linrad support: A high level mixer with parallel J310 as switches.
(March 5 2003)
Two groups of eight J310 each operate as two switches that connect one or the other side of a balanced LC circuit at 70MHz to ground. The midpoint of the 70 MHz coil has a very low level of 70 MHz signal but it will have the 10.7MHz difference frequency with approximately half the peak amplitude of the RF signal at the drains. The output impedance of the mixer is about 17 ohms.

The RX70 unit with its mixer with sixteen J310 transistors provides a similar performance compared to a unit based on VAY-1 mixers. By replacing the J310 FETs with a GaAs FETs it would be possible to improve the mixer IP3 from about 31dBm to about 40dBm according to i.e. U.Rohde QEX Jan/Feb 2003 p 27. I have made a few experiments with MGF1302 GaAs FETs but they were not encouraging.

Figure 1 shows the mixer and IF amplifier. To protect the output from the very high power associated with the low output impedance of the MOS-FET a 4dB attenuator is added at the output.

Fig. 1. Schematic diagram for the 70 to 10.7 MHz mixer.

Fig. 2. The second channel is identical. Only the component numbering differs.