On 11/23/2012 9:38 AM, Bruce Griffiths wrote:
NIST have shown (at least at 10MHz) that the high level mixers they tested are noisier than the ZRPD1. Bruce
Do you have a citation to where they said that? What you quoted doesn't make sense, at least, out of context. We need to clarify phase detector sensitivity specs. For conventional (IE 50 ohm) phase detectors, it is apples vs apples to just go by the volts per radian number. However, mixers like the ZRPD1 artificially triple the voltage sensitivity by operating at 500 ohms, and using transformers to connect to 50 ohm equipment. Doing this doesn't increase the possible signal to noise ratio. Consider this thought experiment. Build your best 500 ohm phase detector and postamp. Now replace with a 50 ohm phase detector and connect 3 postamps in parallel. It is a wash. Of course, you don't have to actually do this. You can simply use an op amp like the LT1028 with very low noise voltage. To actually put a 500 ohm detector on a par with a 50 ohm detector, the 500 ohm detector would need to use 3 diodes in series compared to one in the 50 ohm case. With only one diode per arm, the maximum drive power utilization is considerably lower. Rick Karlquist N6RK _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
