**EMRFD Message Archive 3009**

MessageDateFromSubject3009 2009-05-11 10:53:31 timshoppa Is a diode DBM naturally 50 ohms? Is there something about the diodes in a diode DBM that makes it naturally 50 ohms? I notice that different level mixers - using presumably either different diodes or matched diodes in series - are all specced at 50 ohms. Am I right if I assume that all DBM's have 1:1:1 trifiliar transformers, or are some done with different ratios for special purposes?

Is it just that all the ports have to be terminated at the same impedance, and when you've got any one at 50 ohms then the others have to be at 50 ohms too?

If I terminated all ports at 20 ohms or 100 ohms or 400 ohms, would it still be a good DBM? (With obviously different LO drive levels for optimum performance).

Tim N3QE3010 2009-05-11 11:10:24 Chris Trask Re: Is a diode DBM naturally 50 ohms? >50

> Is there something about the diodes in a diode DBM that makes it naturally

> ohms? I notice that different level mixers - using presumably eitherdifferent

> diodes or matched diodes in series - are all specced at 50 ohms. Am Iright if

> I assume that all DBM's have 1:1:1 trifiliar transformers, or are somedone

> with different ratios for special purposes?and

>

> Is it just that all the ports have to be terminated at the same impedance,

> when you've got any one at 50 ohms then the others have to be at 50 ohmstoo?

>Both transformers are 1:1:1 (or 1:2CT), and in that configuration the

load impedance of the centre tap of the secondary is the same as that of the

primary. The ends of the secondary are each 100 ohms. By placing the diode

ring between the secondary ends, both transformers see their proper load

impeedance as the diodes switch on and off in unison. Ideally, both centre

taps should be terminated in 50 ohms, but just about everyone leaves this

out on the one transformer while the opposite centre tap is the IF output.

Terminating both ports properly provides for the best isolation between the

two secondary ends.

>still be

> If I terminated all ports at 20 ohms or 100 ohms or 400 ohms, would it

> a good DBM? (With obviously different LO drive levels for optimumperformance).

>Yes, it would to a degree, however as you go lower in the impedance the

bulk resistance of the diodes comes in to play. Also, the transformers

themselves are usually designed to accomodate a 50-ohms system so their will

be some amount of contributory degredation from those as well.

Chris

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