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.
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.
6384 2011-06-16 05:05:44 Mike Czuhajewski Re: rf pickup from relays >.... I used RF rated reeds from Coto in measuring interfaces (FairchildDigiKey has them. I recently bought one of their RF relays to repair my HP
>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....
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.)