Discussion of Radio Frequency Mixers.
Much has been said about diode ring mixers, but although these are simple to use, they do otherwise has a limited signal handling capability as does any mixer. The diode ring uses the switching characteristics of the rectifier diode, that is to say the diode is driven to switch on and off by the high local oscillator drive signal input. Although the diode characteristics are also multiplying the signals, the switching effect does however produce harmonics of both the 2nd and 3rd harmonic levels. This in effected also goes to add to the intermodulated products and perhaps reduce the intercept points of the mixers performance.
An active mixer of transistors, bi-polar, Jfet or even VMOS, has the added advantage of a linear region within the transfer characteristics of the mixer, where as a diode ring circuit is a mixer through brut force. Similarly if the active circuit is driven into a non-linear function, the intermodulated products would be produced in greater quantities that would otherwise be perhaps the case with a diode ring.
Using higher voltages for the active circuit power supply, say 28volts, has been used in the pass for active circuit mixers. In any event, the input signal source must not drive the input into a cut-off voltage level, or the pinch off voltage level, or the input signal would be distorted at the mixers or amplifiers output. The multiplying effect of the active device is all that is required to provide the signal mixing action, thus so the local oscillator drive must also not drive the mixer into a non-linear region of the circuit transfer characteristics. In this regard, the intermodulated signal products at the output would not be so prevalent, and the 1dBm compression point would not be then reached or breached.
To handle high input RF signal on the R.F. input, attenuators could be used, or otherwise voltage step down transformers could be used. If the step down voltage is ¼ of the input, then an extra 6dBm in dynamic range would be found. There could be four different secondary coils to the one primary coil of the RF transformer. Each of the secondary’s could be used to supply ¼ of the RF signal to a separate mixer, thus 4 four mixers then adding to the final output I.F. signal through a R.F. transformer output coupling.
It is best however to have an average constant signal at the R.F. input of the mixer stage. For the 1st mixer stage for the 1st I.F. roofing filter, this would require forward AGC control derived from the 1st I.F. mixer output, while the second stage I.F. for the signal bandwidth filtering, this would require an AGC detection circuit before the signal demodulation stage. By combining the both the AGC signals, the “S meter” indication could be determined. Each AGC circuit would have a determined AGC dynamic range, added together would provide the AGC dynamic range for the whole radio receiver. It could then be that the AGC voltages for each stage would have boundary signal voltage levels, so as not to over step the proceeding AGC range of each subsequent AGC amplifier stage.