Re: Harmonic performance - SSB vs CW

Timothy Fidler

Allison the bad guys from the FCC could be reading this (they look like Jake and Elwood in the Blues B ) but they drive around in vans not ex Mt Prospect police cars.. they could shut down the imports if you go telling on poor old Ashar...ON the other hand they could look at those dirty RM amps first too...

Timothy E. Fidler : Engineer BE Mech(1) Auckland , NDT specialist AINDT UT /RT3 , MT2 
Telephone Whangarei   022  691 8405
e: Engstr@...

----- Original Message -----


Sat, 04 Aug 2018 16:44:35 -0700
Re: [BITX20] Harmonic performance - SSB vs CW

I've been hinting at this for a while but unwilling to say it as i'd had too small a sample.
With a sample size of over 4:

Over 20mhz fail for SSB spurs,  CW ok
Below 20mhz if not over driven SSB ok, CW is fail for 80 and questionable for 40.

Other observed bad behavior:  
Excess carrier leakage, this is not leakage around the filter or modulator deficiency.
It is signal routing and ground plane issue plus interconnect between the Raduino
and the main board.  The only thing tried that worked was disable OSC-0 (BFO)
and remove the balanced mod. I used a separate crystal oscillator and modulator
in a mint tin (just like old bitx) and Coax deliver RF to the filter input and audio to
the RX/TX amps, carrier dropped from -29dbC marginal to -50dbc.  A very radical fix.

RF in the audio due to audio switching being done in K3 that has the high power RF output.
Removed the switching and used a 2n7000 mosfet to snub the pop.

DC switching did not remove power for certain from stages resulting pops and howls
added the N3GO 2 transistor fix.

Added needed resistor for Q70 bias improved RX audio overload.

Regulated TDA2822 voltage to 8V so over 12V was not a risk.

The already highly discussed problem of very high power amp gain at 80m and typically
(based on 5 units) 11db less gain at 10M without fixes.  Sneak paths on the ground plane
coupling output to input.  Many changes required to get a decent amp.  in the end
abandoned work due to SSB TX spur issue.

Q90 in the rf path during RX, high RF environment fry Q90 resulting in no TX.  There should be 
some protection for the receiver input.  people worried about using back to back diodes limiting
the over load performance.  I tested this and the base-emitter junction of Q90 to ground caused
the same degradation as a 1n914 diode.  So back to bad diodes would not degrade anything
but would preserve the transistor  Switching the path so the transistor powered or not is not there 
would be better.

As noted and confirmed by Warren leakage around the relays degrade the filters ultimate
attenuation.  I also suspect board layout (ground plane) troublesome.  Measured leakage
was -45db at best, more is needed and two relays at opposing ends can do this.

Some of this was confirmed by literally slicing up my board into 3 sections, power amp,
output low pass filter block and the low level audio and IF sections.  The carrier leakage
was handled by external circuit in a tin.   The modules perform better such as the amplifier
where better stability was noted, or the same (low pass filter blow by) to confirm results.


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