EMRFD Message Archive 11722

Message Date From Subject
11722 2015-10-03 17:25:31 kerrypwr Bridge-T Diplexer Report
Just a report on the final version of my mixer that includes a bridge-T diplexer.

I reduced the diplexer Q to 5; that gave just-more practical values, notably for the small-value inductor in the parallel-tuned section;

http://s8.postimg.org/9mr6kaap1/Diplexer_Schematic_12_8_MHz_Q5.png

The 124nH inductor was wound on a Teflon toroid about the size of a T37; the toroid required only a few minutes work in the lathe;

http://s14.postimg.org/l5csa3rs1/image.jpg

 

The following test results are with the SBL-1 mixer disconnected; I will do VNA and spectrum analyser tests of the complete module later.

The amplifier is the "standard" post-mixer type; bias is 45mA.  It is followed by a 6dB attenuator.

The diplexer is designed for 12.8MHz; the as-yet-unbuilt filter will use 12.8MHz crystals.

The responses of the amplifier + attenuator and of the diplexer + amplifier + attenuator are shown here;

http://s7.postimg.org/j47rwtlqz/Amp_Diplexer_Response_60_MHz.gif

The 3dB bandwidth is about 2MHz.

Kerry VK2TIL.

 




 




11723 2015-10-03 18:22:13 Dave Re: Bridge-T Diplexer Report
  Have you done a return loss plot?

Dave - WB6DHW


11724 2015-10-03 19:17:31 kerrypwr Re: Bridge-T Diplexer Report
Good idea Dave; after all, SWR/RL is a major objective of the diplexer.  :)

In fact I did but forgot to include it in my initial post; this is just the diplexer, nothing else

http://s22.postimg.org/56lov62cx/Diplexer_Alone_SWR.gif

Kerry VK2TIL.

 

11726 2015-10-03 21:46:43 K5ESS Re: Bridge-T Diplexer Report

Seems like making a Teflon toroid core is a lot of unnecessary work.  Most of us don’t have a lathe.  What does it provide over just a cylindrical Teflon or air core?  Maybe I’m missing something.  Open to enlightenment.  

Mike K5ESS

 

11727 2015-10-03 22:05:03 kerrypwr Re: Bridge-T Diplexer Report
Compactness and stability were the main reasons I used a toroid; I did consider a cylinder though.

Material #O toroids are available (I think DIZ has them); they are phenolic with no magnetic properties.

Kerry VK2TIL.
11728 2015-10-04 06:11:37 DuWayne Schmidlko... Re: Bridge-T Diplexer Report
You can get nylon washers in assorted sizes.  Early BITX builds used faucet  washers.
DuWayne KV4QB

11751 2015-10-09 01:38:29 Weddig, Henning-C... Re: Bridge-T Diplexer Report
Kerry,

you can improve the VSWR in the passband (i.e. aroung 12 MHz) if You
tune both resonators !

I have seen that using a too high loaded Q of the diplexer will make it
more difficult to obtain a good match (low VSWR) within the passband of
the diplexer.

Henning Weddig
DK5LV

Am 04.10.2015 um 04:17 schrieb planningpower@iprimus.com.au [emrfd]:
> Good idea Dave; after all, SWR/RL is a major objective of the diplexer. :)
>
> In fact I did but forgot to include it in my initial post; this is just
> the diplexer, nothing else
>
> http://s22.postimg.org/56lov62cx/Diplexer_Alone_SWR.gif
>
> Kerry VK2TIL.
>
>
11752 2015-10-09 02:56:08 kerrypwr Re: Bridge-T Diplexer Report
Thanks Hennig; that is a very insightful observation.

I'm trying to balance SWR with selectivity in the diplexer; it's clear that there are "trade-offs" in this.

Perhaps my idea of obtaining some "free" selectivity at this early point in the chain is not necessarily a good one.

I'm fairly happy with the result; SWR of 1.8:1 in the passband is not too bad; I will have to think about whether I want to trade that for less selectivity.

Your post ties the selectivity -v- SWR relationship very neatly; thanks.

Kerry VK2TIL.
11753 2015-10-09 06:07:26 kb1gmx Re: Bridge-T Diplexer Report
There is an error there.

That is that the diplexer, bridged-T is intended to provide selectivity, this is not the case.
The circuit is there and functions to rout out of band signals to a dump load (resistors) 
and desired signals to the succeeding stage.

Over some frequency range that can be achieved but as you go higher the values become 
impractical for higher Q and the losses greater.

If selectivity is required then a highpass, bandpass, lowpass triad is needed.

Since in most cases the diplexer is followed by gain or selectivity (usual is both in that order)
the need for selectivity is low and the design goal is to effectively terminate the mixer for wide 
bandwidth.

Attempting to achieve termination and selectivity at low loss is a conflicting set of goals.

Allison


11754 2015-10-09 06:56:23 Bill Carver Re: Bridge-T Diplexer Report
Hennig's post was correct. Tune both series and parallel tuned circuits to the i.f. frequency. There's no reason to accept 1.8:1
W7AAZ