EMRFD Message Archive 3056

Message Date From Subject
3056 2009-05-19 01:41:28 ha5rxz Conversion Between 75 Ohm and 50 Ohm Systems
I'm sorry but after trying hard with this particular problem I've had to ask for help, my math skills are just not good enough. I have an amplifier block which is intended for 75 ohm cable TV systems and the specification is as follows:

Maximum RF Input (Single Tone) 50 dBmV
Input Impedance 75 Ohms
Gain 35 dB
Output Voltage 62 dBmV (I think this is a maximum figure)

So, if I fit a 1:1.5 transformer on the input and drive the assembly with a 50 ohm source what is the maximum signal generator output in dBm before I let the magic smoke out?

I only have one of these so I need to get this right first time.

HA5RXZ
3057 2009-05-19 04:59:01 joop_l Re: Conversion Between 75 Ohm and 50 Ohm Systems
3058 2009-05-19 06:25:12 Alan Melia Conversion Between 75 Ohm and 50 Ohm Systems
Hi might I ask "Why worry?" just slot it in a 50ohm system it will work. If
you are interested in fractional dB levels for test purposes then it might
be important. The best wideband solution then is to use a minimum-loss
resistive pad. These can usually be found in profusion at flea markets. A
wideband transformer with better characteristics might be quite difficult to
achieve. If getting all the gain is important, will the input/output
mismatch matter that much??

Alan G3NYK
3059 2009-05-19 11:32:42 ha5rxz Re: Conversion Between 75 Ohm and 50 Ohm Systems
"Hi might I ask "Why worry?""

Because I only have one of these units and the manufacturer has told me that overdriving it WILL kill the module stone dead.

HA5RXZ


3062 2009-05-20 01:54:31 kerrypwr Re: Conversion Between 75 Ohm and 50 Ohm Systems
"Maximum RF Input (Single Tone) 50 dBmV".

dBmV is a rather unusual unit; is the above statement correct?

50dBmV means 50db (100000 times) more than a millivolt; 100 volts!!!

In a 75-ohm system that is 130 watts.

Perhaps it's really "dBuV"; a more-common unit.

In that event, 50dBuV is 100 millivolts; a much-more-likely figure.

Can you clarify this point before we proceed?
3063 2009-05-20 05:32:09 Johan H. Bodin Re: Conversion Between 75 Ohm and 50 Ohm Systems
When talking about voltages, +50dB means 316 times the voltage (the same
as 100000 times the power).

73
Johan SM6LKM

kerrypwr wrote:
> 50dBmV means 50db (100000 times) more than a millivolt; 100 volts!!!
3066 2009-05-20 14:34:28 kerrypwr Re: Conversion Between 75 Ohm and 50 Ohm Systems
Oops; brain fade!!!!!!

I never use dBV/uV; always dBm. I dislike what I perceive as the manipulation that makes dB-voltage line-up with dB-power.

Sorry to have confused things (and myself!!).
3069 2009-05-21 00:42:25 joop_l Re: Conversion Between 75 Ohm and 50 Ohm Systems
3084 2009-05-23 04:09:44 ha5rxz Re: Conversion Between 75 Ohm and 50 Ohm Systems
Confirmed, maximum input to the module is 50 dBmV

HA5RXZ


3091 2009-05-23 08:21:40 jocjo_john Re: Conversion Between 75 Ohm and 50 Ohm Systems
3092 2009-05-23 08:34:43 jocjo_john Re: Conversion Between 75 Ohm and 50 Ohm Systems
3099 2009-05-23 11:39:31 joop_l Re: Conversion Between 75 Ohm and 50 Ohm Systems
3100 2009-05-23 12:47:46 jocjo_john Re: Conversion Between 75 Ohm and 50 Ohm Systems