A Doubly Balanced "H-mode" Mixer for HF

If you have a home-built receiver or transceiver for the amateur bands—or even manufactured equipment—that has proven weak in the front end, this article is for you. You could consider adding a band-pass filter at the input, which is desirable1 for second-order IMD protection; but it will not necessary help the third-order IMD, measured as third-order intercept point or IP3. That is the figure of merit you would want as high as possible to avoid in-band third-order IMD problems. You could also plan to replace diodes having poor secondand third-order behavior with better PIN diodes at the input/output switching points in the front end, increase the current through them or use relays. Yet there are also RF mixers at each conversion.2 Input IP3 or IIP3, can be seen as a cumulative figure, involving all stages in a receiver; or measured in a single stage, such as for a mixer device alone.3 You would desire a receiver cumulative noise figure (NF) as low as practically possible. Observing the values we could get for these two parameters in an active device like a preamplifier, with a gain G, we think about them as tradeoffs because it is difficult to maintain low NF and high IIP3 together. We need to measure the impact on a receiver’s noise-floor performance. Wes Hayward, W7ZOI, describes a figure of merit for receivers named Receiver Factor or RF = IIP3 – NF. The most important equation in a receiver that relates noise floor to NF is: