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Author Topic: New Wideband IQ Demodulator  (Read 4241 times)
KA4POL
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Posts: 2036




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« on: April 05, 2012, 06:13:20 AM »

There is an interesting new circuit available: LTC5585, Wideband IQ Demodulator from Linear. It should be fairly interesting for SDR developments. The frequency range is impressive. So get the soldering irons out or rather heat the ovens up.
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M0HCN
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Posts: 473




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« Reply #1 on: April 05, 2012, 11:09:19 AM »

It is shiny in its way, but would need serious front end selectivity to make it workable as a RX for our use.
IMD3 is just not that impressive (Important on HF) and it is not that quiet (important on VHF).

I looked long and hard at it as a way to simply my cartesian loop TX, before eventually going back to a low fixed IF and a couple of more conventional mixers.

Interestingly they also do a similar modulator, which has possibilities.

Regards, Dan.
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KA4POL
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« Reply #2 on: April 05, 2012, 10:26:37 PM »

I think it's an invitation for experiments.
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NB3O
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Posts: 49




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« Reply #3 on: April 06, 2012, 09:24:16 PM »

Hi Dan, CML Micro is offering an eval board with their CMX998 TX chip which includes a Cartesian loop.
I have one here which I plan to feed into a IC-746's 2 meter TX section.
The mate is the CMX994 RX which seems to have a pretty decent dynamic range for its' power consumption.

http://www.cmlmicro.com/products/index.asp?searchvalue=CMX998&setindex=

73,
Steve NB3O
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M0HCN
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Posts: 473




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« Reply #4 on: April 07, 2012, 05:11:34 AM »

I looked at the 998, but could not locate a source in sane quantities, none of the usual suspects carry the thing that I can see, and distributors usually want to sell me a compete reel.

Note that you will need a micro and some DACs to trim out the DC offsets on the I & Q inputs.

I went for a pair of H mode quadrature mixers driven by two DDS chips to make a 9Mhz first IF, then mix up to (and down from) 85Mhz, before a final mixer pair produces the HF output. 

An interesting thought is that the 5585 and its companion modulator could be used, with a second modulator used to produce the variable phase shift needed to allow the loop to be closed by simply applying DC levels to the I & Q inputs to set the phase of the generated output.

Broadband noise is a concern, particularly as I am trying a HF set, rather then a VHF one.

Regards, Dan.
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G3RZP
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Posts: 4731




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« Reply #5 on: April 07, 2012, 08:14:08 AM »

Dan,

Contact me off line - peter.chadwick@ties.itu.int

I maybe able to help you with the CML part.

73

Peter G3RZP
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NB3O
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Posts: 49




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« Reply #6 on: April 07, 2012, 08:52:35 PM »

Hi Dan, I got the evaluation kits from CML's stateside distributor,  Component Distributors Inc (cdiweb.com) for the CMX994 RX/ 998 TX (P/N EV9942 / EV9980) along with their companion microcontroller PE0002 which is needed to run one or the other. 

The lead times were about a month.

You would need two PE0002 controllers if you wanted to run both eval boards at the same time.

The kits are not the cheapest things in the world, but allow a very quick evaluation since they have downloadable Windows apps and sample scripts for the PE0002.

I am looking forward to pumping some wideband custom waveforms in one and out the other, however I won't have a chance to power them up for another month or so based on the current work sched.

73,
Steve NB3O
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KB8E
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Posts: 24




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« Reply #7 on: April 09, 2012, 10:02:20 AM »

Guys,

FWIW, I'm somewhat familiar with the CMX994; I'm using it in a UHF FM transceiver breadboard. In my case, I'm using it as a transceiver since I don't need the I and Q for FM transmit, just a way to modulate the VCO and reference oscillator of the PLL. You can use the PE0002, but anything with a SPI interface will work. If using a CMX998, I doubt if two PE0002s would be needed; I think the register addresses are different on the 994 vs 998 so one PE0002 could load both. The fractional-N synthesizer on the EV9942 is rather interesting and is fun to play with as well as the 994. I had to rework the VCO to cover the band I wanted and change the front-end SAW filter, but really a nice eval board.

Sam
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M0HCN
Member

Posts: 473




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« Reply #8 on: April 09, 2012, 01:50:19 PM »

Yea, SPI and I2C tools I have, hooking it to a computer to configure the thing is not a major concern.

I am probably going to go with an AD9910 DDS or similar for the LO generators, with the reference clock divided down to clock the I & Q dacs so everything is locked to the same master reference.

Regards, Dan.
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