About a week ago, Joel KB6QVI, tipped me off to the fact that Cecil K5NWA was selling 7.28MHz ceramic resonators on his online site, The Parts Place. Enthused, we both ordered some, and both our orders arrived yesterday. Joel slipped his into a test circuit, which gave him coverage of 7.015 – 7.236MHz. He used a similar circuit to the one I used, though I don’t know his exact component values. Here’s the test circuit I used, which gave me a coverage of 7.080 – 7.268MHz –
Adjusting the values of the 470p and 270p capacitors to, say, 330 and 270, or both to 330, should give a higher maximum frequency. If I were using this in a simple rig, such as a Micro 40, Beach 40, or similar, I’d be using varactor tuning. The 1N4001 (the “poor man’s varactor”) can have a minimum capacitance of as little as 5pF with an 8V bias across it, which would help too. It looks like it could be a useful component for simple 40M phone rigs and with a bit of experimentation, I bet I could increase the maximum frequency and get it close to the top of the US 40M band at 7.3MHz. If you’re in a part of the world in which your 40M band ends at 7.2MHz, then some extra parallel capacitance should do the trick. It’s a 3-pin resonator. If the center pin is grounded, then the internal capacitors are placed in parallel with the resonator. Ignoring the center pin and connecting to the 2 outside pins only, leaves the internal capacitors out of circuit, increasing the maximum frequency.
If you have any interest at all, it wouldn’t hurt to stock up on a few. Resonators for frequencies like 3.58MHz seem to always be in plentiful supply, but ones with frequencies that are useful for the phone portion of 40M are harder to come by. Who knows how long these will be available for?
PS – as I mentioned in the previous post on this subject, these resonators are also available from Mouser, both in the US and the UK. Thanks to Steve G1KQH, for the tip. Here’s the link to them on Mouser’s site. If you set the site to the country you are in, then the link will show you the resonator on the Mouser site specific to your location.
Dave Richards AA7EE 
where do you measure/extract the oscillating frequency from?
Eugene –
I just lay a lead connected to the antenna jack of my receiver close to the circuit and listen for the signal on the receiver. If you wanted, you could take an output via a low value cap (47pF or similar – in that range) connected to the hot end of the 1K resistor in the emitter lead of the transistor. The transistor can be any general purpose low signal NPN type, such as 2N2222, 2N3904 etc.
Dave
AA7EE
cool, i’ll have to try building one
The proof is in the testing.
Mouser stock going quicker than that vortex takes your bath water down the plug hole.. I hope we will see some interesting 7MHz projects written up on your Blogs and sent into Sprat for members to build?
73
I don’t have any immediate plans to use mine Steve, but wanted some for the parts drawers “just in case”. It would be great if the availability of these resonators were to inspire a few more designs for QRP rigs though. Any takers? 🙂
I have no rush either Steve, but I bought up a few, if G3MFJ runs at GQRP out I am sure we can barter for a few eggs. Other info for Mouser, the UK price of the final order has VAT added to the final bill.. and rem plus carriage.. if under £50!
73 Steve
Mine arrived all they way from Texas US today, delivered to the UK 2 days from date of order. Miles is no barrier for parts for this hobby any longer thanks to the old internet, I will report back next week on my Blog the test results..
PS Mouser only have 57 left, but 2000 on order!
http://g1kqh.blogspot.co.uk/
73
That’s the same amount of time mine took to get to me in California Steve! Love the analog multimeter on your blog that you got from banggood. I have a large old analog one from Radio Shack that I’ve had for around 25 years. It still works fine, but doesn’t look quite as cool as yours.
So, with all the people buying these resonators, I wonder how many are getting them for QRP rigs?
Dave
AA7EE
I guess some have jumped on the 7.28MHz band wagon to make a Buck or two Steve? Not received the MF500B Multimeter yet, it is still stuck between here and China somewhere.. I will review that too when it arrives Phew! Too much to do not enough hours in the day.
Cheers
Mouser now lists these as obsolete!. Looks like we all got in at the right time Dave! I have still got to get around to running the test circuit with the 3 values I mamanged to obtain inc the 7.28Mhz ones.
Holy Moly Steve – Mouser had several hundred of them just a few days ago. I wonder if somebody bought a load of them for resale?
Dave
AA7EE
He who hesitates is lost! In this case we did publicize them, just a little bit 🙂
Steve – as of just now, Cecil has 444 available. Perhaps he bought up Mouser’s supply? http://www.thepartsplace.k5nwa.com/index.php?route=product/product&path=68&product_id=199
The idea of using the 7.3728 resonators as Dave mentioned in his DSB rig post seems fine as well but I will probably grab some of Cecil’s 7.28 ones as well. Mouser has over 800 of the 7.3728 ones in stock and you can pull them down into the band. Just be careful to not go out of band as it will be easier to do with these. http://www.mouser.com/ProductDetail/ECS/ZTA73728MT/?qs=DbvhANEtCxDj6UotdtZjkA==
huh, Cecil also has some 400+ 7.3728 resonators as well and he quotes “Tunes over a wide frequency range with 100 pf capacitor a shif of 7170KHz – 7410KHz was noted by Patrick W9PDS and a stability of 50Hz. Note that for a lower top frequency, put a fixed small polystyrene capacitor in parallel with the adjustable capacitor, which can be made smaller.”
This has certainly stirred the old thread back into action! I have 7.16MHz, 7.20MHz, and the 7.28MHz ones, all which I obtained from Mouser. The 7.37 would be no good here in the UK, and I doubt not that stable at the bottom end of their range? Happy Resonating!
73 G1KQH
Yeah Steve, 40M is tough. I saw some pretty neat looking 40M kits from the UK recently but was then like “wait, I can’t transmit this mode on that freq.”.
Yes Dave, we certainly talk different English both sides of the atlantic, a lot less bandwidth to play with here, and no 220MHz they gave that one away to DAB radio.. Never mind eh? 😦
73 G1KQH
I do have one question related to the topic but somewhat ashamed to ask as my theory is rusty as heck and I just don’t have much experience with ceramic resonators. However, what is the purpose of the third pin compared to those with only two? I note Dave’s schematic doesn’t reference the third pin. Is the center pin just a ground?
With CR’s that have a center pin, that is an indication that there are internal capacitors. If you don’t connect the center pin to anything, then you will be leaving the internal caps out of circuit. That’s what I do, then if I want to introduce external capacitance, I can do it with external NPO caps. Don’t be ashamed John – I didn’t know this until very recently either! I believe the internal caps are connected from each side of the resonator to the center pin.
Dave
AA7EE
ahh, make sense now. I’m going to try my best to replicate your version of the Micro 40. Interestingly enough, I didn’t realize Cecil had a stock pile of 7.28’s til I saw comments in this older post “trending” on your blog today. I was going to go with Mousers 7.3278’s but they are cheap enough I may just get 5 of each from Cecil.
Excellent! Build it bit by bit John. I built the LM386 AF amp first, and made sure it worked (finger to the input pin, and if there’s a loud buzz, it’s working!) Then I built the VXO and buffer – you’ll want to fiddle around with the cap values to get the coverage right. Once that’s working, all you have to do is add the NE602 mixer and you have a 40M receiver! Then you can add the mic amp and TX/RX switching, so you can verify that you have a very low-level DSB signal on transmit. Then the driver and final, and you’re done!
It took me quite a while, because I like to take my time on these things. It’s not quite as easy as I made it sound in that above description, but that sequence of steps worked well for me. Let me know how it goes.
Dave
AA7EE
I’m going to build the VXO first. I just got a new digital storage scope and I have fond memories of my old friend, NV1B, and I playing with various oscillator circuits from Doug Demaw and other authors 25 years ago. We’d walk around his yard with various oscillator circuits on bread boards to see how far away we’d get before his TenTec rig would no longer pick us up.
Slowly getting back to kit and homebrewing… http://i.imgur.com/kTKoKe6.jpg
Just ordered 5x 7,28 and 5x 7.3728 resonators from Cecil, best $7 ever spent. ❤
Take good care of those John – you never know when they will be unobtainium! Nice shot of the DMM and scope too – you have a more up to date workshop than I do! Actually, I don’t have a workshop – I just build here at my desk.
Dave
AA7EE
I do use the kitchen table from time to time, much to the XYL’s disgust! 😦
73 G1KQH
Hi Dave ! Do you have ballpark values for the two caps so that I might tune the entire cw portion on 40 meters ? Enjoyed your post. Thanks for sharing. WD4LNW
I do not I’m afraid Doug
73,
Dave
AA7EE
Any recommendations on what value of small polystyrene capacitor in parallel with the variable capacitor do you recommend to drop the frequency down by roughly 0.060mhz?
Off the top of my head, I’m afraid not – am busy running around today. Trial and error works well 🙂 W8DIZ sells NPO’s in a wide variety of values if you don’t have many on hand.
Dave
AA7EE
The was an excellent article in the current Sprat 169 about this. Observations with ceramic resonators in Super VXO mode…
I remember looking at it briefly Steve and, IIRC, the author noted that paralleling up CR’s has the effect of limiting the frequency range instead of increasing it, as is the case with crystals. Interesting………