EMRFD Message Archive 6375

Message	Date	From	Subject
6375	2011-06-07 20:25:11	Ashhar	rf pickup from relays
i was putting in a bank of 6 top-coupled band-pass filters for a transceiver project. they were switched by regular DIP 12V DPDT relays. as i was putting the arduino board through it's paces, switching the filters, i realized that even when the filters were switched off, the some signals were coming in. i first thought they were birdies, but they were turning to the correct side and they weren't tuning any faster than normal. so, we had a leak. then, i held a 0.1uf all over the place trying to find where the leak was from. i realized it was picking up from the relay coils. so, i strapped bypasses on both sides of all the relay coils and it went away. i never realized that relays could act as such active antennas. i post this just so others will beware. - farhan (VU2ESE)
6376	2011-06-08 03:53:27	Alan Melia	rf pickup from relays
Hi Farhan, I guess these were DIL reed relays. I used RF rated reeds from Coto in measuring interfaces (Fairchild Sentry IC tester) in the 80s and these had a screen between the coil and the reed. I am not sure whether these are available still. We were not looking for the kind of isolation you need in a receiver. Alan G3NYK
6378	2011-06-08 23:12:14	Ashhar Farhan	Re: rf pickup from relays
i opened one up and checked. they are mechanical relays. i suppose the coils are radiating RF that's being picked up by the antenna leads. i was hoping that as all the points are at low impedance, RF pickup wouldn't be a problem. however, i suppose i failed to understand that the low impedance is only at the port of the networks. the impedances can be pretty high at specific points within a building block. - farhan
6384	2011-06-16 05:05:44	Mike Czuhajewski	Re: rf pickup from relays
>.... I used RF rated reeds from Coto in measuring interfaces (Fairchild >Sentry IC tester) in the 80s and these had a screen between the coil and >the reed. I am not sure whether these are available still.... DigiKey has them. I recently bought one of their RF relays to repair my HP 8640B signal generator. (One of them has a footprint and pinout identical to the original relay.) Like the original, the Coto replacement is coaxial with the reed switch running through a metal tube which has wires at both ends for grounding to the circuit board. (I have x-rays of both relays, and in the case of the old one which I could afford to dissect I also have photos.)