In e.g. VCO design, assuming a less than ideal varactor/varicap, is there a particular resonant circuit/tank topology or ratio of components where given a base operating frequency with no control voltage applied, the VCO frequency deviation either side is maximized with respect to change in varicap capacitance?
Hmm well besides just having the varicap as the only capacitance in circuit.. Freq. ~ 1/sqrt(L*C)... I'm not sure what else you are looking for. (I've never made a varicap VCO.) George H.
Hi George, thanks for getting back, to clarify I meant "tank" in a kind of a general sense for resonant circuit topologies. You can have different kind of tanks like series, parallel, ones with tapped inductors or tapped capacitors and so forth.
I was curious if any particular topology of that type when used as the oscillator frequency determining network was more amenable to maximizing deviation with respect to the tuning capacitance value than some other one
On a sunny day (Fri, 9 Aug 2019 12:39:39 -0400) it happened bitrex wrote in :
If you want the varicap to have maximum effect it middle range value should be the only capacitance around in the LC tuned circuit. If there are other Cs in the circuit such as coil winding capacitance, trimmers, feedback capacitors, Cce, Ccb, etc. then the effect of the varicap is reduced. You can use varicaps in parallel though.
You seem to be proposing to forward-bias the varicap?
A schematic sketch would help.
Some capacitive multiplier that worked at AC? If it's opamp frequency, I wonder if one of those two opamp GIC's (general impedance converter) could make the capacitance look bigger.... That doesn't really help though, cause the frequency range is related to the proportional change in C. As Jan said keep all other stray C small.
George H.
Either side of what?
As others have suggested, it's not clear that you're asking for anything besides "make the variable part be as much of the total as you can".
If you want more dF/dV, you can always run it in the GHz then mix it down. Deviation of many decades, if you like.
You can probably come up with a tank where dF/dV is arbitrarily high in some local range (but tending to constrain the average value over the full control range, I would suspect).
You can certainly come up with one where the F(V) curve goes through hysteresis loops (more commonly described as, jumping from one resonant mode to another). The dF/dV would be, well, you might argue it's infinite at the edges of such a loop, but "undefined" might be more accurate.
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website: https://www.seventransistorlabs.com/
Or put variable feedback on the cold end. That way you could use a pot, a fast op amp, and a fixed cap. ;)
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
The problem is that more or less I have an inductor of a given size that's pretty much fixed by physical requirements, and a variable capacitor that's in the some-pF range that it is, and only wants to vary so much in response to a signal.
But I don't want to run the tank at the frequency it wants to run at given the constraints so I have to add some series or parallel fixed elements to bring it down in frequency. But then naturally I don't get enough dF/dV as I'd like because when the frequency comes down so does that.
I didn't know if there was a particular resonant circuit/oscillator topology that would let me cheat there somehow (there are a lot of ways to connect up RLCs in a feedback loop and make them oscillate and I don't know the dF/dV for all of them, if one of the elements is variable, off the top of my head) or if I should resort to down-converting as TW suggested, or perhaps do it indirectly by running the main oscillator at a high frequency and generating a CV for a VCO in a PLL or somesuch.
it's slow enough freqs (MHz) that I can use some op amps.
Oooh I see what you're saying. C multiplier. Oooooh that might work.
On a sunny day (Fri, 9 Aug 2019 18:06:17 -0400) it happened bitrex wrote in :
74HC4046? Works from 3V upwards. Always have some in stock here.
I assume that you are asking how to maximize the tuning range.
On an LC circuit with a fixed L in order to get a specific frequency range, the capacitance range must be squared. So to get 1:3 frequency range, you need a 9:1 capacitance range. Minimizing any stray capacitances helps extending the high frequency limit.
On the high capacitance side, when he control voltage is low, compare this to the RF voltage present. The RF voltage will change the capacitance during the RF cycle. In the worst case the capacitance diode becomes forward biased during the RF-cycle, loading the resonance. The phase noise performance may suffer badly.
One thing that may help is using a dual capacitance diode (common cathode), In this case one diode is reverse biased and acts a capacitor while the other diode is forward biased acting as a resistor and during the opposite RF cycle the role of the diodes are reversed. This distorts the RF waveform symmetrically.even at zero control voltage.
At least with only a single capacitance diode there should be a low limit for the control voltage compared to RF voltage, so that the diode is always reverse biased.
The RF voltage can be an issue at least in receivers. Some high quality receivers drop the front section impedance level to 25 or 12 ohms instead of the ordinary 50 ohms. This also drops the RF voltage compared to lowest capacitance diode control voltage, reducing the risk for spurious mixing products, mainly the second order.
In an oscillator, you may have to look at the RF voltage levels compared to the capacitance control voltage.
Of course if you can also change the L e.g. by a variable DC magnetic field on some suitable ferrite material will also help extending the frequency range.
On a sunny day (Sat, 10 Aug 2019 07:57:42 +0300) it happened snipped-for-privacy@downunder.com wrote in :
If you do not have 2 varicaps I have uses transistors Ccb and Cbe
I hAVe nOt AnALyZeD IT BuT IT wOrKS
Yeah, that part has about the crappiest oscillator in history.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Well depends on what you define as 'crappy' It is great for audio modulation and demodulation (PLL) and a zillion other things. No idea what the OP wants to do, and if it MUST be a LC. You better respect the CD4046 74HC4046 and 74HCT4046, it is more popular than the 555 almost ? ;-)
The metal-gate 4046 is quite a nice part for what it is. Its oscillator tunes more than 100:1 with pretty good linearity. Its two real warts are the very poor predictability of the oscillation frequency and the deadband in the phase-frequency detector PD2, helpfully positioned right at the servo point. You can fix that by adding a resistor to ground to pull the servo point off the deadband, but there's nothing you can do about the poorly controlled oscillation frequency except make the deviation range very large.
All the HC versions have _horrible_ oscillators: they're wildly nonlinear (up to 5:1 changes in slope), and they just quit oscillating for control voltages below a volt or so. (Some slightly better, some a bit worse.) A couple of the glorified versions (7046 and 9046) claim that their PDs don't have the dead band. However, the dead band is easily worked around and their oscillators still stink on ice.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
On a sunny day (Sat, 10 Aug 2019 05:38:11 -0400) it happened Phil Hobbs wrote in :
snif snif, will have to throw all those 47HC4046 away... Anyways raspberry can also make all sorts of frequencies. Somebody even makes DVB signals with one, WITHOUT any external parts added...
The original 4046 oscillator worked over a large range of voltages, and the Philips 74HC and 74HCT didn't. The 74CT9046 did have an improved pahse detector.
-- Bill Sloman, Sydney
Am 09.08.19 um 18:39 schrieb bitrex:
It would help to know your VCO design.
If it is the usual VHF VCO, then almost all use a CC circuit: collector at RF ground, maybe through a small resistor to get some output energy, emitter high impedance, most often capacitively loaded and some series resonance in the base. That is the textbook case of an oscillating follower.
If you measure into the base, you see a capacitor in series with a negative resistance. That un-damps anything inductive in the base connection unless there is a large enough positive resistance to cancel the negative one. If the circuit is meant as an amplifier that is the job of the base/gate/grid stopper.
If you have a varicap in the base LC, its tuning range is reduced by the series capacitance of the BE impedance. That may be quite small. I once did try to pull down the range of a Z-Comm VCO. That worked to some extend, but there was a frequency I could not get below, even with large ceramic caps. That was the frequency of the base resonator stripline and the BE capacitance.
There are 2 oscillator books by one Randall Rhea; he described the working of the VCO quite well. It seems that the transistor type is quite important for both the negative resistance and also for the series BE capacitance. Rhea is the author of the Genesys system that is now part of Agilent ADS, eh, Keysight.
On the positive side, the varicap inductance is absorbed by the stripline.
There are newer varicaps that are optimized for large capacitance ratio. Take a look at Infineon, for example.
At low tuning voltages, dC/dV is greatest. Unfortunately the Q is worst there, also. Avoid having them in forward direction even for a short part of the cycle. The usual trick with anti-series diodes helps.
Rohde says that many small diodes in par. yield better phase noise than a large one because of averaging. While I'm normally happy to plow with the power of a thousand chicken, I'm not really convinced. It cannot make a difference when I connect several subdiodes onchip vs. onboard.
cheers, Gerhard
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