On Tuesday, April 3, 2012 2:56:15 AM UTC, Dave M wrote:
K7ITM wrote:
On Mar 18, 7:05 am, dave wrote:
On 03/18/2012 04:20 AM, JIMMIE wrote:
I am trying to design a circuit that will enable me to switch H pad
attenuators in and out of a line but just cant get my head wrapped
around it. I have come up with one method of doing it but it seems
a little complicated. I would greatly appreciate it if someone
could direct me to a circuit diagram.
Jimmie
Bridge the pad with a DPST switch or relay. Switch "on" pad "off".
Errrrp. That leaves the pad in-circuit across the line...not good in
most situations. Better if the switch also disconnects the pad from
the line. You probably need a 4PDT switch: the four moveable
contacts to the input and output lines, the four fixed contacts on one
side to the four terminals of the pad, and the four fixed contacts on
the other side wired so the line connects straight through. You can
do it a bit more simply by bridging the pad with the DPST as
suggested, but also adding a SPST that is N.O. if the DPST is N.C.,
with the SPST opening up the shunt path in the H-attenuator.
If you leave the H-pad in the circuit, bridging it with a DPST and not
opening up the shunt path, assuming the pad is the same impedance as
the line (=R in the following...), you shunt the line with 8.7*R for a
1dB pad (not terrible...), 2.9*R for a 3dB pad (not so good), and
0.95*R for a 10dB pad (pretty awful).
Cheers,
Tom
This is probably what you're looking for:
http://www.arrl.org/files/file/Techn...df/9506033.pdf
With careful construction, you should get pretty accurate results at HF and
VHF frequencies. Using SMD resistors in the construction should yield even
better performance than described in the construction article, since it
looks like the author used metal or carbon film resistors, which are
somewhat inductive. Use of a PCB and good shielding between the sections
should give you a really good attenuator.
Granted, the article describes a Pi-type attenuator, but the technique
should be applicable to an H-type circuit as well..
--
Dave M
A woman has the last word in any argument. Anything a man says after that is
the beginning of a new argument.
Close Dave, I need to modify this for H type balanced pads so I am thinking 4PDT switches.
Thanks for your suggestion and time.
Jimmie