Re: Testing 1:1 Ferrite Transformers

Michael Hagen
 

I answered some of the questions, method.

I made a tight setup with everything connected with SMA connectors on a tiny PCB.

Shorten leads on the transformer.  Now much better! SWR about 1.3:1 ?  This was all done at 12 MHZ with 1 MHz span.

Inductance on 3/8" ferrite toroid 6T #30 was measured at about 18uH.

So I am expecting it hard to get much better.  My SMD resistor 0805 47 Ohm,  and I forgot to measure it.

So this is why PCBs need to be small and laid out well.  Cheap SMA connectors from eBay sure make testing easier.

Impedance matching as most have stated, is the big deal in RF design.  Getting 50 ohms is tough.

73's

Mike, WA6ISP



On 2/21/2018 8:03 AM, Michael Hagen wrote:
I had a few days playing with my crystal filter PCBs.
I spent most of Sunday learning about my Siglent SA.  Spectrum Sunday!
I had in stored out in the shed, a sin.  I don't have room for all this stuff.

I used the tall 12 MHZ crystals.  From tests a year ago, I determined from all the testing that the cap value was 240 pF.
This is the 8 crystal filter, just like the one in uBit.  The PCB has 2 Pin Molex and matching transformers at each end.
I used Molex connected RG174 with BNCs about 1 ft long.

I made connections to the PCB for the crystals with "Machine Pins".  Nice fit.    Again tested the crystals in an Oscillator so all 8 were selected to under 70 Hz.   I did notice trimming the leads upped the frequency some.  They are trimmed to about 1/4".
I have 1:2 transformers on the 2 hole cores. I connected them as auto-transformers.
I saw about a 1.7 KHz filter with at least 4 dB ripple in the pass band.
I have tried 220pF and 270pF.
So, I started testing transformers.  I set up the SA with a Directional Coupler to measure Reflection Cooef.
Thus SWR.   I tried winding different transformers.   At 12 MHz, the best SWR I could get was 1.6 :1?
Bifilar twisted around a toroid, and used full transformer (isolated not auto connection) with a 47 Ohm SM
on the output.  With an SMA 50 Ohm Dummy I get reflection of -22 dB (really good).  With the transformer
setup I get about 12-13 (SWR 1.6).

Is this the best it gets?   How should I otherwise match my crystal filter.   Is that causing the ripple.
I thought I was driving the filter with about 200 Ohms?

What say the real RF guys?

Thanks, Mike, WA6ISP
 

 
-- 
Mike Hagen, WA6ISP
10917 Bryant Street
Yucaipa, Ca. 92399
(909) 918-0058
PayPal ID  "MotDog@..."
Mike@...

-- 
Mike Hagen, WA6ISP
10917 Bryant Street
Yucaipa, Ca. 92399
(909) 918-0058
PayPal ID  "MotDog@..."
Mike@...

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