Originally Posted By: peter
Is that the inside of a boxed commercial product? It looks to my untrained eye as if their sweatshop needs to get better orphans -- that's some pretty haphazard-looking soldering, especially considering this is RF.

Peter

All those coiled lines are feed inductors to provide DC with a high impedance to the RF traces. There is a shunt cap at the end of each one where the DC comes in. The crappy soldering of those caps shouldn't affect the signal path.

Originally Posted By: mlord
Originally Posted By: mlord
Mmm.. one thing it needs, is an added DC-blocking capacitor on each of the four inputs, to prevent the +12V switching voltage from running up to my antennas (and possibly shorting out there).

Any suggestions as to the appropriate value there for passing UHF/VHF television frequencies?

Mmm.. The reference design I found for a 2GHz switch uses 56pF blocking caps.. so perhaps I'll try something close to that. But better suggestions are more than welcome!


I would put in 1000pF to be safe. Even at 200MHz, the 56pF will be about 14 ohms. At VHF channel 2 the 1000pF would look like 3.5 ohms.

I think these switches normally pass 900-2000 MHz, so the feed inductors are probably tuned to about 1500MHz. You'll want to check your lowest channel to see if you have too much insertion loss. Also, these switches may not have the best isolation between ports (18-30 dB).

You may be able to remove the 12V supply if you want. You would remove the output transistor feeding the feed inductors and put your 5V control line there. You would want to scale down the 2.2Kohm by the common port to keep the same current through all the diodes as you would with a 12V supply (1kohm would probably work). Maybe put a 1000pF and 0.1uF on top of the already placed caps at the inductors to provide more filtering to the 5V supply.

If you REALLY want more isolation, you can hack this board to be the same as the schematic you posted earlier from the other message board. You would need +/- 5V though and another switch to get diodes:

  • Make the 2.2kohm resistor 0ohm.
  • At the feed side of the series diode on each leg, place another diode from the first diode pin to ground, anode on ground. There is plenty of ground by the diodes to scrape off some resist to make a close connection.
  • In series with each control signal, place a 1kohm resistor.

The port you want enabled would be given a +5V signal, the others would be given -5V. The series diode would be turned on for the desired port, and for the unused ports the series diode would be turned off and the shunt diode turned on, providing more isolation.

Also, if you find the insertion loss too high at lower channels, you could cut the printed coils away and place discrete inductors. Digikey has some 0805 or 1206 muRata parts that should bridge the gap. This would be hard to tune without a network analyzer, but you could try some parts between 0.1uH and 1uH to see what gives the flattest response.

edit:
I wish I had thought of this; up until a week ago I had access to a network analyzer, and I have some switches laying around. I could have played with some values and given you insertion and isolation graphs.


Edited by Folsom (01/05/2009 22:43)