[Elecraft] An idea for the K2

[John Magliacane]

> I would like to know more about using a switching mixer for the BM.
> If the circuitry does not get too complex or introduce problems it would
> be worth trying. Please let me have more information.
> 
> 73
> Stewart G3RXQ

Imagine that you apply audio to a transformer that has a center tapped
secondary winding.  (With respect to the center tap, the output of the
transformer will consist of two independent audio signals, equal in
amplitude, but 180 degrees out of phase with one another.)

Now imagine that you ground the center tap, and connect a SPDT switch to
the free ends of the secondary.  The center pole of the switch doesn't
connect to the transformer, but serves as the "output" of this circuit.

Finally, imagine that the switch is toggled back and forth at an RF
rate while audio is applied to the transformer.

The end result will be a double sideband suppressed carrier RF signal
on the output of the switch.

In real life, we use OP-AMPs to provide the two out-of-phase audio signals
instead of a transformer, and CMOS analog switches, such as a CD4066 or
CD4053 (or better) in place of the mechanical SPDT switch and apply RF
to the control pins.

The end result is the same.

I use this approach *A LOT* for doubly-balanced mixing, synchronous
detection, phase detection, and modulation through the MF spectrum with
excellent results.

A quick Google search didn't turn up many simple examples of this approach,
but "Bat Detectors" (ultrasonic downconverters used to chase bats), and
"Cave Radios" (LF SSB transceivers with magnetic loop antennas used for
underground explorers) that do appear on the net use them quite often.
I used a switching demodulator and modulator in my 1200 baud BPSK Pacsat
Modem (http://www.amsat.org/amsat/articles/kd2bd/Pacsat_Modem/).

More involved examples can be found in Software Defined Radio designs
utilizing Commutating Detectors (so-called Tayloe Detectors).

A high-quality DSB modulator is quite simple compared to these designs.


73, de John, KD2BD


=====
Visit John on the Web at:

	http://www.qsl.net/kd2bd/


	
		
__________________________________ 
Celebrate Yahoo!'s 10th Birthday! 
Yahoo! Netrospective: 100 Moments of the Web 
http://birthday.yahoo.com/netrospective/