EMRFD Message Archive 5992

Message	Date	From	Subject
5992	2011-04-02 07:18:01	Brooke Clarke	Mixer Diodes
Hi Chris: The keys to diode selection for mixers involves the barrier height of the diode and it's space charge current limits. The former is easily measured by applying 1 ma forward current and reading the Vf. This is the parameter that determines the needed LO drive level. The Fluke DMMs seem to use 1 ma when in diode mode. http://www.prc68.com/I/DMM.shtml A pulse technique is needed to measure the latter parameter because if the test current was applied continuously the diode would be destroyed. The higher the space charge current limit the higher the power levels that the mixer can handle. For more on the idea see "Space Charge Limited Current" at http://www.prc68.com/I/Diodes.html. Note a number of WJ mixer app notes that did not understand this are listed. If the diode is replaced by a switching device, like a FET, space charge current limiting is still a factor. But it may not be a data sheet parameter so needs to be tested. -- Have Fun, Brooke Clarke, N6GCE http://www.PRC68.com
5993	2011-04-02 07:28:19	Chris Trask	Re: Mixer Diodes
> > The keys to diode selection for mixers involves the barrier > height of the diode and it's space charge current limits. > The former is easily measured by applying 1 ma forward > current and reading the Vf. This is the parameter that > determines the needed LO drive level. The Fluke DMMs seem > to use 1 ma when in diode mode. > http://www.prc68.com/I/DMM.shtml > > A pulse technique is needed to measure the latter parameter > because if the test current was applied continuously the > diode would be destroyed. > The higher the space charge current limit the higher the > power levels that the mixer can handle. For more on the > idea see "Space Charge Limited Current" at > http://www.prc68.com/I/Diodes.html. Note a number of WJ > mixer app notes that did not understand this are listed. > If the diode is replaced by a switching device, like a FET, > space charge current limiting is still a factor. But it > may not be a data sheet parameter so needs to be tested. > Thank you for pointing this out. It seems to be true for bipolar devices used in commutating (aka Gilbert Cell) mixers. I ran into this some years ago and have focused on diode-based mixers since then. The slow turn-on and even slower turn-off times of FETs and bipolars are serious obstacles for high frequency switching performance, especially when they result in all devices being in the on state simultaneously. That is a major source of IMD degredation. Chris Trask N7ZWY / WDX3HLB Senior Member IEEE http://www.home.earthlink.net/~christrask/
5995	2011-04-02 08:17:00	ha5rxz	Re: Mixer Diodes
It seems that we have an interesting debate between two knowledgeable men, on the one hand Chris N7ZWY with his diode based mixers and the other Giancarlo I7SWX with the semiconductor switch based designs. I cannot think of a better way to expand our knowledge of the subject. The finest work I know of on HF receiver front ends* is that carried out by Martein PA3AKE with his amazing design. I wonder if he considered diode switched mixers in his research? Peter HA5RXZ * With the work done by Bill W7AAZ and Harold W4ZCB coming a very close second