21 - Diving Deeper Into The Diplexer

Part 2 of the Mixer Mini-Series.

Nick M0NTV explores the design and construction of a Diplexer for the IF port of a Double Balanced Diode Ring Mixer.

I then took these values and also modelled the design in LTSpice. When it came to physically building it I tested the series resonant filter and the parallel resonant filters separately first of all with an S21 sweep on the NanoVNA. This is how I usually test all my filters these days.

[HA-a]

Thank you for the video.

It would be useful to note the toroid mix values used in the band pass filter. From the pictures it looks like mixture 6 (yellow) toroids were used, which can have a relatively high Q factor compared to ferrite core mixes like 43. The SWR curve of the bandpass filter may be a bit less steep with a more broadband toroid. Nonetheless, the diplexer seems like the right solution. Thanks again for the interesting video.

[MXYM]

This is the best explanation on this subject I have seen. Thanks.

[alanmacdonald]

In the grand scheme of things, building the diplexer really isn't that much work, the results of the tests are speaking for themselves! You, my friend, are doing an excellent and inspiring job with the testing and explanation (I wonder who taught you about rejectors and acceptors ;-).
Great work as always Nick, look forward to the next one.

[edbeckerich]

Nick, back again to say how great your videos are, man I learned a ton from this one, and can't wait for the next one!!

[cthoadmin]

There's something here I don't understand. I get the part where you say the output from the mixers IF port must be 50 ohms to provide a good match and stop reflections back into the mixer. Couldn't you just feed the IF output into an impedance matching inductor, designed to match the input impedance of the next stage? Filtering would then be done in this next crystal filter (?) section... What have I missed here? Learning LTSpice at the moment, maybe I'll try to model it.

[GFUW]

Genuinely looking forward to the 'so what?' video Nick. Does it make any real world difference? Thanks for sharing.

[Luappy]

One thing you could do with explaining is how you arrived at the series LC values (23.9pF and 5983nH). At 13.3MHz these show a Z of 50.005102963016Ω and w of -0.81855029821357 degrees. Might be interesting to plot (using step params in LTSpice) how changes in L and C affect the w and how this affects the SWR.

[Steve-GMHUU]

👍Thanks for posting, very interesting and useful.

[johnwest]

Excellent presentation. Clear and concise, with what matters most to the way I learn - WHY use a diplexer. Why it's better than the other circuits. Until your presentation I never understood exactly what a diplexer did, or how. Thank you.

My only other comment, (for those who work with much higher frequencies, ) is that resistors don't stay resistors at higher frequencies. One needs to take their inductance (mostly) and their capacitance (possibly) into consideration depending on the frequency of operation. That's where the NanoVNA becomes the most valuable design tool you can have, since ALL components, including resistors have some inductance and capacitance, and it can be dealt with using the NanoVNA's. They have opened up microwave frequency design and construction to the amateur radio hobbyist.

[Luappy]

Nicely explained video. i've been meaning to build a diplexer for my hb DBM for a while now.

[smonair]

Hi, very interesting video and made me consider the diplexer I made for the Eamon Skelton EI9GQ transceiver. I followed Eamon's circuit but moved the IF frequency up from 10.7MHz to 11.2MHz to match the IF filter. So the capacitance and inductance values will require tweaking a bit. LTSpice simulation shows the mismatch I have to be quite small but I will put my VNA on it to check. Looking forward to your next video, Nick. Best 73's de Steve M0SHM.

[LydellAaron]

Thank you so much. Diplexers are an important topic. Have you done a video on phase rotators?

[IanMcCrum]

Hi Nick, I ran the simulation (with a .NET command) and changed the 23.9pF to 22pF - the values for ZIn and Zout changed a lot, the peak is now 150 Ohm at. You may need to "tune" the diplexer to get it to work or pick a different Q value. Thank goodness for NanoVNAs!

[nevmarr]

Hi Nick, another option to put on the IF might be a Wes Hayward termination insensitive amplifier, what'd think? I've seen/read in lots of places that DBM like to see 50R but I've never worked out why. What is it about the architecture of a DBM that makes the IF 50R? Perhaps others could comment too?

[chadx]

Nice explaination.
Check out Mini-Circuits Labs Reflectionless Filter ( XLF-151) series. These are a low pass versions instead of bandpass.
Question: for wide band Data use what is the phase response in the passband, how bad does it distort the IF signal.

[davidbrodie]

Hi, thanks for the video, I found it really interesting and it moved me on a lot with a project I'm playing with. Suffice to say that my mixer IF is currently terminated in a band pass filter and I think I'll be inserting a duplexer now!

[johnwest]

I was re-watching this excellent video, because that's what it is, and I'm a slow learner. :)
I do have a question for you, although a bit peripheral to the point of diplexers. I see that you mount your toroids directly on the copper-clad PCB material. I've watched some other videos that show how the inductances of them change with proximity to metals and dielectrics, so I've been trying to 'suspend' mine off the PCB's as much as reasonable, either by using heavy leads attached at the coils, or with small pieces of Styrofoam for the toroids to sit on, but your circuits appear to be quite accurate as built and affixed directly to the PCB, with little or no tweaking. Is this because the relatively low frequencies of operation are more tolerant to minor changes in inductance?

BTW, I'm going to get one of those UV glue pens. That's an excellent tip. It allows one the control to make sure everything is properly positioned at the moment the glue sets, and I have numerous improperly positioned parts to demonstrate the need for one.

[twobob]

Small Correction "The 'Entire Universe' is as lazy as possible." ;) Not just RF

[mfwwhelan]

Hi Nick,
Excellent clear presentation. I've built several QRP transceivers but none with a diplexer. Next time.
Quick question: I also use Me Squares. What glue do you use to attach them to the ground plane?

Mike, EI6AU

[drstrangelove]

Thank you, good video. What is the black wire on the right at 0:08? A DC bias supply for a diode? (but there's no diode, there is there?)