How do i reduce the drivelevel of the crystal to 100 uW? - Page 1
Forum section: electronics / beginners

author: Scarlett133
author_rank: Contributor
timestamp: January 26, 2025, 10:21:59 am
content: How do i reduce the drivelevel of the crystal to 100 uW? Possibly with an Resistor?

author: Circlotron
author_rank: Super Contributor
timestamp: January 26, 2025, 10:57:28 am
content: Reduce the supply voltage probably. My guess is half supply voltage will reduce power output down to one quarter.

author: Scarlett133
author_rank: Contributor
timestamp: January 26, 2025, 12:24:24 pm
content: No i meant the crystal oscillator, it needs 100 uW typically to much and it will break, i would try to use some resistors but i dont know which yk

author: Marco
author_rank: Super Contributor
timestamp: January 26, 2025, 02:46:38 pm
content: There is no voltage amplification, the output power is set by the output voltage swing, the crystal power is also set by the output voltage swing (and crystal resistance at resonance). There is no knob to turn to lower voltage/power across the crystal without lowering the supply voltage.If you want to use some tiny crystal with a lower voltage swing, but have the same output power to the antenna, you'll need a different circuit.

author: CaptDon
author_rank: Super Contributor
timestamp: January 26, 2025, 03:40:15 pm
content: First off, that is a horrible circuit, probably need an RF bypass capacitor on the key side of the RFC, second, it is a horrible circuit. Why reduce crystal drive to 100 microwatts and how do you know the crystal can't endure substantially more? 100 microwatts is .1 milliwatt. I don't know of any crystal that physically couldn't handle at least 1 milliwatt and 10 milliwatts being far more typical, a factor of 10X or 100X your spec. Is power conservation an issue here? Supposed to run on a AA cell for a year? B.T.W., F.C.C. part 95 regulations specify 100 milliwatts D.C. input power so if you are making a little unlicensed QRP transmitter you have lot's of room to increase the power. What is the goal of this project?

author: Scarlett133
author_rank: Contributor
timestamp: January 26, 2025, 04:34:35 pm
content: Well i was trying to build something small and Simple, i looked at JLCPCB Parts and they only have so low power Crystals yk

author: SteveThackery
author_rank: Super Contributor
timestamp: January 26, 2025, 05:41:31 pm
content: Quote from: CaptDon on January 26, 2025, 03:40:15 pmWatch crystals. For example the LFXTAL002997 has a maximum power rating of 1.0uW.

author: ferdieCX
author_rank: Regular Contributor
timestamp: January 26, 2025, 06:00:59 pm
content: Hi,You could find this AN useful

author: D2236
author_rank: Regular Contributor
timestamp: January 26, 2025, 06:09:49 pm
content: Sometimes drive current is really important, like the case with accurate time-keeping. You should always know the specs of the crystal, to work with the typical ESR value stated in the datasheet.https://www.we-online.com/en/news-center/blog?d=oscillatingcrystalsparametersThis can also be measured if you have the equipment available, like a VNA. With the ESR known, you can compute the power in the crystal with measuring drive signal voltages applied over the crystal.Here is advice from the late Jim Williams on crystal current concerns.

author: TimFox
author_rank: Super Contributor
timestamp: January 26, 2025, 06:33:31 pm
content: How do you know the drive level to the crystal? 700 mW is apparently the total power to the transistor circuit.If you have the crystal specs and you are running at series resonance, then you can calculate it from the AC voltage across the crystal and the crystal's series resistance.

author: CaptDon
author_rank: Super Contributor
timestamp: January 26, 2025, 08:48:38 pm
content: I looked up the HC49 style crystals at 16MHz and I see they do spec 100uw and 500uw drive levels. I guess they maintain a certain frequency and aging tolerance at those very low drive levels. I need to look through my stash of similar crystals and see just how much drive it takes to wreck one. I have run many styles of crystals from 100KHz to 20MHz fundamental frequency in my 18vdc powered Pierce oscillator and never hurt one. I don't know what the apparent drive level is, but it is surely closer to at least 1mW or more. I guess I'll have some fun trying to wreck a crystal.

author: Scarlett133
author_rank: Contributor
timestamp: January 30, 2025, 02:54:42 pm
content: I built the transmitter and it works like an charm, i think its AC drive level, like from the amplification of the Transistor

author: CaptDon
author_rank: Super Contributor
timestamp: January 30, 2025, 03:21:23 pm
content: I can't imagine what the drive level of the old ham radio style (FT-243?) crystals were? A single tank tunable 6AG7 oscillator (Marked as Grid Drive) driving an 807 with 75 watts input, 50 watts output. Makes me wonder about an untuned Pierce crystal controlled power oscillator using a 6C4 producing around 100 milliwatts of drive into a buffer / amplifier. When we say 'Drive Level' are we talking about the apparent power dissipated by the crystal?? I can see 'Drive Level' could have some ambiguous meanings? In the vacuum tube Pierce oscillator each side of the crystal could be moving 40 volts pk to pk in reference to chassis ground, but at resonance the crystal impedance is very low so I guess the calculated I2R loss is also low. You could take one of those old 'diode / microampmeter / antenna' field strength meters and hold it against the case of those old crystals and peg the meter during key down.

author: Scarlett133
author_rank: Contributor
timestamp: January 30, 2025, 03:47:44 pm
content: Ill try that and share the results

author: TimFox
author_rank: Super Contributor
timestamp: January 30, 2025, 04:32:40 pm
content: Quote from: CaptDon on January 30, 2025, 03:21:23 pmI have a fuzzy memory of a similar 6AG7-807 circuit with an FT-243 crystal that had a #47 (iirc) pilot lamp in series with the crystal that visibly glowed.

author: iMo
author_rank: Super Contributor
timestamp: January 30, 2025, 06:01:13 pm
content: In your circuit you cannot go down with the power at the crystal when you want 100mW (my assumption, doublecheck what is allowed in your country) into the antenna, because the transistor at 7MHz needs some "input power" as well. The power gain at 7MHz could be say 10 of that transistor.If you want lower crystal drive level you have to add a separate oscillator with a second transistor (low power), and the output of that oscillator you have to couple into the base of that "final" transistor (aka power amplifier "PA").That is the proper way how to build a CW morse qrp 40m band transmitter. Single transistor ones do not work well and the people on the 40m band will soon get unhappy..Btw., I would recommend you to find your local amateur radio club - go there, discuss, and then apply for the ham radio operator license..Below a typical two transistor transmitter with some qualities (and people will copy you around the globe).For 40m (7MHz) you have to use different values of L and C.You would need a ham operator license for it (its output power could be from say 1W to 8W based on settings and parts used), however..PS: and the same for 40m band..

author: Scarlett133
author_rank: Contributor
timestamp: January 30, 2025, 06:37:16 pm
content: Thanks i soon will, just want to experiment a little, i would never try to transmit without an filter anyways, always dummyload!

author: iMo
author_rank: Super Contributor
timestamp: January 30, 2025, 06:38:51 pm
content: Quote from: Scarlett133 on January 30, 2025, 06:37:16 pm

author: Scarlett133
author_rank: Contributor
timestamp: January 30, 2025, 06:48:39 pm
content: Guys i made an Improvement, with an real Colpitts Oscillator now

author: CaptDon
author_rank: Super Contributor
timestamp: January 30, 2025, 11:04:32 pm
content: Tim, wow, I forgot about those 'crystal current' light bulbs!! I also remember some military gear that had a selector switch with an 'Xtal Current' position and the meter scale was 0-50ma. The 'Xtal Current' meters on Radar receivers was a measure of local oscillator injection into the mixer and crystal health 1N21 for the 1-2GHz stuff and 1N23E(R) for Xband. Times have changed.

author: Scarlett133
author_rank: Contributor
timestamp: January 31, 2025, 01:16:48 pm
content: Thats a lot of current!

author: ftg
author_rank: Frequent Contributor
timestamp: January 31, 2025, 02:14:36 pm
content: Quote from: CaptDon on January 30, 2025, 03:21:23 pmWell, the single tube crystal controlled power oscillators were called "rock crushers" for a reason.A style not really compatible with small modern crystals.Quote from: CaptDon on January 30, 2025, 11:04:32 pmAnd in hobbyist use 1N21 works also for X-band (and C-band).