[hpsdr] PennyWhistle bias adjustment for minimum distortion

[David McQuate]

If the bias adjustment resistors are set for minimum current, the 
amplifier will be operating
in class C -- guaranteed non-linear -- which would be fine for CW and FM 
modes.
The amp can be driven hard into compression for max power out.
(However, if the Tx gain and Drive controls are set too high, there will 
be overflow in
computing the I & Q samples sent to Penny, and the transmitted signal 
will be awful.)

For SSB, or AM we want a linear amplifier -- minimum distortion.  
Penny's manual
suggests setting the bias so that each transistor's drain current is 450 
mA (no signal).
It has been suggested that a spectrum analyzer might be used to tweak 
the bias
for minimum intermodulation distortion while amplifying a two-tone signal.

Who has a spectrum analyzer?  Anyone having Mercury has a spectrum analyzer.
Other than an attenuator no additional hardware is needed.   Feed 
penny's output into
an attenuator rated 20W or more.  Attenuators may be cascaded so the 
total attenuation
is at least 50 dB.  Connect the attenuated signal to Mercury's input.

I've placed a version of KISS Konsole having a two-tone capability on 
the SVN--

svn://206.216.146.154/svn/repos_hpsdr_kiss/branches/WA8YWQ/KISS Konsole

KK already as the capability of full duplex operation with transmitter 
and receiver on the
same frequency.  On the Setup form, Transmitter tab, uncheck the two 
boxes labeled,
"Mute Rx on PTT" and "Only Tx on PTT".

When you click the "Two-tone" button, you'll see in the spectrum display 
the two transmitted
tones, and probably the intermod products.  (Selecting 48000 sample 
frequency will increase
the frequency resolution in the displayed spectrum.)   Carefully adjust 
the amplifier bias controls
to minimize  the distortion products (but don't let the amplifier power 
supply current go too
high).

I'm far from expert in use of the SVN, so if you have trouble accessing 
this version of KK,
please let me know & I'll try again.

HPSDR has BITE! (built-in test equipment)

73,
Dave / WA8YWQ