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Author Topic: Synch Detectors  (Read 12572 times)
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VE7 Kilohertz
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« on: February 07, 2005, 04:14:47 PM »

Last year sometime or maybe even earlier, there was a great discussion about synchronous detection, circuits boards etc. There were a few people who tried the Yaseu replacement brd for the FT1000 etc. Some built up their own circuits.

I am determined this time, to actually get something built for my Racal 6790, which already has a product detector, but just needs the VCO portion added to make it synchronous.

See schematic of the IF here on page 9.  

http://kahuna.sdsu.edu/~mechtron/PremRxPage/ref_mans/ra6790gm/section7schematics.pdf

So, let's pic up the discussion again. What are you using for a synch detector, and what ever happened to the Yaseu board? Did it work? If it did, what is the p/n of the board etc. If you built your own, what circuit did you end up building?

Cheers

Paul
VE7KHz
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wavebourn
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« Reply #1 on: February 07, 2005, 04:20:10 PM »

Actually, PLL works as a very narrow filter. I once tried to use a very high Q tuned cirquits and CW EM filter, results are better than with PLL.
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w3jn
Johnny Novice
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« Reply #2 on: February 07, 2005, 06:43:32 PM »

Frank GFZ knows a lot of about the sync detector as used in Racals, but it's basically very simple and isn't a PLL at all.  They use a FM IC and take the limiter output (all AM is stripped off and you essentially get the limited carrier of the signal) and shoot it into a product detector (as the LO).  That way the LO is always in phase with the signal since it really is the signal (amplified by about 100dB and limited).  No PLL lock times to worry about here!

73 John
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WA1GFZ
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« Reply #3 on: February 07, 2005, 07:50:12 PM »

Hi,
Pete SOV and I found the loop filter too fast in the FT1000 board. The lock time suffers when you slow it down. Also you have to pull the resonator to 455 KHz. Pete used a variable cap and I used a pair of chip caps to center the response. I think you have to pull it up 5 KHz.
I fed the two outputs into a MOT DSP module that Pete provided the code
and I hacked into working. It is cool.
Today there is freeware to make your PC sound card to do the demodulation. The I/Q channels go into the stereo inputs.
Tom Vu had problems with his at high levels of modulation but I don't know if he played with the loop filter.
I'm working on a phase detector that uses a 74AC74 and 74AC00 that divides the signals by two to get 50%duty cycle then the gates are set up to act like an XOR phase detector. The advantage of this is the VCO is sitting in the center of the tuning range when no signal is present. Other phase detectors drive the VCO to one end of the travel or sweep it around. I have not built it yet or decided how wide to allow the tuning
but it does what I want so far on paper.
Thr Racal set up is hard to beat when set up properly as the MC1357
FM demodulator generates a carrier for the MC1496. The MC1357
must be properly tuned for it to work well on AM. It does not HAVE I/Q outputs though.
Rob K2CU gave me the idea by suggesting I look at the old 4046 phase detectors.  The XOR is an interesting phase detector. I will use a VCXO running at 1820 KHz.  with dividers to get 0 and 90 degree LO signals.
This is one of many projects hiding on the bench.
Check out Rob's wide range detector noted here. This is another cool AM demodulator. fc
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Ian VK3KRI
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« Reply #4 on: February 08, 2005, 05:59:08 AM »

Quote from: w3jn
Frank GFZ knows a lot of about the sync detector as used in Racals, but it's basically very simple and isn't a PLL at all.  They use a FM IC and take the limiter output (all AM is stripped off and you essentially get the limited carrier of the signal) and shoot it into a product detector (as the LO).  That way the LO is always in phase with the signal since it really is the signal (amplified by about 100dB and limited).  No PLL lock times to worry about here!

73 John


Ive played with this using an MC3357 FM chip.  Stick 455kHz into the limiter section AND into the  Gilbert cell that normally gets the qudrature output of the  limiter delayed by the tuned cct.   It seems to work, but it was a bit sensitive to input levels.
Maybe one day I'll dig back through the strata of unfinished projects and stick it in a receiver for real....
                                               Ian VK3KRI
Oh and while I remember , other chiips from the family 3359 etc have an internal cap between gilbert cell and limiter output that results in lovely feedback through the limiter when you try this trick.This of course genereates lots of oscillations on various frequencies , which makes them a no-go.  I had hoped to use one of the family that has the logarithmic S-meter outputs for agc/S-meter.
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WA1GFZ
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« Reply #5 on: February 08, 2005, 08:19:07 AM »

Ian,
I found the MC1357 has a wide range of inputs possible in the Racal circuit. I actually stuffed an extra filter module in my hot rod RA6830
between the output of the IF amplifier and input of the detector module.
The A8 slot on the motherboard was just wide enough to fit a module with mechanical filters. The extra 10 dB of loss added didn't change the radio performance. It took 3 cuts and 5 jumpers to get the control to the extra
filter module.
The old CA3039 has a nice log output. I built a simple spectrum analyzer
many years ago.  fc
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w3jn
Johnny Novice
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« Reply #6 on: February 08, 2005, 08:24:05 AM »

THe NE605/615 is a combination NE602 and NE604 - gilbert cell mixer/oscillator and amp/limiter/demod/RSSI.  Has just about everything you need on the one chip.

As Frank noted, though, there's no I/Q available - although there could be if you built a simple 90 degree phase shifter for the LO and fed another gilbert cell mixer.  WA1SOV has a good page (i found it while GOOGLEing) on using a Analog Devices I/Q demod chip.  He also has a very nice analog allpass filter design so you can get 90 degree phase shifts from 50-15K Hz.  This is the way to do it if you want to be able to listen to either sideband, or both in mono, or both in stereo.  One of those projects I need to get to, as I have almost all of the parts to do it.

73 John
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WA1GFZ
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« Reply #7 on: February 08, 2005, 09:12:15 AM »

John,
I/Q is possible with the Racal with a buffer amplifier power splitter and phase shifter like a quad hybrid. Once you have 0 and 90 degree
LO it only takes two mixers. Still locking the LO to the carrier is the hardest part of the job. The Racal method has one problem as I see it.
The internal IF noise through the filters and IF gives the MC1357 enough signal to generate a constant carrier, BUT the phase noise of this carrier increases as the signal goes into the mud. This makes the adjustment of the MC1357 very critical.
There is no PLL to lock so the LO gets clean as soon as the signal comes out of the noise. I have never observed a problem with this but wounder what will happen into a DSP. The FT1000 board would drop out of lock
and you would hear the signal swoop back into lock after a long drop out.
Slowing down the loop to reduce distortion made the lock problem worse.
This is what drove me to the XOR phase detector that puts the LO in the center of the range when no IF carrier is present. it would drop out of phase lock but the lO would remain at close to 455 kHz. Best performance would be when the incoming signal was at the center frequency of the PLL VCXO in my case. As the signal comes out of the mud it would then regain phase lock. The required frequency shift would be small so it would allow a slow loop. Then later maybe add a circuit to change loop filter
time constant when in or out of lock. Chuck K1KW demonstrated the SE3
loop switch and the slowest loop sounded the best. That function with automatic switching would be cool. Many high performance RX synthesizers do this as an automatic function so it is possible.
Any thoughts anyone??? fc
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VE7 Kilohertz
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« Reply #8 on: February 09, 2005, 10:11:44 AM »

Hi Guys,

Excellent discussion and ideas. It's sort of all coming back to me now.

Looking at the 6790 schematic, the 1496 input in particular pin 8, would it make sense to insert a PLL like a 564 or 568 etc. at that point to simulate the carrier? That would get us the phase lock on the carrier but still not allow for ISB reception. Would this buy us anything in conditions of severe QSB? Would it cause more problems than solve them?

I don't know what the cct looks like for the 6830 but I imagine it is similar to the 6790. I guess the other way to get synch detection is to build a complete unit and run it from the IF out. I managed to get a AD607 eval board which has the AD607 and support ccty already installed including very nice gold plated SMA I/O connectors to all the important points. The cct can be seen here in the data sheet.

http://www3.telus.net/radioboy/Pages%20from%20AD607_c.pdf

I think something like Pete's (WA1SOV) cct is a good starting point and only needs the addition of the PLL to make it true SD. Has anyone built this cct using the AD607?

Cheers

Paul
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WA1GFZ
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« Reply #9 on: February 09, 2005, 10:21:55 AM »

Paul,
I have a 607 but have not built it yet. The 6830 circuit is the same as the 6790 circuit just packaged smaller. look at the lo generated from the
MC1357 with a spectrum analyzer and scope to see what happens to the lo
as the signal goes into the noise. Then think what a pll would do. as it locks and unlocks with various loop filter constants. my point is to have the vco parked close to CF when there is no input signal so it locks easier.
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