VCO stability
Jay Kickliter
Posts: 446
I'm new RF design, so please bear with me. I have a situation where I want to do FM on one frequency only. I've read a lot about Phase-Locked Loops, but it seems to me that it's pretty difficult to tune their charge pump filters to get a lock yet not fight with the FM.
My question is if it is feasible to use some sort of a precision voltage reference, divide it to get the voltage for the desired carrier frequency, and sum that voltage with the information signal to feed into the VCO? This is assuming a commercial VCO, not a homemade Colpitts oscillator. Almost all of the circuits I see out there are either unstable (subject to frequency drift), or use a PLL.
If PLL is the only way to go, can someone recommend a chip that is easy to interface with, and will be good for the 2-meter band?
My question is if it is feasible to use some sort of a precision voltage reference, divide it to get the voltage for the desired carrier frequency, and sum that voltage with the information signal to feed into the VCO? This is assuming a commercial VCO, not a homemade Colpitts oscillator. Almost all of the circuits I see out there are either unstable (subject to frequency drift), or use a PLL.
If PLL is the only way to go, can someone recommend a chip that is easy to interface with, and will be good for the 2-meter band?
Comments
All PLLs rely on low-pass filtering in the feedback loop for stability. If the audio modulating frequency summed into the VCO's control input is above the filter's cut-off frequency, you should be able to produce FM without the PLL fighting to regain control. It will simply filter out the modulation and use the average frequency to maintain a lock. Of course, if the filter's time constant is too long, it will take longer to QSY and lock onto a new operating frequency, so there is a small tradeoff.
I'm not sure what to recommend for a VHF VCO. Most of my experimenting has been in HF bands:
One thing I cannot recommend is using a Propeller counter in PLL mode. Although it will get you to the 2-meter band, and you can do FM, there's too much phase jitter, which causes spurious sidebands throughout the entire band (and outside) that cannot be filtered.
You might be able to generate your FM signal at a lower frequency using the PLL/FLL method noted in my thread, then run it through a mixer to reach 144+ MHz.
-Phil
Ettus Research at www.ettus.com is a really interesting supplier.
These days, it is very challenging to build RF items from scratch and there is a heck of a lot that Ettus does to eliminate the drudgery of involved. That leave more time and energy to explore what is one the airwaves, how a better antenna might help, and so on.
The key word here is 'low budget'.
http://www.minicircuits.com/products/vco_sm_5v.shtml
Use one of those with a PLL chip:
http://www.fujitsu.com/global/services/microelectronics/product/assp/pll/
You will need an amateur radio license to transmit on the 2m band, of course.
-Phil
You can use a fixed frequency and phase modulate it with a network of variable capacitance diodes and inductors of the appropriate values for the frequency you are using. This will be demodulated by an fm rx as if it was fm, If you are transmitting low speed data, this will not work as it does not have dc capability.
Actually, I already have a VCO on order I'm going to evaluate. It's the one I linked to in my first post. It's pretty close to the 2 meter band.
For now I want to use traditional hardware, and maybe try using the Prop for greater control later. I'm not building anything specific; just learning. However, I do like your idea of generating the signal on the Prop and mixing it up.
Oh, just yesterday I saw that my FCC Technician call sign was posted. I took the test about a week ago.
is one-half or less of your lowest modulating frequency.
-Phil
Can you post a schematic for the transmitter?
-Phil
It's a little different than most of the FM circuits I've found on the internet. Most seem to have the top cap in parallel with the transistor and and the bottom cap from the emitter to ground.
Here's the most comprehensive document I've found reguarding PLL's: http://web.itu.edu.tr/~pazarci/pll/NSC_PLL_Handbook_DeansBook_4_01.pdf A little too comprehensive, it's a bit much to digest.
Alternatively a series of harmonic frequency multiplier stages can be used, but thats more complex than a PLL divider chip these days.
This won't work with broadcast FM though as its very wideband and crystals can only be pulled a small amount. For narrowband FM at VHF frequencies its pretty good (except that you need special crystals with a simple PLL). Using a sigma-delta PLL with fractional multiplication would fix that, but is more complicated (and may generate more spurious outputs).
If I read you correctly, what you propose is to have a VCO running at the final VHF frequency, and divide that down to a feedback frequency that the Prop can handle for doing the VCO control. Right? That should work. Just keep in mind that any frequency or phase errors will be multiplied by the divider ratio. 'Still worth an experiment, though!
-Phil