Balun 300Ω to 75Ω?

[mole42uk]

Hi,

I am modifying a Heathlkit IG-37 stereo FM generator which has a switched attenuator on the RF output which is connected to a weird 300Ω socket on the front panel.

It would be much more convenient if I replaced this with a 75Ω co-ax socket. Not only can I easily get co-ax plugs but it would also suit the majority of my FM radios if I could just connect a 75Ω aerial lead.

I think a balun is called for, but I have no experience of designing such a thing. I want to wire it behind the panel and connected between the attenuator and the new socket. Can anyone help? A partial circuit diagram is here:

[Ambientnoise]

I expect that a small transformer (made from a couple of ferrite beads or maybe a salvaged ferrite toroid)with a 2:1 ratio would do the trick. 2:1 turns ratio translates to 4:1 impedance ratio. Try say, 6:3 turns. From memory this was what simple baluns consist of. Beads will not handle much power, a few mW at a guess. Alternatively, a T-attenuator could be used asymmetrically but this would be lossy, of course.

Rgds

Ken

[Dave Moll]

Would something like this serve your purpose? If you need to change gender of the coaxial connection, connectors for this are available for mere pence.

As it is a generator and so a bit of signal loss won't matter I would just use the 'hot' terminal (the one connected to R51) and ground as your unbalanced output. To make it 75ohms (ish) put a 150 ohm across that terminal and ground as well, but quite honestly it will have no real difference.

[Herald1360]

The output impedance of that attenuator circuit isn't either 300ohms or balanced so just change the socket to anything you like. BNC or Belling Lee or whatever. If you connect straight to R50 and ground the output impedance will be somewhere near 50ohms.

The attenuator steps look roughly like 20dB.

[emeritus]

I agree with the observations about not needing a balun etc, but out of interest ( and the lack of anything good on TV) I dug out a copy of the ITT Reference Book for Radio Engineers which I knew has tables of normalized minimum loss impedance matching pads ( "T" -type "L" pads with only one series and one shunt resistor). For a 4:1 ratio the theoretical loss is 11.44dB, and for 300Ω to 75Ω the series and shunt resistor values are 259.8Ω and 86.61Ω respectively.

Calculating suitable values of parallel resistors is tedious, and it gave me an excuse to play with Excel, something I haven't needed to use for calculations before. Suitable parallel resistor combinations from the preferred range values are, with less than 1% error, 360//910, 300//2k0, or 270//6k8 for the series arm, and 100//620, 100//680, or 91//1k8 for the shunt arm. Any of these combinations should be equivalent when using 1% tolerance resistors, but the best theoretical matches are 270//6k8 [= 259.7Ω] and 91//1k8 [= 86.62Ω], differing from the ideal calculated values by less than 0.05%.

[mole42uk]

Originally Posted by Herald1360

<snip>If you connect straight to R50 and ground the output impedance will be somewhere near 50ohms.

The attenuator steps look roughly like 20dB.

Thanks Herald!

The front panel legend suggests the attenuator is 20dB per step.....and I've ordered a co-ax socket for the output. Tests will occur!

This http://resistor.gock.net/ is very useful.

[JohnBHanson]

This is a resistor calculator I wrote a few years back.
http://86.26.73.21/cgi-bin/resistor-comb