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Old 27th Mar 2020, 11:27 pm   #1
Wendymott
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Default AN612 Balanced Modulator

Hi Peeps.... I cannot remember where I saw it.. but the AN612 was recommended as a DBM rather than the SA612. The AN612 is a 7 pin SIL chip, but all the data I have is in Chinese or whatever.
I bought 10.. as you do..and they arrived very quickly from China,considering the current situation.
I made a test jig as per photo 1. I cannot see what VR1 (4K7) does..VR2 balances the sidebands out (pre 9 mhz filter)... but VR1 seemingly had no effect.. I also rigged it up as an Audio modulator with 9 Mhz carrier and there seemed to be no effect there too... I added the Chip internal diagram for your perusal.
It obviously is there for a reason........... PS Note.. I was looking on the spectrum analyser... not the scope... if its a modulator linearising pot.. as an after thought.
Thanks in antisipation
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Old 27th Mar 2020, 11:50 pm   #2
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Default Re: AN612 Balanced Modulator

I suspect each pot is a twiddler for the DC offset of one of the inputs.

If so, then one will null LO feedthrough to the IF, the other will null RF feedthrough.

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Old 28th Mar 2020, 1:34 am   #3
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Default Re: AN612 Balanced Modulator

NTE1249 (same as AN612) has an english datasheet, but it wont tell you anything other than basic specs.
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Old 28th Mar 2020, 12:07 pm   #4
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Default Re: AN612 Balanced Modulator

Thanks Terry.... Its more than I had.... The pin description now gives me a clue as to their functions..... I am going to look for the "odd" application note..but I know what I want to do.
In these troubled times...I need to keep on keeping on

Yes David.... now I understand the functions
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Old 28th Mar 2020, 12:31 pm   #5
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Default Re: AN612 Balanced Modulator

The AN612 was successfully used as the balanced modulator in various Uniden export SSB CB radios like the Cobra 148GTL-DX and the Stalker 9 from the early 1980s. You could try copying the AN612 modulator circuit from these radios. They did have a reputation for having very clean SSB.

I think I have the Uniden AN612 modulator circuit somewhere on a bit of scrap PCB. I also have homebrew eval PCBs for the MC1496 and the NE/SA612 modulators. Lots of people use the NE/SA612 as a DSBSC modulator but I wouldn't recommend it for anything beyond a basic QRP radio. There are much better alternatives.

A good benchmark for comparison would be a DSBSC modulator based on the Minicircuits TUF-1 diode ring mixer.
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Old 28th Mar 2020, 1:18 pm   #6
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Default Re: AN612 Balanced Modulator

Hi Jeremy. Thanks for the info... I will compare a SBL-1 RDM... I have a couple, with the SA612 I have hooked up at the moment. As you may remember I have used the MC1496 in a few projects quite successfully...but I wanted to try something new-ish.
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Old 28th Mar 2020, 3:49 pm   #7
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Default Re: AN612 Balanced Modulator

It's definitely worth a play because the AN612 should perform much better than the SA612. The AN612 can be run at a higher current and voltage and it should give much less distortion. I think it will also be easier to null the carrier on the AN612 although the carrier does move up and down a small amount once modulation is applied. At least that's how I remember it when I used to tinker with SSB CBs in my student days.

Unlike the SA612 the AN612 output is also buffered so it can drive a lowish impedance at the output. I think the CB circuits used to load it with 1 or 2k ohm but it can probably drive 500 ohms without introducing much more in the way of distortion.

By contrast, the SA612 runs at a very low current and it really is meant to be used for smaller signals. However, it can be used as a modulator and some manufacturers (Elecraft?) use it like this but the performance is always going to be compromised when compared to the traditional alternatives.
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Old 28th Mar 2020, 4:27 pm   #8
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Default Re: AN612 Balanced Modulator

Hi Jeremy and others..IF... you get the Carrier and L/O inputs crossed.... hmmm.. not saying..VR1 will not null... however if they are connected CORRECTLY.. I can get a great carrier null...Im going to make a triband front end for 6 / 4 and 2 which is the final intention and see where it goes
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Old 28th Mar 2020, 4:53 pm   #9
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Default Re: AN612 Balanced Modulator

On the classic CB circuits the LO/carrier is fed to pin 3 and the audio to pin 1 of the AN612. This looks correct when looking at the internal circuit for the AN612 as this is the conventional way to drive a Gilbert cell.

Your schematic in post #1 has the LO going to pin 1 and this doesn't look right to me.
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Old 28th Mar 2020, 5:04 pm   #10
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Default Re: AN612 Balanced Modulator

Some of the Cybernet export SSB CB radios also used the AN612 modulator and I think they used two carrier nulling trimmers in a similar way to your original circuit. So I suspect that both of your trimmers are there for the carrier null when used as a balanced modulator. Are you trying to use it as an RF mixer? I'm not sure what frequency range it can operate over when used as a mixer.
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Old 28th Mar 2020, 8:58 pm   #11
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Default Re: AN612 Balanced Modulator

Hi Jeremy..... Probably as a modulator and RX Mixer...I could use two independent parts but a bit wasteful.. As it happens I did two photo's earlier which I now attach.
I tried two different versions of "passive" RDM's and was not impressed. Its all a matter of the end result..again I itterate Im not looking at pro performance.. good enough is good enough.
I will be fitting two 9 Mhz filters one for Fm and one for SSB...and a few mini relays for switching various circuit routes.
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Old 28th Mar 2020, 10:14 pm   #12
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Default Re: AN612 Balanced Modulator

Quote:
I tried two different versions of "passive" RDM's and was not impressed.
A decent diode mixer can perform quite well as a modulator so maybe you could get more success from a different type. Some Minicircuits diode ring mixers aren't best suited to this application and also it's really easy to degrade the performance (a lot) if the port terminations aren't managed well.

The AN612 chip is a good solid choice in my opinion and it will probably be close to the performance of a diode modulator in terms of IMD and it may well give better carrier suppression with the two trimpots.

It is possible to predict the TUF-1 mixer performance as a modulator as long as the port impedances are well controlled at 50 ohms. The datasheet shows that the LO to RF isolation is typically over 70dB at 10MHz and the output IP3 is about +8.5dBm. With a +7dBm LO power, the leaked LO level at the RF port should be about (+7 - 70) = -63dBm or better. With sufficient two tone AF drive to deliver two tones each at -20dBm at the output of the modulator the IMD3 terms seen either side would be (20+8.5)*2 = 57dB lower than these two wanted tones.

With a leaked carrier at -63dBm the carrier would appear to be about 43dB lower than the two wanted tones. There will also be similar -20dBm tones at the unwanted sideband. I'd hope the carrier suppression would be a bit better than this in reality and once a typical SSB filter is fitted the carrier suppression could go down another 10 or 15dB. I'd consider this to be a reasonable estimation of what to expect from a diode mixer based modulator.

Your AN612 performance looks to be quite similar although it looks like you have a crystal filter after the modulator. Your AN612 should be a lot better than the SA612 mixer/modulator chip.
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Old 29th Mar 2020, 8:31 am   #13
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Default Re: AN612 Balanced Modulator

There is also the possibility of a cheap 'fast bus switch' and a little bit of transformer artistry. Cores are cheap, magnet wire is cheap. You can have superb performance for pocket money. Look up 'H-mode mixer' and you don't have to buy minicircuits transformers.

Get some fine enamelled wire and use marker pens to colour a few short lengths in different colours (the old coloured 'Verowire' stuff was a useful size and ready done) Twist them together to form a miniature rope. Use a suitable tiny toroid (8-Ish mm) and wind say seven turns on FT43. separate the ends of the rope and use the colours to sort which wire is which.

I've built hexafilar transformers this way to give a centre tapped primary and two centre tapped secondaries. Not difficult, quick and cheap.

The two centre tapped secondaries feed the switches.

The centre of the primary is grounded, and the signal goes in one end. The other end is left floating. You might say that half of the primary isn't needed and represents a waste of wire and effort. But it serves to improve balance by balancing the capacitive effects in the winding. It's a neat trick that isn't generally known about... a winding to nothing.

The fast bus switches ground the ends of the secondaries under the control of the local oscillator, so the signal from the mixer emerges differentially on the two centre taps of the secondaries. Use a balun to combine them. As a modulator/upconverter, this is the DC coupled port, so you'd put audio in here as a balanced signal from a pair of opamps, LO/carrier would go to the switch control pins, modulated RF would come out of the primary of the fancy transformer.

You can get superb carrier rejection with one of these and far better linearity than you need, but as it's cheap and uses very little power, it's great fun.

David
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Old 29th Mar 2020, 1:40 pm   #14
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Default Re: AN612 Balanced Modulator

Thanks Jeremy and David... All noted... not sure where I am going yet.. but the AN612 looks favourite.. If I use 10T presets on the balance pots rather than the open presets at present I am sure the carrier leak will improve.
Anyways.. early days yet..
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Old 29th Mar 2020, 1:51 pm   #15
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Default Re: AN612 Balanced Modulator

Yes, I think the AN612 is a good choice. See below for a couple of CB circuits that use this chip. The Uniden circuit uses TR27 to deliberately upset the carrier balance at pin 1 of the AN612 when in CW/AM/FM mode and this forces the modulator to produce a strong carrier for these modes. The other circuit is from an old Cybernet export CB radio and it uses two trimmers to help null the carrier for SSB. It also adds an external buffer amplifier Q13 before the crystal filter.
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Old 30th Mar 2020, 12:21 am   #16
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Default Re: AN612 Balanced Modulator

If you do an image search on AN612, it will pop up some gems of other stuff, including practical circuits (as opposed to the datasheet test circuit).

Here is one (you will need Google translate) http://www.ja3gse.sakura.ne.jp/modulator.html

I find the Japanese experimenter sites will quite often have info on the (to us) more unusual IC's.
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Old 30th Mar 2020, 2:58 pm   #17
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Default Re: AN612 Balanced Modulator

I've just realised I have a few of these buried 'in stock' somewhere in my old component drawers. I'd forgotten all about them and I've been using NE612's for years.
Reading up on them, they do seem a lot beefier than the Philips device and can provide a greater output - very useful when used as a TX mixer. The NE612 is a bit weedy in this respect and demands a lot more post-mixer amplification to useful power levels, which can be interesting on the stability front.
The lack of an on-chip oscillator is no great drawback, I always build my own anyway so I can put some filtering in line.
Time to dig one out and set up the test gear, just the job when we're not allowed out! Oh yes.
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Old 1st Apr 2020, 10:50 am   #18
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Default Re: AN612 Balanced Modulator

Hi Terry.... Mucho gracias I have basically copied the second circuit I think we must have done the same search..I used 10T trim pots for a much closer carrier cancellation..I still have some things to do on it. The I F Amp is next... or shall I go front end ?? either way it has to be done..THANKS for your input

Andrew... yes they are quite useful
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Old 1st Apr 2020, 5:46 pm   #19
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Default Re: AN612 Balanced Modulator

Ooh, you've built it properly, Wendy!
I've dug out my old stock of AN612's and done a very quick & dirty lash-up on a scrap of 'previously enjoyed' PCB.
At a carrier frequency of 6 MHz and -17dBm, audio at 1 kHz triangle wave, we get 1.2v p-p into the 2k impedance of a 6 MHz ceramic filter terminated with 2k.
That's a lot more than I've got out of an NE612. 11v supply.
Anyone know what frequency these will go to? The NE612 will mix reasonably well up to about 500 MHz, although the osc doesn't stretch much beyond 200 MHz.
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Old 1st Apr 2020, 6:31 pm   #20
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Default Re: AN612 Balanced Modulator

Forgot to mention I had unbalanced the modulator to make it produce AM.
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