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Old 17th Jul 2019, 11:37 pm   #1
Bazz4CQJ
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Default Use of a Ceramic Resonator in a BFO

A search of the net reveals one or two projects where people have used a 455kHz ceramic resonator (usually encountered in the form of a 3-pin filter), in place of a quartz crystal or tuned circuit, as the basis for a stable BFO. The rationale is that it offers better stability than may be easily obtained from a tuned circuit but is more easily pulled than might be the case with a crystal.

There's also been some use of ceramic resonators in VFO's.

I wonder if anyone has been down this route or can see any pitfalls?

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Old 17th Jul 2019, 11:59 pm   #2
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Default Re: Use of a Ceramic Resonator in a BFO

Can’t see why not do it. Has to be more stable than the BFO kit I had when I was a kid. That was basically an IFT and a transistor and some biasing components.
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Old 18th Jul 2019, 5:27 am   #3
Terry_VK5TM
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Default Re: Use of a Ceramic Resonator in a BFO

Using ceramic resonators in BFO's and VFO's has been around for years (not so much these days with all the you beaut DDS/Clock chips around).

One gotcha, be wary of any DC across them, they start to get noisy and can fail.
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Old 19th Jul 2019, 11:09 am   #4
G6Tanuki
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Default Re: Use of a Ceramic Resonator in a BFO

The big problem I can see here is that the ceramic resonator will work on a fixed frequency, whereas a BFO/CIO really needs to be tunable so you can put it on the appropriate side of the signal.

Will the stability degrade if you try 'pulling' it VXO-style? Depending on the style of resonator you may be able to buy 100 of them cheap off Alibaba, check their resonant frequencies, and then use the two most-widely-differing ones for your sideband-select switch.
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Old 19th Jul 2019, 12:50 pm   #5
G8HQP Dave
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Default Re: Use of a Ceramic Resonator in a BFO

I think you can pull them in a VXO further than a crystal.
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Old 20th Jul 2019, 12:38 pm   #6
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Default Re: Use of a Ceramic Resonator in a BFO

http://www.pan-tex.net/usr/r/receivers/sbfo.htm

I recently built this two terminal resonator circuit on stripboard and it works fine. So far withiout adding the transformer.
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Old 20th Jul 2019, 3:44 pm   #7
merlinmaxwell
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Default Re: Use of a Ceramic Resonator in a BFO

455kHz resonators are used in lots of remote controls BTW.
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Old 21st Jul 2019, 5:13 am   #8
Bazz4CQJ
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Default Re: Use of a Ceramic Resonator in a BFO

Quote:
Originally Posted by vosperd View Post
http://www.pan-tex.net/usr/r/receivers/sbfo.htm I recently built this two terminal resonator circuit on stripboard and it works fine. So far without adding the transformer.
Don m5aky
Thanks for that info; that's one of the circuits that I'd found in my search. Wonder what you are using it for? I want to improve my beloved Realistic DX-160 , kindly given to me by Hybrid Tellies some time ago. Curiously, I spend more time tinkering with that than I spend using my Icom 718!

If I do replace the existing BFO (with its LC circuit), I must check the level of drive the BFO feeds in to the product detector so that I can replicate that. The oldest BFO circuit using a ceramic resonator that I found was a 1988 circuit from Rad Com, again, using a FET.

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Old 21st Jul 2019, 8:45 am   #9
Julesomega
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Default Re: Use of a Ceramic Resonator in a BFO

Lowe used them (2-pin) in their receivers, starting with the HF-123. They are tuned 1.25kHz for USB and LSB. More stable than most L-C types
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Old 21st Jul 2019, 2:55 pm   #10
Bazz4CQJ
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Default Re: Use of a Ceramic Resonator in a BFO

I think that most of the resonators I have in stock are three-pin devices, so the circuit below is interesting.

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Attached Files
File Type: pdf 3-pin BFO.pdf (33.5 KB, 32 views)
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Old 21st Jul 2019, 3:36 pm   #11
mickm3for
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Default Re: Use of a Ceramic Resonator in a BFO

Hi used one of 456khz pulled by .5khz for upper sideband carrier ins osc . Does not drift at all Mick
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Old 21st Jul 2019, 3:42 pm   #12
Bazz4CQJ
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Default Re: Use of a Ceramic Resonator in a BFO

Mmm... sounds like this is a good way to go for a BFO!

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Old 22nd Jul 2019, 8:30 am   #13
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Default Re: Use of a Ceramic Resonator in a BFO

I struggled with this when I built my homebrew 160m transceiver. The design is a straightforward single superhet with an IF of 455 kHz, so obviously it needs carrier oscillators for generating the AM and SSB signals. The one on 455 kHz for AM was fine, just a cheap Murata 455 ceramic filter in a Colpitts osc, but the one for SSB had to be pulled down to 453.5 kHz. I went through many ceramic filters before I found one that would pull far enough in the right direction and start reliably. A lot of 'optimising' of the feedback capacitors was called for.
However, it all worked fine in the end and the rig is still in use.
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Old 22nd Jul 2019, 9:19 am   #14
Bazz4CQJ
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Default Re: Use of a Ceramic Resonator in a BFO

I wonder (suspect) if it is the case that the resonators have greater temperature sensitivity than a crystal, so the tempco of the capacitors in the circuit may need to reflect that? I must checkout which resonators I have in stock; I think I have about 4 different types, all bought some time ago for project which (as yet) never proceeded.

Additionally, I'm interested in making the BFO tune with a varicap diode.

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