Isolator in VCO output
It will reduce the effects of impedance mismatch, but not eliminate them entirely. You should get about 20dB of isolation.
If you use a circulator don't forget to terminate the unused connector
Peter
Gain is cheap (at low powers) so you are better off using a 10 dB pad + 10 dB gain and you'll easily get 30 dB of isolation, cheaper & smaller than a circulator.
I'm sorry what do you mean with a "gain" ? I'm not familiarized with RF lab accesories.
In ADS there is a circulator and a isolator. What should I fill out in the fields Z1, Z2, Z3 or what are the most important parameters ? My intention is to place one of these devices between a VCO and a 50Ω load.
I didn't know you were talking about simulation. An isolator would be ideal, just leave the default values; 100dB is plenty of isolation.
For a real VCO circuit an isolator is impractical so one would use an isolation amplifier, which is just attenuator+amplifier+attenuator. The goal is high reverse isolation, however not much foward gain is needed.
thanks, and what about the Z values in the ADS components ?
It doesn't matter the amplitude of my signal, I just want the frequency against voltage control that's why I asked for isolators and circulators.
I beleive just leave tham blank and it will default to 50 ohms. If you are just doing a simulation then you don't need an isolator if you are just terminating the VCO with 50 ohms.
well, actually with a capacitive load there is no problem but with a 50Ω load emulating a spectrum analyzer there is no output signal. I do not have an output buffer that's why I'm looking for a device that allows the circuit not to be charged.
OK, I assumed the VCO could drive 50 ohms. You can use a VCVS with high input impedance (1E9), a gain of 2, and a 50 ohm output impedance. That should give you a frequency independant, unity gain buffer with high input impedance and 50 ohm output impedance.
ok thanks, it's a good idea for simulation, but I have the VCO already fabricated and need to implement a solution in laboratory,
Sounds like you need a high impedance probe. I have one of these and it works well, but may be too big for your application if it is an IC.
http://cp.literature.agilent.com/lit...2696-92000.pdf
For ICs I have made a poor man's high impedance probe. I took a tugnsten needle probe and clipped the tip off about 1mm from the end. Then took a 1k ohm SMT resistor and silver epoxied it between the tip and probe. This was primarly for DC probing of RF traces. It may be usefull to a few GHz. There will be lot's of inductance and loss but considering a spectrum analyzer has 100 dB dynmaic range it may be usefull for simple measurements.
I'm sure there are high impedance active probes for IC use but they are probably expensive.
Maybe a simple scope probe will work too.