Need a budget LO - Page 1
Forum section: electronics / projects

author: katzohki
author_rank: Frequent Contributor
timestamp: March 17, 2014, 04:10:29 pm
content: Let's say I'm designing a radio transmitter that I want to operate at 140MHz. Taking my input signal (sine wave, FM modulation or something) I would need to mix that with ~140MHz signal in order to shift my data up to the transmitting frequency. And say that I want to be able to tune it in a range, maybe 5MHz. All this I can handle fine, but for generating the 140MHz to mix it up. Sure I could buy a VCO from mini-circuits for $12, but I want to keep this project on a really low budget.Am I missing something really obvious here? What's the simple and cheap way to generate this LO of ~140MHz?

author: I_may_be_drunk_right_now
author_rank: Contributor
timestamp: March 17, 2014, 04:14:50 pm
content: Software Defined Radio

author: katzohki
author_rank: Frequent Contributor
timestamp: March 17, 2014, 05:41:24 pm
content: Ah, well a software defined radio would really be more of an end-product. And depending on how it functions, a SDR could need a LO for downmixing as well.

author: fcb
author_rank: Super Contributor
timestamp: March 17, 2014, 08:11:40 pm
content: 1. Colpitts (with varicap tuning..)2. Crystal based oscillator (harmonic)

author: katzohki
author_rank: Frequent Contributor
timestamp: March 17, 2014, 08:51:24 pm
content: I was looking at the Hartley oscillator earlier. I don't have a lot of experience with oscillators. I see that you can use an opamp in place of the transistor for these oscillators and I was going to ask if the OPA355 would be a good one for the frequency range I'm thinking of, but I think maybe a better question is: "What is the advantage to using the opamp?"I should also ask how do I ensure (or better, calculate) the frequency stability? I'm not sure I want to get into designing my own PLL, but there's no point in having the frequency drift all over the place either.I'm really trying to avoid having big ugly inductors or variable capacitors on there, so I think a varicap is a good way to do it.Thank you, I'm sort of heading in the right direction I think.

author: dannyf
author_rank: Super Contributor
timestamp: March 17, 2014, 08:59:06 pm
content: A few options:- a digitally controlled oscillator. That that difficult to build and offers the best resolution.- a pll oscillator: many such options, from ti, on or silabs. Generally good frequency range but resolution will depEndend on the chips.- a dds: best of both words.Some mcus have digitally controlled oscillators or pll oscillators with pin out. A potential solution.

author: katzohki
author_rank: Frequent Contributor
timestamp: March 17, 2014, 09:56:08 pm
content: Danny, it seems like all the single chip solutions I've looked at run around the ~10$ price range. Maybe I'm searching for this in the wrong way? I was hoping for a cheaper solution.I am probably going to need a microcontroller. I was thinking of a PIC10F320 or an ATTiny or something. Any recommendation on a MCU that has a nice output for this sort of application?

author: dannyf
author_rank: Super Contributor
timestamp: March 17, 2014, 10:59:06 pm
content: Try TI's CDCE family. Fairly inexpensive and easily controlled with a mcu.

author: PeteJM
author_rank: Contributor
timestamp: March 17, 2014, 11:00:14 pm
content: Quote from: katzohki on March 17, 2014, 04:10:29 pmSo your making a VHF Radio Transmitter? Whats your callsign?

author: Marco
author_rank: Super Contributor
timestamp: March 18, 2014, 02:02:27 am
content: Quote from: dannyf on March 17, 2014, 08:59:06 pmSo how would you do this? Take something like a MAX2606 (cheap enough to skip making your own VCO), use a clock divider to feed a micro and use a DAC to tune it?

author: KJDS
author_rank: Super Contributor
timestamp: March 18, 2014, 09:09:00 am
content: I'd use a BFR93 based oscillator. Forget using an op-amp for a VHF oscillator, phase noise will be worse. It will need to be part of a PLL, chose a suitable analog devices part and simulate the performance using ADs PLLSymhttp://www.analog.com/en/rfif-components/pll-synthesizersvcos/products/dt-adisim-design-sim-tool/resources/index.htmlI'm sure there's enough hams that have trodden this path already for something suitable to have been published.

author: katzohki
author_rank: Frequent Contributor
timestamp: March 18, 2014, 10:00:51 pm
content: Ahah, well I guess the cat's out of the bag. Yes, I am a ham, KJ6 EZQ. I am also an engineer, but RF design is not an area that I am strong in.I was thinking of a foxhunt transmitter actually.Part of the struggle I'm having is not so much how to do it, but how to do it cheaply.This MAX2606 looks highly promising, at a glance...

author: dannyf
author_rank: Super Contributor
timestamp: March 18, 2014, 10:15:56 pm
content: Since the tuning range is small, you can simply use a transistor as the oscillator whose frequency is controlled by a diode fed by a mcu.incremental cost, if your system already has a mcu, is nil.use a dual gate mosfet plus agc for a broader running range.

author: katzohki
author_rank: Frequent Contributor
timestamp: March 22, 2014, 06:14:15 pm
content: I had some space on a prototype board, so I went ahead and put a colpitts oscillator and bought the parts. We'll see what happens once I get the boards made!

author: SArepairman
author_rank: Frequent Contributor
timestamp: March 22, 2014, 08:13:57 pm
content: how many of these things are you building? why not just go for the expensive part if its a one off custom device?

author: katzohki
author_rank: Frequent Contributor
timestamp: March 27, 2014, 10:56:32 pm
content: Quote from: SArepairman on March 22, 2014, 08:13:57 pmAh, that's quite a good point, but I'm trying to look at this from a practical design point of view. Who knows, maybe I'll make a few dozen if it turns out to be really successful.