IF oscillator with ceramic filters - Page 1
Forum section: electronics / rf-microwave

author: MrWifiHifi
author_rank: Regular Contributor
timestamp: June 03, 2022, 09:04:45 pm
content: Hello. I wonder if it is possible to build an oscillator for IF using ceramic filters. I would need it for frequencies 10.7MHz, 6.5 and 5.5MHz ... What circuit could be used and would it be possible to detune the given oscillator with varactor diodes for measuring IF circuits?

author: David Hess
author_rank: Super Contributor
timestamp: June 04, 2022, 02:11:02 pm
content: Quote from: MrWifiHifi on June 03, 2022, 09:04:45 pmA quick search reveals several examples like:http://www.flashwebhost.com/circuit/ceramic_filter_bfo.phphttp://www.tompolk.com/inventions/455_KHz_Oscillator/455_KHz_Oscillator.htmlhttp://www.agder.net/la8ak/12345/n19.htmhttps://www.qrp4u.de/docs/en/Oscillators/index.htmhttp://www.seekic.com/circuit_diagram/Signal_Processing/Oscillator_Circuit/Ceramic_filter_oscillator_circuit.htmlBased on the above, I think one option is to short the input and output pins and treat the ceramic resonator as a two terminal crystal. Then one of the many voltage controlled crystal oscillator circuits can be used.

author: MrWifiHifi
author_rank: Regular Contributor
timestamp: June 05, 2022, 01:25:58 pm
content: I think, I could use this kind of oscillator, but my question is, how to connect capacity diode (single or double) to the oscillator circuit.Also, I mean to ask, would it be possible, to use same kind of circuit with LC circuit for AM IF frequencies, also with capacity diodes...?

author: BigBoss
author_rank: Regular Contributor
timestamp: June 05, 2022, 01:37:40 pm
content: Quote from: MrWifiHifi on June 05, 2022, 01:25:58 pmCeramic Resonators have relatively low Bandwidth so Tuning around the center frequency will be very limited.( Quality Factor is very important for an oscillator, Q factor of a ceramic resonator is maximum at the center frequency but decreases rapidly around in a narrow bandwidth )If you need a wideband oscillator around the center frequency, using a ceramic resonator is not a good idea. Design a VCO with a PLL will be a better option.

author: David Hess
author_rank: Super Contributor
timestamp: June 05, 2022, 05:06:53 pm
content: Quote from: MrWifiHifi on June 05, 2022, 01:25:58 pmThe varactor diode replaces a load capacitance. There are plenty of examples of VCXOs online also:https://www.electronics-notes.com/articles/electronic_components/quartz-crystal-xtal/vcxo-voltage-controlled-crystal-xtal-oscillator.phpQuoteSure:https://electronics.stackexchange.com/questions/241352/250-350-mhz-voltage-controlled-oscillatorQuote from: BigBoss on June 05, 2022, 01:37:40 pmTuning range is very limited but enough for the application in mind. In applications requiring modulation, a frequency multiplier after the oscillator is typically needed to get enough change in frequency.QuoteFor high tuning resolution, DDS is preferred these days instead of fractional-N or multiloop PLL systems. At higher frequencies, the DDS becomes either the reference or summing frequency as part of a PLL. A DDS feeding the PLL allows for lower phase noise and faster locking because the reference frequency can be higher.

author: bd139
author_rank: Super Contributor
timestamp: June 05, 2022, 05:20:41 pm
content: It'll work fine if you're sweeping a narrow IF.Modifying the original circuit, the objective is to change the load capacitance. You would have to experiment with the varactors but 1SV149 is a good start. 1N4007 are fairly good as well for these applications if the 1SV149 has too much capacitance but don't expect much in the way of linear tuning from either.You can skip the top varactor if you want as the resonator acts as a DC block.Using a 10 turn pot for the 10k one gives you a decent resolution.If you want to decrease the Q and increase the tuning range of the resonator, paralleling 1-3 of them works but it drastically affects stability and drift of the oscillator.I have build VXOs using this circuit that are fairly stable but that was with crystals not resonators. You can usually "pull" a crystal 20KHz each way quite happily.

author: MrWifiHifi
author_rank: Regular Contributor
timestamp: June 07, 2022, 06:48:00 pm
content: In the end, I decided to use the LC connection to generate IF frequencies. I thought it would be better with the crystal, but probably not.But I can't decide between two oscillator connections ... Can you advise me? I can' decide between this and colpitts.

author: MrWifiHifi
author_rank: Regular Contributor
timestamp: June 07, 2022, 07:42:38 pm
content: My idea is to build a device for aligning radios, which could be connected to an oscilloscope and act as a polyscope. I know it's an old type of device, but I have parts from drawer stocks for building it. I know that you can buy devices like NanoVNA and the like today, but at the moment I don't have much money for that. I already have an idea of ​​what the device should contain, but I have a problem with the design and construction. I'm throwing a block diagram here, like I'd like to have.

author: MrWifiHifi
author_rank: Regular Contributor
timestamp: June 07, 2022, 08:42:10 pm
content: The device in the block diagram should work as follows:- The heart of the circuit is a VCO with switchable bands for AM and FM. It can be swept using a generator ramp, which is triggered by a blanking pulse from the oscilloscope.- There is also a circuit to display the desired frequency on the HF envelope. I forgot to draw that a ramp signal enters this circuit, which is gradually compared with fixed frequencies. If the current ramp is at the same level as the fixed frequency, the circuit will output a pulse which will appear as a brighter dot on the curve. In this way, for example, 3 points (frequencies) on the curve could be set, according to which the curve would be tuned. The circuit can also be expanded to display multiple points.- There would be two buffers at the output of the HF generator, one for the HF signal output, the other for the counter.

author: MikeK
author_rank: Super Contributor
timestamp: June 07, 2022, 09:32:05 pm
content: Quote from: MrWifiHifi on June 05, 2022, 01:25:58 pmIn case it wasn't known...The "problem" with using a varactor to tune the frequency is that it requires a wider voltage range to achieve the capacitance range that it's capable of.