Message	Date	From	Subject
3009	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 N3QE
3010	2009-05-11 11:10:24	Chris Trask	Re: 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? > 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. > > 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). > 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 ,----------------------. High Performance Mixers and / What's all this \ Amplifiers for RF Communications / extinct stuff, anyhow? / \ _______,--------------' Chris Trask / N7ZWY _

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

> 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?

>

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.

>
> 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).

>

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

,----------------------. High Performance Mixers and
/ What's all this \ Amplifiers for RF Communications
/ extinct stuff, anyhow? /
\ _______,--------------' Chris Trask / N7ZWY
_