VCO Driving a LPF-Stopband refection problem
VCO Driving a LPF-Stopband refection problem
(OP)
I have the output of a MiniCircuits ROS 1500 VCO (output approx 9dbm) feeding into a MiniCircuits LFCN-1325 low pass filter. In the stopband the VSWR of the filter is 40. Since this will cause large refected signals in the stopband, and since the first harmoinic of the VCO is at about 0dbm, will there be any problems in the VCO with non linearities creating unwanted signals. I know there is a frequency pulling issue, but for my application frequency pulling is acceptable.
By the way, I can't afford to use a pad to eliminate the problem because I need to keep the VCO signal level high enough to drive a mixer.
By the way, I can't afford to use a pad to eliminate the problem because I need to keep the VCO signal level high enough to drive a mixer.





RE: VCO Driving a LPF-Stopband refection problem
I am not sure what this "frequency pulling" effect is you are mentioning. Perhaps you could explain.
By the "first harmonic", I assume you mean the second harmonic in standard terminology. The fundamental would be the first overtone or the first harmonic according to standard terminology.
A VSWR of 40 means a reflection coefficient pretty close to unity. Then any reflected signal will get absorbed by the VCO (whose output reflection coefficient is not stated). Have you tried this system or are you trying to decide if it will work on paper before buying? Trying it should resolve the problem. Failing that, ask mini-circuits.
Suppose you take the reflection coefficient of the filter down from 1 to 0.5 by buying a better one. The VSWR has gone from 40 to 3, but the reflected signal only halved.
RE: VCO Driving a LPF-Stopband refection problem
RE: VCO Driving a LPF-Stopband refection problem
I am laying out a prototype PCB at this point. I talked to MiniCircuits and they were not sure how much of a mismatch their VCO could tolerate. They said that " a VSWR of less than three should be OK". When I probed the underlying mechanism of why mismatch caused problems I did not get any coherent answer.
As to your better filter suggestion, any filter with with good stopband rejection will have a VSWR of at least 20 in the stopband. One colleague of mine suggested I use a diplexer to absorb the stopband signals. This would probably work, but would add to the circuit complexity.
Your suggestion of a 7dbm mixer is probably the best solution. Any feel for how low below the local oscillator spec I can go and still get linear mixing? Maybe 7dbm is enough for the 10dbm mixer I want to use?
Thanks for your suggestions
RE: VCO Driving a LPF-Stopband refection problem
RE: VCO Driving a LPF-Stopband refection problem
<nbucska@pcperipherals DOT com> subj: eng-tips
RE: VCO Driving a LPF-Stopband refection problem
Thanks for the explanation of the frequency pulling. I checked the mini-circuits glossary and it agrees with what you said. What I didn’t understand is where it comes into your thinking. Your load is constant at a given frequency so the frequency pulling affect is just a frequency calibration error, which is pretty uncertain in a VCO anyway. You said it wasn’t a problem so why am I still going on …
When you said that a filter necessarily has lousy VSWR outside its passband I felt uneasy about the factualness of this assertion. Take a look at the Mini Circuits application notes section on filters, the note on constant impedance filters. The net result is that it is possible to make (and presumably to also buy) filters with reasonable return loss outside the passband.
I was just thinking of the internal construction in terms of paralleling a low pass and a high pass filter with equal corner frequencies. The high pass drives a dummy load and this will absorb the unwanted HF, rather than reflecting it.
I can’t quite get my head around your desire to use a level 10 mixer with a 7dBm signal. Why not just use a level 7 mixer? At 3dB below the designed mixer level you should not get any problems, just 0.5dB-1dB conversion loss. (See mini-circuits app note "How to select the proper mixer").
RE: VCO Driving a LPF-Stopband refection problem
RE: VCO Driving a LPF-Stopband refection problem
The main reason I want to use a 10dbm mixer is that I happen to have a few of these in my parts stash.
Your suggestion of the series RC is basically the diplexer scheme I mentioned earlier. It should work but adds a little complexity. I am trying to keep the design as simple as possible. Maybe the right approach is for me to layout the PCB to allow the extra components, and then try the circuit first without the the RC loaded to the board.
I still would rather find an explanation for the mechanism that describes what happens in the VCO when there is a large reflection coefficient. Then I could decide more intelligently what the right approach should be.
Thanks again for your help.
RE: VCO Driving a LPF-Stopband refection problem
RE: VCO Driving a LPF-Stopband refection problem
Sounds interesting. Is there a reference text you can refer me to that elaborates on this? I'm a long time engineer but relatively new to RF.
Thanks
RE: VCO Driving a LPF-Stopband refection problem
RE: VCO Driving a LPF-Stopband refection problem