Convert series resonant to parallel resonant circuit - Page 1
Forum section: electronics / projects

author: richsf
author_rank: Contributor
timestamp: May 21, 2025, 01:09:38 pm
content: Some background context: I've including an image of the a second local oscillator of a radio, with a PLL front end that converts the tuned frequency in 5kHz steps to a 45MHz IF which has a 30kHz bandwidth 45MHz monolythic crystal filter. Then the 2nd LO tunes +/- 2.5 kHz using a D-A and varactor diode (D19 along with L6) to 'Pull' the crystal oscillator in order to tune within the IF down to 0.1 kHz resolution (purportedly). The CPU to D-A IF tuning is open loop and calibrated in an alignment procedure. Its a Radio Shack DX-394B, though I think the schematic of the LO is all one needs to look at for this question.The crystal X1 was missing from the set from a prior owner. A 44.545 MHz crystal was obtained from Mouser, however the 44.545 spec is for its parallel mode. This circuit seems to expect a series mode crystal at 44.545 MHz, so the parallel mode spec crystal is oscillating at a frequency too low, not surprisingly, and can not be pulled with the existing circuit to 44.545 without stopping the oscillator.Unable to find a suitable series spec'd crystal ( great if someone can ) I am thinking maybe it is possible to convert this circuit to operate with the parallel mode crystal. I don't have RF experience, so could use some insights.I'm sure this was a 3rd overtone design, and D19 and L6 help select that mode as well as calibrate the tuning. I don't understand L7 -- in conjunction with C77 / 22 p it probably would suppress some unwanted spurious oscillation mode at a specific frequency, maybe that was the purpose. I'm thinking this is a Colpitts oscillator, is that correct ?I've looked for VCXO parts (nothing suitable found) and considered just building a new parallel mode LO.The colored markup is mine, just to help my eyes follow the schematic. LOLThanks for any help or educational remarks.

author: Terry Bites
author_rank: Super Contributor
timestamp: May 21, 2025, 01:37:48 pm
content: https://www.edn.com/crystal-series-and-parallel-resonances/Yes, but the resonant frequency will be shifted a bit. How much of that can you live with?

author: richsf
author_rank: Contributor
timestamp: May 21, 2025, 01:53:03 pm
content: Thanks for the response. Actually it is the goal to shift the resonance higher. The circuit is using a parallel resonance specified crystal as though it was a series resonance crystal. So, the resulting frequency is too low.

author: TimFox
author_rank: Super Contributor
timestamp: May 21, 2025, 04:17:32 pm
content: The normal model for an oscillator crystal is a set of series resonances (fundamental and odd-order overtones) in parallel with each other and a shunt capacitance (including the holder and oscillator circuit capacitance).This gives a fundamental-mode series resonance, a fundamental-mode parallel resonance, etc. in a graph of reactance vs. increasing frequency.The parallel resonance frequency is slightly higher than the series resonance for each mode: the equivalent series capacitance within the series-resonant mode is much lower than the shunt capacitance.At each overtone, there will be a series-resonant mode, and a nearby parallel resonant mode from the additional shunt capacitance with that series-resonant circuit.Note that the odd mechanical overtones areapproximatelyan odd integer multiple of the fundamental, but not an exact harmonic.However, harmonic distortion in the oscillator active circuit will give output frequency components at integer multiples of the actual oscillation frequency.Normally, an additional tuned circuit in the oscillator circuit is required to select a desired overtone frequency.

author: antenna
author_rank: Frequent Contributor
timestamp: May 22, 2025, 03:48:21 am
content: In that circuit, L6 will lower the resonant frequency. Remove the slug and try again. If that don't work, short across L6 to effectively remove L6 from the circuit. Brass screws can lower the inductance if placed where the slug is, but that's not the best solution.L7, from the literature I have read, looks a little small. It is chosen to resonate with C77, but not at the desired overtone, rather somewhere between the desired overtone and the fundamental. There was a pdf about crystal oscillators shared here (Crystal_Oscillator_Circuits_Krieger_Matthys.pdf), and that book says to make L have a reactance equal to what a resistor would be based on the formula RC=.5/f with C in uF and F in MHz. So, if .5/44.5=.011236, then divide that by .000022uF, you get 510ohms (reactance in this case). 510/(2pi*44.5MC)=1.8uH. If attacking L6 doesn't get you there, maybe swapping L7 for 1.5uH to 1.8uH would help. This may not work, but that's what I remember reading.

author: richsf
author_rank: Contributor
timestamp: May 22, 2025, 12:32:27 pm
content: Thanks to all. And I need to read through pertinent parts of that book, it looks promising. Also, I'm going to try to set up a simulation of the circuit so I can learn more about it. I saw several videos of how to find the model parameters using a nanoVNA which I have. I didn't find a manufacturer's model.AND, this circuit is now working with a mod similar to one of your suggestions, a 100 pF cap was put soldered from the X1 L6 junction to ground. So now it is very close to the correct frequency, however the LO pulling voltage is ineffective. So, OK for stations on a 5kHz boundary, but not so good for SSB of course. It is a kludge, but OK for now.Thanks

author: antenna
author_rank: Frequent Contributor
timestamp: May 22, 2025, 08:54:07 pm
content: Try putting it across just the coil instead When you go from the crystal to ground with a capacitor, you are also bypassing the varactor diode (which is what does the fine adjustments).Also, by using 100pF in series with the varactor (once you move it), you will be decreasing the overall capacitance range of that varactor diode because two capacitors in series are less capacitance than either by itself. Try bypassing just L6 with a larger capacitor, maybe 1nF, so its effects on the varactor are minimized.The sole purpose of L6 is to shift frequency down from the value printed on the crystal because capacitors canl only increase it. The sole purpose of the varactor is to make the crystal frequency adjustable, so don't bypass that.Just swap out that capacitor with 1nF (or a short jumper wire), and only go across L6 with it..

author: antenna
author_rank: Frequent Contributor
timestamp: May 23, 2025, 05:09:53 am
content: Adding to my last post:If you really wanted to do it right, you could physically remove L6 and use its spot to place a trim cap to ground and then have the varactor parallel to that instead of in series. This would allow for initial alignment with the trimmer and good varactor control.

author: richsf
author_rank: Contributor
timestamp: May 23, 2025, 12:41:02 pm
content: Antenna, Thanks for the suggestion, I'll have to think about that. I really don't know enough about this area and the book Krieger_Matthys is very helpful. For educational reasons, I think I'll either try to simulate this or breadboard it, pretty simple circuit. I wish the manufacturer would provide a model of their crystals, but I did see a video of how to find the 4 model parameters using a nanoVNA. Another learning experience.From this thread's mention and the book I understand the purpose and value of L7 in the emitter now. Progress ! L7 is about having frequency dependent impedance instead of frequency independent resistor, so that the 3rd harmonic oscillation is favored. Since we are now using a 44.545MHzFundamental, maybe that could be replaced with a resistor, but on the other hand it probably does no harm. The original design had both R88 / 1k and the inductor L7 in the emitter path.

author: antenna
author_rank: Frequent Contributor
timestamp: May 24, 2025, 02:11:10 am
content: I highly doubt that a 44.545MHz crystal is available as a fundamental, how are you coming to that conclusion? I don't think they make fundamental crystals that high. About 20MHz is the highest fundamental crystals go (they get too thin to be practical above that).The 1k resistor is for DC biasing and serves a couple functions. It stabilizes bias against temp changes, allows for setting the operating point independent of transistor gain (with a divider bias), and it establishes an RC time constant. The time constant is set to ensure the emitter is up to the proper level within a couple cycles (or the oscillation will not start).When switching from a fundamental to an overtone, the inductor is needed for reasons you have already read about, but it doesn't eliminate the DC biasing requirements. Simply replacing the resistor with an inductor brings the emitter to DC ground, which you don't want. The book simplifies things a bit. The author figures that someone reading that book is well familiar with DC biasing and chose not to focus on that.Consider your circuit for a moment. The bias resistors puts 2v on the base. One diode drop later (the b-e junction) puts about 1.3v on the emitter. Can you imagine the current needed to put a constant 1.3v across an inductor with low DC resistance? Ouch. Later in that book, you will find a section about startup problems, and that will talk about the need for the oscillator to be in its active region to ensure startup. With the emitter grounded, its not in its active region.I've spent a good amout of time measuring crystal parameters and putting them in simsmith to see the effects of changing what's in series with them. I recommend modeling it, the insight you will gain playing with that model will change your oscillator game forever. Once you get the basics down and can make oscillators work on your first try, you can then move on to something funner. That nanoVNA you have, with a dc blocking cap (must have), you can look at the S11 of the oscillator and see the frequencies where the resistance goes negative (where the circuit has gain), and fine tune that negative resistance dip to your frequency ensuring other modes of oscillation at different spurs don't start. Sometimes you can sacrafice a little negative resistance where you need it to eliminate it where its causing problems.Sorry I rambled on so long.

author: aeg
author_rank: Regular Contributor
timestamp: May 24, 2025, 02:57:11 am
content: These days you can get AT-cut crystals up to about 50MHz fundamental.

author: antenna
author_rank: Frequent Contributor
timestamp: May 24, 2025, 03:45:04 am
content: Quote from: aeg on May 24, 2025, 02:57:11 amThanks, I did not know that. I would love to see what it takes to wreck one just to know how they hold up. Do they handle the same power levels compared to their overtone counterparts?I guess he could just jump L7 and see if it runs at the right frequency. Easy enough to test without pulling components.edit: might have to drop that 1k resistor a little if it dont work though.

author: richsf
author_rank: Contributor
timestamp: May 24, 2025, 01:20:00 pm
content: I have to go back and digest your longer post. I appreciate the help.Here is the mouser # I am using. They have another frequency that could be used for a 3rd harmonic which I wish I had bought. They are tiny. 100 micro W max spec sheet says.https://www.mouser.com/ProductDetail/774-406I35B44M54500Can't complain about the price either.

author: TimFox
author_rank: Super Contributor
timestamp: May 24, 2025, 02:34:50 pm
content: Don’t confuse “third overtone” with “third harmonic” for crystals.See my post higher up.A 40 MHz crystal is probably 3rd overtone, at a slightly different frequency than the third harmonic of the fundamental resonance mode.