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Registered Member #207
Joined: Sat Feb 18 2006, 05:14PM
Location:
Posts: 45
Hello,
I am attempting to construct a device to demodulate digital RF data. The signal path of my RF receiver is a standard superheterodyne setup whereas channel selectivity is accomplished by mixing an RF signal with a LO frequency to produce an IF frequency. Naturally, the downconverted IF is filtered and passed onto a demodulator.
For reference, my LO frequency must operate between 25 and 65MHz.
Here's my problem: The RF digital data I'm trying to demodulate uses a scheme that encodes symbols with a +/-600Hz, and +/-1800Hz FSK from the center frequency, essentially resulting in four different symbols. Because of this, I'm under the assumption that my LO frequency must have significantly less instability/distortion than +/- 600Hz, otherwise some of my symbols will be masked by what could be poor LO performance. Is my assumption correct?
Also, if this is the case: How does one generate a LO frequency that is precise/stable enough to accomplish this task? Should I head down the road of using PLL driven with a VCXO that's then connected to an external VCO for frequency synthesis? Or something to that effect?
What's the correct way? Who are your favourite manufacturers for these sorts of devices?
Registered Member #51
Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263
There are cesium (or maybe rubidium) frequency standards available on ebay fairly cheap. I am not sure you need this accuracy though, perhaps someone else can comment on that.
Maybe a good temperature compensated crystal would be enough for your needs?
Registered Member #16
Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554
A DDS VFO is the way modern ham radios operate at those frequencies. Drift is less than 1Hz if your VCO is oven controlled, and tuning steps can usually be done in steps of 100Hz or so (sometimes 50Hz).
Registered Member #207
Joined: Sat Feb 18 2006, 05:14PM
Location:
Posts: 45
Thanks for the insight, guys. A couple more things I want to point out:
The LO frequency needs to be adjustable. However, after it's set, it doesn't need to be reset, simply maintained. A microcontroller interface for configuration is always nice though. There are a lot of PLLs on the market that you can interface to digitally. I just can't seem to find a PLL/VCO combo that's suitable in this frequency range AND has the stability/phase distortion/jitter requirements I'm in need of.
So, something like a TCXO alone wouldn't be suitable unless it had an attached PLL and VCO, as stated above. The selectivity is really important.
Also, the output doesn't need to be a sine wave, or clipped sine. Due to the nature of the mixer i'm using, a DC square wave (50% duty cycle) can be used.
I'll look into DDS VFO's to see how easy they are to incorporate into my design. But, if anyone else has any ideas or experience the the field, i'm all ears!
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
DDS is the way to go. Hams are fond of the old AD9850, and we're currently tinkering with the AD9954 at work.
If a DDS chip by itself doesn't have low enough phase noise, you can make a hybrid synthesizer that incorporates both a DDS and a PLL. Again, radio hams like to build these kinds of things, and I've seen plans for them in the public domain.
Nowadays it's quite common to basically hook the antenna or IF up to a fast ADC, and do all the demodulation digitally. The DDS, mixer, etc. are all just cores inside a FPGA, or one of the many digital downconverter ASICs that are available. If you do this at the IF stage, you can use sub-Nyquist sampling to make your life easier.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
For my ssb transceiver I'm using an Si570 programmable clock which should be accurate enough for your application. For a quick/lazy start I bought this kit
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Generating extremely accurate Local Oscillator (LO) Frequencies
