Forums

Opamp frequency mixer

Started by bitrex July 4, 2015
So as a hobby project I'm thinking of building a tiny little theremin 
using some of those Soviet subminiature pentodes that are really cheap 
on eBay.  Unfortunately, it seems that the most complicated part of the 
project would be the frequency mixers, as I don't think they really made 
subminiature heptodes and a "proper" mixer would probably require two tubes.

I'm thinking about going solid state for the mixers in the first 
iteration at least...since the RF is so low I could probably just use a 
dual opamp with sufficient GBW.  There was this article but of course 
all the links to the schematics are broken:

http://electronicdesign.com/analog/make-frequency-mixer-op-amps

Can anyone suggest an opamp mixer topology for the low 100s of kHz?
On 2015-07-04 1:30 PM, bitrex wrote:
> So as a hobby project I'm thinking of building a tiny little theremin > using some of those Soviet subminiature pentodes that are really cheap > on eBay. Unfortunately, it seems that the most complicated part of the > project would be the frequency mixers, as I don't think they really made > subminiature heptodes and a "proper" mixer would probably require two > tubes. >
This would be the genuine Soviet method: http://www.thereminworld.com/files/pages/99/images/article1-1.png http://www.thereminworld.com/files/pages/99/images/article1-2.png But why heptodes? Here they show one with just ordinary transistors so it could also be done with triodes: http://www.thereminworld.com/files/pages/99/images/article2.png They have also been built with ordinary triodes and pentodes: http://www.rcatheremin.com/38rockmore.php http://www.rcatheremin.com/38sept.php
> I'm thinking about going solid state for the mixers in the first > iteration at least...since the RF is so low I could probably just use a > dual opamp with sufficient GBW. There was this article but of course > all the links to the schematics are broken: > > http://electronicdesign.com/analog/make-frequency-mixer-op-amps > > Can anyone suggest an opamp mixer topology for the low 100s of kHz?
If it absolutely has to be semiconductors I'd use the MC1496. -- Regards, Joerg http://www.analogconsultants.com/
On Sat, 04 Jul 2015 13:53:15 -0700, Joerg <news@analogconsultants.com>
wrote:

>On 2015-07-04 1:30 PM, bitrex wrote:
[snip]
>> >> Can anyone suggest an opamp mixer topology for the low 100s of kHz? > > >If it absolutely has to be semiconductors I'd use the MC1496.
I'd go with the MC1494... if you can get it. It has the level shifting built-in... quite adequate for audio, and immensely eases the pain of doing your own level-shifting. ...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.
On 7/4/2015 4:53 PM, Joerg wrote:
> On 2015-07-04 1:30 PM, bitrex wrote: >> So as a hobby project I'm thinking of building a tiny little theremin >> using some of those Soviet subminiature pentodes that are really cheap >> on eBay. Unfortunately, it seems that the most complicated part of the >> project would be the frequency mixers, as I don't think they really made >> subminiature heptodes and a "proper" mixer would probably require two >> tubes. >> > > This would be the genuine Soviet method: > > http://www.thereminworld.com/files/pages/99/images/article1-1.png > http://www.thereminworld.com/files/pages/99/images/article1-2.png > > But why heptodes? Here they show one with just ordinary transistors so > it could also be done with triodes: > > http://www.thereminworld.com/files/pages/99/images/article2.png > > They have also been built with ordinary triodes and pentodes: > > http://www.rcatheremin.com/38rockmore.php > http://www.rcatheremin.com/38sept.php >
Nice! Thank you!
>> I'm thinking about going solid state for the mixers in the first >> iteration at least...since the RF is so low I could probably just use a >> dual opamp with sufficient GBW. There was this article but of course >> all the links to the schematics are broken: >> >> http://electronicdesign.com/analog/make-frequency-mixer-op-amps >> >> Can anyone suggest an opamp mixer topology for the low 100s of kHz? > > > If it absolutely has to be semiconductors I'd use the MC1496.
Ah yes, I'd forgotten about that one.
On 7/4/2015 5:06 PM, Jim Thompson wrote:
> On Sat, 04 Jul 2015 13:53:15 -0700, Joerg <news@analogconsultants.com> > wrote: > >> On 2015-07-04 1:30 PM, bitrex wrote: > > [snip] > >>> >>> Can anyone suggest an opamp mixer topology for the low 100s of kHz? >> >> >> If it absolutely has to be semiconductors I'd use the MC1496. > > I'd go with the MC1494... if you can get it. It has the level > shifting built-in... quite adequate for audio, and immensely eases the > pain of doing your own level-shifting. > > ...Jim Thompson
Looks like a nice chip, but if this exercise turns out well I might make a PCB and do a small quantity run...and yeah, oooof. Mostly non-stocked here, and while there seem to be some available new from China, they're really expensive! :-(
On Sat, 04 Jul 2015 16:30:24 -0400, bitrex wrote:

> So as a hobby project I'm thinking of building a tiny little theremin > using some of those Soviet subminiature pentodes that are really cheap > on eBay. Unfortunately, it seems that the most complicated part of the > project would be the frequency mixers, as I don't think they really made > subminiature heptodes and a "proper" mixer would probably require two > tubes. > > I'm thinking about going solid state for the mixers in the first > iteration at least...since the RF is so low I could probably just use a > dual opamp with sufficient GBW. There was this article but of course > all the links to the schematics are broken: > > http://electronicdesign.com/analog/make-frequency-mixer-op-amps > > Can anyone suggest an opamp mixer topology for the low 100s of kHz?
I second the thought of using a MC1496 or MC1494 if you're going to use semiconductors. The NE612 may also suit, but you'd need level shifting. But you don't do tube projects unless you're planning on using an excess of parts. See if you can find a pre-1964 ARRL Handbook and look through it for mixers, or do a search on "tube mixer circuits" -- there's a lot more ways to do it than with heptodes, that's for sure. You could make a 180 degree shifter and summer by making an appropriate transformer (probably at least mildly resonant at good toob impedances) from signal A, with the secondary center-tapped and fed with signal B. Feed a pair of triodes with the transformer outputs, and poof! you have a single balanced mixer. Or, you could use one miniature heptode. -- www.wescottdesign.com
On 7/4/2015 5:06 PM, Jim Thompson wrote:
> On Sat, 04 Jul 2015 13:53:15 -0700, Joerg <news@analogconsultants.com> > wrote: > >> On 2015-07-04 1:30 PM, bitrex wrote: > > [snip] > >>> >>> Can anyone suggest an opamp mixer topology for the low 100s of kHz? >> >> >> If it absolutely has to be semiconductors I'd use the MC1496. > > I'd go with the MC1494... if you can get it. It has the level > shifting built-in... quite adequate for audio, and immensely eases the > pain of doing your own level-shifting. > > ...Jim Thompson >
The LM13700 can be used as a four-quadrant multiplier...it has a unity gain bandwidth of 2MHz and a slew rate of 50 V/uS...seems like it should be fast enough to use at low RF?
On 2015-07-04 2:24 PM, Tim Wescott wrote:
> On Sat, 04 Jul 2015 16:30:24 -0400, bitrex wrote: > >> So as a hobby project I'm thinking of building a tiny little theremin >> using some of those Soviet subminiature pentodes that are really cheap >> on eBay. Unfortunately, it seems that the most complicated part of the >> project would be the frequency mixers, as I don't think they really made >> subminiature heptodes and a "proper" mixer would probably require two >> tubes. >> >> I'm thinking about going solid state for the mixers in the first >> iteration at least...since the RF is so low I could probably just use a >> dual opamp with sufficient GBW. There was this article but of course >> all the links to the schematics are broken: >> >> http://electronicdesign.com/analog/make-frequency-mixer-op-amps >> >> Can anyone suggest an opamp mixer topology for the low 100s of kHz? > > I second the thought of using a MC1496 or MC1494 if you're going to use > semiconductors. The NE612 may also suit, but you'd need level shifting. > > But you don't do tube projects unless you're planning on using an excess > of parts. See if you can find a pre-1964 ARRL Handbook and look through > it for mixers, or do a search on "tube mixer circuits" -- there's a lot > more ways to do it than with heptodes, that's for sure. >
With tubes it doesn't have to end up in excess, except for power consumption, of course. When I was a kid I built a receiver that could demodulate AM as well as SSB. It contained only two tube stages and very few other parts.
> You could make a 180 degree shifter and summer by making an appropriate > transformer (probably at least mildly resonant at good toob impedances) > from signal A, with the secondary center-tapped and fed with signal B. > Feed a pair of triodes with the transformer outputs, and poof! you have a > single balanced mixer. >
For 180 degree shifting all you need is one triode as a follower. Make the anode resistor the same as the cathode resistor. Then the anode carries the 180 degree signal.
> Or, you could use one miniature heptode. >
I only know the combination ones as available (for a lot of money): http://www.boiaudioworks.com/2G21NOS The DK96 is probably completely unobtanium by now: http://www.radiomuseum.org/tubes/tube_dk96.html -- Regards, Joerg http://www.analogconsultants.com/
On 7/4/2015 5:06 PM, Jim Thompson wrote:
> On Sat, 04 Jul 2015 13:53:15 -0700, Joerg <news@analogconsultants.com> > wrote: > >> On 2015-07-04 1:30 PM, bitrex wrote: > > [snip] > >>> >>> Can anyone suggest an opamp mixer topology for the low 100s of kHz? >> >> >> If it absolutely has to be semiconductors I'd use the MC1496. > > I'd go with the MC1494... if you can get it. It has the level > shifting built-in... quite adequate for audio, and immensely eases the > pain of doing your own level-shifting. > > ...Jim Thompson >
Also, I have about a hundred LM13700s I picked up dirt cheap in the DIP package somewhere around here in a box ;)
On Saturday, July 4, 2015 at 11:24:21 PM UTC+2, Tim Wescott wrote:
> On Sat, 04 Jul 2015 16:30:24 -0400, bitrex wrote: > > > So as a hobby project I'm thinking of building a tiny little theremin > > using some of those Soviet subminiature pentodes that are really cheap > > on eBay. Unfortunately, it seems that the most complicated part of the > > project would be the frequency mixers, as I don't think they really made > > subminiature heptodes and a "proper" mixer would probably require two > > tubes. > > > > I'm thinking about going solid state for the mixers in the first > > iteration at least...since the RF is so low I could probably just use a > > dual opamp with sufficient GBW. There was this article but of course > > all the links to the schematics are broken: > > > > http://electronicdesign.com/analog/make-frequency-mixer-op-amps > > > > Can anyone suggest an opamp mixer topology for the low 100s of kHz? > > I second the thought of using a MC1496 or MC1494 if you're going to use > semiconductors. The NE612 may also suit, but you'd need level shifting.
Anything the MC1496 can do, the AD633 can do more painlessly. The AD834 and AD835 can do it a lot faster. -- Bill Sloman, Sydney