ANOTHER DIRECT CONVERSION HF SSB TRANSCEIVER
|PLL VCO||DDS VFO|
|Tayloe Mixer/Detector||Exciter & Receiver Audio|
The Tayloe Mixer Board
The Tayloe Sampling Mixer is the device that fired my imagination and triggered my "n-th" attempt at building a phasing type direct conversion transceiver. My visit to the web-site QRP2001 gave me the push start and I have not regretted my experimentation with this exciting device. I got myself a couple of 74FST3253 dual fast bus switche IC that function as a 1 in 4 analogue multiplexer and some Advance CMOS IC's ( 74AC74 and 74AC86) and set about building my first Tayloe Detector receiver. This version required a Local Oscillator injection of twice the working frequency driving a counter that generates the two square wave signal which are 90 degs out of phase for driving the Tayloe Mixer. The results obtained were very pleasing in terms of sensitivity and large signal handling capability. Plus, I got by with a simple low-pass filter on the RF input for selectivity.
I finally changed the design to a balanced input and one requiring an LO injection of four times the working frequency. The balanced input feature was an influence from part 4 of the series "Software Defined Radio for the Masses" by Gerald Youngblood AC5OG which was published in QEX March/April 2003. Check the "Articles" page on this link and look for Part 4 of the series. Using this LO drive of 4 times the operating frequency clocking a Johnson Counter allowed an easier means of providing the two square wave signals with greater accuracy in quadrature, with a 50 % on-off duty cycle which is necessary for attaining maximum unwanted sideband attenuation.
The Local Oscillator is fed to an Exclusive Or gate which in turn clocks the D flip/flop, a 74AC74 wired as a Johnson Counter. From the Q output of one half and the Not Q output of the other half come signals at 1/4 the frequency, 90 degs. out of phase that clock the input RF signal to each of the four switched outputs of the 74FST3253 once every cycle. These square wave signals are in quadrature and actually apply two bit binary code of 00, 01, 11 and 10 in sequence to the 74FST3253 providing the 1 in 4 multiplexing.
By changing the levels on the inputs of two other Exclusive Or gates we can decide whether the Q output of one D flip/flop 'A' leads or lags the output of the Not Q of the other D Flip/flop 'B' and this provides Upper or Lower Sideband selection. The fourth X-Or gate through the switching of input levels drives the chip enable pins of the 74FST3253 chips to control the transmit/receive functions of the Tayloe Mixers.
The block diagram to the left is an example of the 80 meter situation where the 3.5 MHz RF signal is fed to the FST3253 and the signal is sampled four times per cycle through the sequential switching of the device. This switching is produced by the two 90 degs. phase-shifted LO signals that drive the detector. Four audio signals emerge in poly-phase, 0, 90, 180 and 270 degs. out of the FST3253 and these go to the relative inputs of the low noise deferential amplifiers that function as post mixer amplifiers. Only the 0 and the 90 deg. output signal phases are utilized. Note that the NE5532 is power with +5volts and the half voltage bias is derived from the signal input side of the FST3253.
The detailed operation of the Dan Tayloe N7VE Mixer is well documented on the Internet.
On the exciter side, four phases of audio signal, 0, 90, 180 and 270 degs. from the transmit audio phase-shift board drive two TL082 amplifiers that apply the AF signal for modulation by the Tayloe transmit mixer. The operation of the mixer in this case is opposite to that of the receive function. Here the two signal in quadrature clocks the four switches in sequence, sampling the audio on the inputs every cycle of the 3.5 MHz LO. The combining of the signals in the output results in the attenuation of the unwanted sideband at an RF level. Pins 9 and 7 of the receive FST3253 and the transmit FST3253 are configured in push-pull and connected to a tri-filar wound toroid transformer that provides a 1:4 impedance match (See fig. 5).
While I do not have the means to accurately measure the performance of the modulator, the carrier and unwanted sideband suppression appear to be acceptable