Twin Triode Mixer Circuit For Ssb Frequency Conversion

push-pull circuits and the tubes into exact amplitude and phase balance, about 20 db of injection frequency cancellation is all that can be depended upon. With suitable trimming adjustments the cancellation can be made as high as 40 db, however, in fixed frequency circuits.

The Twin Triode Mixer The mixer circuit shown in figure 28 has about 10 db lower distortion than the conventional 6BE6 converter tube. It has a lower voltage gain of about unity and a lower output impedance which loads the first tuned circuit and reduces its selectivity. In some applications the lower gain is of no consequence but the lower distortion level is important enough to warrant its use in high performance equipment. The signal-to-distortion ratio of this mixer is of the order of 70 db compared to approximately 60 db for a 6BE6 mixer when the level of each of two tone signals is 0.5 volt. With stronger signals, the 6BE6 distortion increases very rapidly, whereas the 12AU7 distortion is much better comparatively.

6AS6's

6AS6's

6as6 Preamplifier Schematic

Figure 29 BALANCED MODULATOR CIRCUIT FOR SSB FREQUENCY CONVERSION

Figure 29 BALANCED MODULATOR CIRCUIT FOR SSB FREQUENCY CONVERSION

Figure 30

RESPONSE OF "N" NUMBER OF TUNED CIRCUITS, ASSUMING EACH CIRCUIT Q IS 50

Figure 30

RESPONSE OF "N" NUMBER OF TUNED CIRCUITS, ASSUMING EACH CIRCUIT Q IS 50

In practical equipment where the injection frequency is variable and trimming adjustments and tube selection cannot be used, it may be easier and more economical to obtain this extra 20 db of attenuation by using an extra tuned circuit in the output than by using a balanced modulator circuit. A balanced modulator circuit of interest is shown in figure 29, providing a minimum of 20 db of carrier attenuation with no balancing adjustment.

Selective Tuned Circuits The selectivity requirements of the tuned circuits following a mixer stage often become quite severe. For example, using an input signal at 250 kc. and a conversion injection frequency of 4000 kc. the desired output may be 4250 kc. Passing the 4250 kc. signal and the associated sidebands without attenuation and realizing 100 db of attenuation at 4000 kc. (which is only 250 kc. away) is a practical example. Adding the requirement that this selective circuit must tune from 2250 kc. to 4250 kc. further complicates the basic requirement. The best solution is to cascade a number of tuned circuits. Since a large number of such circuits may be required, the most practical solution is to use permeability tuning, with the circuits tracked together. An example of such circuitry is found in the Collins KWS-1 sideband transmitter.

If an amplifier tube is placed between each tuned circuit, the overall response will be the sum of one stage multiplied by the number of stages (assuming identical tuned circuits). Figure 30 is a chart which may be used to determine the number of tuned circuits required for a certain degree of attenuation at some nearby frequency. The Q of the circuits is assumed to be 50, which is normally realized in small permeability tuned coils. The number of tuned circuits with a Q of 50 required for providing 100 db of attenuation at 4000 kc. while passing 4250 kc. may be found as follows:

Af is 4250-4000=250 kc.

fr is the resonant frequency, 4250 kc.

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