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Octave x Octave band receiver design

I've been getting reminded of just how awful old-time RF signal processing stuff is.

I have a system with two VCOs, f_x and f_y, whose output is at 2f_x +

2f_y. f_x and f_y both vary over a full octave. I need to do a vector receiver for this.

So, I needed to construct 2 f_x + 2 f_y + f_IF.

After several tries, I wound up with

f_x -> LPF -> 20 dB coupler -> pad -> doubler -> pad -> LPF -> HPF (the order matters) -> amp -> LPF = 2f_x

f_x -> LLPF->20 dB coupler-> pad-> PLL offset by +12.5 MHz -> LPF -> pad

-> doubler -> pad -> LPF -> HPF (the order matters) -> amp -> LPF -> pad = 2f_y+25 MHz

these go into

mixer (+7dBm LO, -6 dBm RF) -> pad -> LPF -> HPF = forest of spurs.

I'm currently throwing together a 1:1 PLL to clean this mess up.

If I'd had enough of the right filters, I could have done all of this by double conversion and got rid of most of this crap, but oh, well.

The real version is going to use DDS/PLLs to do all of this, of course.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs
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What are you using for a doubler? I've been using an AD834 fed by sin

70MHz. ...Jim Thompson 
--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

Have you looked at a simple FPGA implementation? like Altera's CYCIII, for what? $5 everything is in there [I think] and such low power can be portable.

Reply to
RobertMacy

It's all Mini Circuits stuff from my drawer. The doublers are FD-2, and the eventual LO frequency ranges from 225 to 425 MHz.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

It has to produce output at well over 400 MHz, with low spurs.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

Why does the order matter and why the 20dB coupler? Another way to do this may be a tracking filter or switched filter after the doubler. Especially since an octave is only 2x. Of course you'd have to characterize (calibrate out) the phase shift of it because you want a vector receiver but you'll have to also do that with LPF/HPF. Beingcheap I probably wouldn't spring for more than a lone diode for the doubler.

The best way to generate a clean 2f_x is a PLL, using it's own output divided by 2 as a reference.

Make sure it locketh on the right peak and stayeth on that peak. Even if the power briefly browns out.

I found this to be a very helpful filter design software:

formatting link

It's free but you get an initial nag screen until you buy their meter. Which supposedly is also quite good but I don't have that one.

And a nice front panel in brush-look gold-colored bling anodization :-)

No kidding: My mountain bike buddy has handlebar clamps with that look and a genuine cajun guy with a dog approached us on a trail "Maian! Thait goul shuaah look faaaiin!". A very friendly old man. We chatted for at least 1/4h but were unable to convince him that this ain't gold.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

The order matters because the source and load impedances are different, and neither is really 50 ohms. The filters may be odd-order as well, I'm not sure.

The original theory of this was that I would just cruft something together out of my junk drawers, to prove the concept, and then do the more elegant version. But chains of old-school signal processing parts do get ugly, even with all the RF-port levels backed way off to reduce the n*f_RF spurs. Just the interacting LO-port spurs are evil enough.

Yup. I don't have the fast logic handy, though, and this is all hand-wired stuff for now.

Yup. I usually use ye olde Schmitt-trigger-wrapped-round-the-loop-amp acquisition aid (which I thought I invented in about 1982, but which was published about 5 years earlier). I can make the Schmitt generate an offset big enough to pull the loop out of lock on the smaller peaks.

Thanks. Good to download it soon, before the site goes away--the AADE guy's health gave out, or so we heard here.

Sell it to him!

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

Sometimes best to place buffers inbetween, just source followers. Then you lose 6dB at each one. Or use TV buffer amps that can be opened to switch the source resistors to 50ohms. Those are usually around 0dB with source termination.

That can become really frustrating when spur levels depend on how much torque one puts on the nuts of the SMA connections.

Sounds like an RF automatic-clutch. So far I think I only used controlled ones, "wrong peak ... go one more".

What happened? We just lost our dentist for a similar reason, he had to close shop. Of course that was exactly when I broke a tooth ...

Nah, he is a man of modest means and prefers hiking with his dog. The mountain bike my buddy rides is a Specialized Enduro, an expensive downhill machine with double-crown fork and the whole works.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

No time for that level of smarts on this one. Next round is all DDS, which will have its own problems since I've never actually designed with one before, but at least they'll be different ones. ;)

Dunno, he closed the site 'for health reasons'. I just looked, and he's back part time, selling kits only. I ordered one myself today, so we'll see what it's like.

BTW the nag screen goes away if you just launch and close the app ten times.

;)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

Depends on the frequency range. If it's tens or hundreds of MHz you can get "ready-to-bake" chips. Adhering to the layout guidelines as best as possible almost guarantees a success. I try not to push the loop filter as close to the upper limit as mfgs suggest.

Try to stay away from VCOs that have a whole lot of digital control integrated because those can be less than cleanly.

It never did for me and I've used it lots. But glad he's at least back in the saddle.

[...] 
--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

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