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May 2021 - Homebrew Double Balanced Mixers

In April I wrote about testing Schottky diodes using the method described by  Charlie Morris ZL2CTM in his youtube video. The purpose of this was to make my own double balanced diode mixer as Charlie had done.

This is something I've never done before so much reading entailed after the simple mixer construction as shown in Charlie's video.
Mixer Circuit



Just like with antennas, if you don't have a good impedance match then you get reflected waves... it seems the same is true with mixers. So they need to be properly terminated.

In his October 2012 PW article, George G3RJV makes a double balance mixer in a mint tin as a utility device. It is not dedicated to a particular intermediate frequency and so he provides I.F. port termination with a resistor. (shown below). He says "Ideally a full diplexer circuit should be added to the output – but (he said) this simple 50Ω termination works for most applications".
G3RJV DMB

 

This was my experimental mixer.

My Mixer

I designed it as a "test bed". The idea being that I could try various sets of diodes by creating "plug in" matched sets. The use of a dab of different coloured nail varnish identifies which set it which.

The intermediate frequency port (in my example) has the chosen intermediate frequency of 9MHz.
Ideally, all frequencies would see a 50 ohm termination - and therefore there would be no reflected waves back into the mixer to cause undesirable effects like noise.

But while a resistor is fine for all other frequencies, we ideally need the diplexer to be "opaque" to 9MHz and therefore allow the following stage to provide the termination.

There are a number of diplexer designs, but the one that I tried was recommended by Bob Burns G3OOU on his website.
G3OOU diplexer

Bob's  website shows a pretty symmetrical device and the tuned circuit values are chosen such that they resonate at the I.F. frequency.
This is described elsewhere as an excellent bandstop/bandpass diplexer originally popularized by Joe Reisert W1JR

Essentially, the signals coming from the mixer pass through if they are at the intermediate frequency and are terminated by the 50 ohm resistors otherwise.



As with the mixer, I built my version of the diplexer as a form of test bed with plug in tuned circuits so that I could later develop a diplexer for any frequency that I wanted before making a permanent version. The 50 ohm resistors are located underneath the PCB in the form of surface mount devices.
My Diplexer

The Toko coils are from the junk box but Bob Burns gives values in his article.


I set up my test equipment as shown below to measure the mixer performance.
Test Equipment Setup

Charlie Morris ZL2CTM had set his testsignal levels as follows. 12 MHz (LO) was 1.4 volts (+7dBm) and the RF port was set to 0.20 volts. My signal generator presents 50 ohms on each output.
On the output of the diplexer I connected a 50 ohm resistor to simulate the impedance that a following stage would present - and  this was then measured.

In the previous article I show that I selected two sets of diodes. One using a meter with three digit precision and the other set with my new meter that has four digits. As described above, I was able to swap between the sets. This was especially useful because later I was able to compare those with a matched set from Hewlett Packard that a friend had given me. When he originally bought them they cost £10 per set!

Matched with 3 digit meter

HP diode matched set

Matched with 4 digit meter

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February 2021
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March 2021
Charlie's VFO and other Arduino projects
(Occasional Arduino Pain).

March 2021
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April 2021
Testing Schottky Diodes
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April 2021
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(for the G3RJV DVD)

May 2021
Homebrew Double Balanced Mixers
(using the ZL2CTM method)

May 2021
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(but still doing useful work)

November 2021
Circuit Boards and the Dremel
Easy PCB's