Page 243 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

TABLE 14-3 Out-Frequencies for Simple and Harmonic Mixers

Second-Harmonic Third-Harmonic
Frequendes to Mixer SimpLe Mixing Mixing Mixing

Fosc and FRF FOS( ± FRF 2Fosc ± FRF 3±Fosc ±FRF
Now let's return to the one-transistor mixer. We can compare the ratio of
conversion transconductance to small-signal transconductance, this time including
mixing at harmonics of the oscillator frequency. Table 14-4 shows that the

conversion gain for harmonic mixing is almost the same as the conversion gain for
simple mixing when the oscillator injection voltage is about 500 mV peak sine wave
into the baseemitter junction. However, one down side to having high-amplitude
injection voltages into the mixer is that more filtering may be required at the
mixer's output to filter out all the signals of fundamental and harmonic frequencies
related to the oscillator frequency.

TABLE 14-4 Transconductance Ratios for a Single-Transistor Mixer

Oscillator Signal
g m_conversion! 9mQ
(Peak Sinusoidal FundamentaL
gm conversioj9 mQ 9 m_conversio./9 mQ
Voltage) Frequency Second Harmonic Third Harmonic
0.013 0.242 0.030 0.0025
0.026 0.446 0.107 0.018
0.052 0.698 0.302 0.093

0.078 0 .810 0.460 0.197
0.130 0 .893 0.642 0.379

0.182 0 .926 0.736 0.505
0.260 0 .949 0.810 0.625
0.520 0.974 0.902 0.779
So the bottom line IS If harmonic mixing IS needed In a one-transistor mixer, make

sure that the oscillator voltage is at least 130 mV peak into the base-emitter
junction. gm_coovecs;oo/gmQ is known from this table and gmQ = I cQ / 0.026 volt,
gm_coove,;;oo can be found. For example, if the oscillator injection voltage is 0.052 V
peak into the base-emitter junction, then for the second harmonic of the oscillator

signal mixing with the RF signal, we have

/ g
g nl_COl1vcrsion.-2nd mQ = 0.302

and thus

gm_con"crsion.-2nd = gmQCO.302)

If ICQ = 1 mA, then gmQ = 0.0384, and then