Scope Testing Modulation - Retro 75 - Why No More?

[W1UJR]

February 22, 2011 - Scope Testing Modulation On the Retro 75 Transceiver

Posted a video of my modulation testing on the Bruce's Bench page.
I was surprised to find that modulation was only possible to 60-70% before distortion occurs.
Perhaps this is the norm with this modulation scheme?

Measuring set up used - Retro 75 transceiver built by W1UJR, Elecraft DL1 Wideband Dummy Load,
Elecraft CP1 Directional Coupler, Elecraft 2T-gen 2-Tone Test Oscillator.
Also used the Elecraft W1 140W Computing Wattmeter and SWR Bridge for initial power measurement and set up.

Power output was good, found power output about 2.5 watts with an unmodulated carrier, does rises to about 4 watts at 60-70% modulation.
Waveforms were recorded on Tektronix THS720A Digital Oscilloscope.

Wonder if anyone has discovered the limiting factor here?
RKW mentioned the LED being used as a limiter, thoughts?
Frankly, does it really matter that much with 2-4 watts of carrier?

Photos and video at -->> http://www.w1ujr.net/bruce's_bench_2011.htm

[WD5JKO]

Bruce,

  What you see is pretty normal with the Retro-75. Remember we do not speak in sine waves, and the AGC loop is designed for speech. If you connect the microphone, and with proper mic phasing, you should get to 90% on peaks. Also make sure yours has resistor R28 grounded as per the schematic. This effects the AGC circuit time constant.

The issues you bring up were overcome in my unit on the other Retro-75 long winded thread. The hardware in the box you got is capable of > 130% modulation with the rework I detail at the following link:

http://pages.prodigy.net/jcandela/Retro75/

I like your setup! Boy my bench rivals the class E guys...

Jim
Wd5JKO

[W1RKW]

Bruce,
I concur with Jim.  Using a tone will cut modulation back to the level he states however, voice modulation can approach 100% quite easily.  My Retro is still without the LED in place but I use a scope to watch the waveform.  Without the LED one can go beyond 100% slightly but you need to watch the modulation if you want to remain clean.  Probably not a big deal at 2 watts but what the heck.