Ensemble RXTX Builders 2

Posted on Feb 6, 2014

This stage adds the TX Mixer to the board and provides the modulation of the Dividers` output signals by the 4 I and Q signals from the TX OpAmps (Quadrature Sampling Exciter - QSE). The result is a double sideband RF waveform that will be coupled into the PA Stage. Let`s suppose you were working 40m with a SDR center frequency of 7. 056 MHz. You

Ensemble RXTX Builders 2
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have tuned your SDR (via the software) so that you are listening to a CW station on 7. 066 MHz (10 kHz up from the center frequency). If you were to listen to the I and Q signals from the hardware RX via an amplifier or headphones (assuming there were no other signals and little noise) you would hear a single 10 kHz CW signal on both channels. Your ears may or may not be able to detect the fact that the tone in one ear is 90 degrees out of phase (I. e. , In quadrature) with the tone in the other ear. By comparing the phase difference between the I/Q signals the SDR RX software determines that this signal is 10KHz higher than the center frequency and displays the CW signal as a broken line on the waterfall. It also frequency shifts this signal at the output of the soundcard so it is heard at as normal CW tone from the speakers. Now, assume you decide to transmit on that frequency where you have placed your cursor (7. 066 MHz). Your SDR software will emit a 10 kHz signal (the delta between the center frequency and the selected TX frequency) in both the I and Q outputs, again, in quadrature. This will go into the SDR hardware on the TX side, through the TX OpAmps and into the TX Mixer, where it will beat up against the LO center frequency of 7. 056 MHz. The input to the mixer is 10KHz from the soundcard; the output from the mixer is the RF product of the LO and the input 10 kHz signal. If you were to listen to the input 10 kHz...

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