Re: wideband RF impedance matching

steve wrote:

> Tim,
>
> Yes, the impedance I gave was for the signal input. I'm trying to
> create a 0-500MHz frequency generator and I've got a 250-500MHz VCO, so
> I was going to mix the VCO output with a 250MHz LO signal from a
> MAX2620 fixed frequency oscillator.
>
> Thanks to everyone for their replys, they've been really helpful!
>
> Steve

The signal inputs go straight to the emitters of the switching transistors,
by the look of it. The impedance they give would be for small-signal
operation e.g. in a receiver. Do you mind if there are harmonics in the
input current? You could drive it hard, if you want lots of output power.
The impedance would be different then but it doesn't matter, all that
matters is the input current will come out the top of the mixing cell
chopped up by the LO waveform. If you just use a centre-tapped
mini-circuits transformer or something like that, with a resistor from the
centre tap to ground to sink 20mA or so out of the centre tap, that ought
to do. You wouldn't need much swing at the mixer inputs to fully switch
the current from one side to the other, 100-200mV ought to do it. You will
however need plenty of current so a transformer which steps down the
voltage and steps up the current should help. The only problem is that if
you drive it hard like this, then you will get other mixing products such
as 3LO +/- 3RF, which would be hard to get rid of. If you don't want these
then you could put resistors in between the transformer and the input pins
so that the current is sinusoidal again. The 3LO, 5LO etc. frequencies
will still be present in the LO however, because the limiting action of the
LO buffer will create them. For that reason, with your VCO, I would try
using a 500MHz fixed oscillator, so that 3LO+/- fVCO will be so high that
you can filter it out effectively.

You might want to reconsider the oscillator frequencies, I don't know if it
matters for you if there is some VCO leakage in the output. If you used a
750-1250MHz VCO and a 750MHz fixed oscillator, then it would be easier to
get rid of any LO and VCO leakage.

Chris
.

Relevant Pages

  • Re: wideband RF impedance matching
    ... the impedance I gave was for the signal input. ... create a 0-500MHz frequency generator and I've got a 250-500MHz VCO, ...
    (sci.electronics.design)
  • Re: wideband RF impedance matching
    ... >load) to an AD8343 mixer which has a 2.7+6.8j input impedance. ... The impedance you quote is for the signal input at 50MHz only ...
    (sci.electronics.design)