SM 5 BSZ - I/Q mixer for direct conversion radio
(Sept 28 2001)

I/Q mixer for direct conversion radio

A direct conversion radio needs a very good mixer. A strong signal near the passband center will produce strong audio signals at low frequencies (i.e. 1kHz). The mixer has to have a very low second order intermodulation to avoid a spur at twice the frequency,(2 kHz).

If the second order spur is allowed at -40dB below the strong undesired signal a standard schottky mixer (7dBm LO) may be fed with about -13dBm. The output with a 50 ohm load is then about -20dBm. If this output level is amplified to near saturation of a Delta44 A/D converter, the noise floor at the amplifier input has to be at -125dBm in SSB bandwidth to only degrade performance by 3dB. I do not know how to make a DC to 100kHz amplifier with a noise figure below 10dB for a 50 ohm source impedance, but that is what is required - and then the second order spur will be only 40dB below the interfering signal.

The simple HCMOS mixer in fig 1. produces a second order spur at -50dB when the level is adjusted for a standard low noise op-amp to degrade the dynamic range by less than 3dB. I have used Fairchild RC 4136 which is specified 10nV/sqrt(Hz). The -50dB second order spur is produced with an input signal level of +3dBm. The output voltage is 90% of the input voltage if the load impedance is 1 kiloohm. (The output impedance is about 300 ohms) This mixer is by far superior to a standard schottky mixer and it is probably about 20dB better than a high level schottky mixer.

Fig 1. A quadrature nixer using 74HC4052. The control inputs are fed from a synchronous counter that divides a local oscillator by four. As a result the switch loops through all four positions during a period of 4 local oscillator cycles, see fig. 2. For each switch position only one connection is made between the antenna and either output. This way the make before break operation of the switch will not cause shortcircuits across the coil as would be the case using 74HC4053 as in an older design. The dynamic range of the 74HC4052 design is superior to that of the 74HC4053 design. The coil is bifilarly wound on a toroid and easily allows a bandwidth of 1 to 30MHz. The inductance of the coil must be high enough to provide an impedance well above 50 ohms for one winding at the lowest frequency. The stray capacitances must be low enough to provide an impedance well above 50 ohms for the highest frequency. One alternative is to use the transformer from inside a schottky diode double balanced mixer. If a trifilar winding is used, use one for the input signal and connect the remaining two as shown in the figure.

Fig 2. The waveforms at F and 2*F will rotate the 4052 switch through its 4 positions with equal times at each position if the waveforms are generated from a synchronous counter running at 4 * F
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