Impedance matching of rf circuit

[Gab_RF]

hey there, i'm using a vhf modulator that give the rf output with 75-ohm, i connected it to an amplifier of 50-ohm input impedance. Is the impedance in this situation matched? if not how can i match this network?

 

[thylacine1975]

> Is the impedance in this situation matched?
No... since 75≠50, the amplifier and modulator aren't matched.

Personally, I wouldn't worry too much about it. Yes, you *could* fix it with an LC matching network or a transmission line transformer relatively easily if your modulator is only operating over a narrow frequency range (one VHF channel?), but for the ~0.2 dB maximum mismatch loss you could possibly reclaim it's not worth it.

The only other possible benefit to be had (of matching the two) would be the removal of reflections at the impedance discontinuity if there was a substantial length of transmission line (e.g. coax) between the modulator and amplifier. If the modulator is for a video system, such reflections could appear (if the transmission line is long enough) as "ghosting" type artifacts. If these aren't a problem, I wouldn't bother

 

[Fadi Mkhayel]

hi MR thylacine1975,i'm facing the same problem while taking from uhf video modulator to a uhf aplifer, at the receiver the screen isn't clear and some how scrambled, i have the same impedance mismatch, thanks in advance.

 

[thylacine1975]

Heya guys - this page (http://www.ham-radio.com/k6sti/match.htm) describes the resistive approach to joining your amplifier/modulator systems. This is probably the approach I'd recommend, as it's inherently broadband and just works
The downside though, is that's it's lossy - and at 5.7 dB it's significant - but given the (usually high) output level of a modulator this might be OK. If it's not, then your other primary options are (in my order of preference):

* a transmission line transformer (often referred to as a 'quarter wave transformer') - e.g. http://www.qsl.net/w4sat/qtrwavtr.htm,
* L-network (e.g. http://home.sandiego.edu/~ekim/e194rfs01/jwmatcher/matcher2.html), or
* a transformer (such as http://www.datasheetcatalog.org/datasheet/macom/TPX-75-4N.pdf).