep 21, 2018 at 07:35 PM, Howard Fidel wrote:
I've been sitting here on the sidelines for a while. I was hoping to have a complete solution finished by now. However, life gets in the way. I unexpectedly got 2 consulting jobs, plus my teaching job has eaten up most of my free time. However, I think it is worth putting my 2 cents in now.