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Saturday, March 26, 2016

Switching to a Resistive Splitter on the Frankenstein R2


I did a little work on the Frankenstein R2 phasing receiver.  I noticed that I had a lot more noise and hum on 30 meters and above than I did on 40 and below.  I don't think it was common mode hum -- switching to a battery supply didn't help much.  But when I took a look at what happened to RF signals between the antenna connector and the input to the DC receiver mixers,  I noticed that the signal level seemed to vary quite a bit with frequency. 

There were only two circuits in there:  a 1.7 MHz high pass filter (to knock down AM broadcast interference) followed by a simple bifilar toroidal transformer signal splitter.   My guess is that the 1.7 MHz high pass filter's response was being messed up by the bifilar toroid transformer that I was using as a signal splitter. There might have been some unplanned-for resonances between them. This might have had the effect of knocking down the higher freq signals, making any noise in the receiver (probably from the digital VFO) more apparent.  Also, I noticed that I had this toroid too close to the digital VFO box and to the DC power plug for the Arduino/Si5351 combo -- that might have been sending some noise into the DC receiver input. 

To make a long story short, I took out the toroidal signal splitter and went with a resistive splitter like the one above.  This seems to have helped quite a bit.   I know it adds some additional loss -- about 3 db over the toroidal transformer, right? 

Another possibility:  While rummaging around I found a little 3-terminal TV signal splitter.  It is marked "5-900 MHz" but I'm guessing it would be fine down to 1.8 MHz.  Any thoughts on trying that?  

7 comments:

  1. Replies
    1. In 6 db loss compared to a toroidal transformer spliter?

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  2. I have one, like your original, a toroid. Cheapo, not very good even for TV.

    ReplyDelete
  3. A resistive splitter will add

    3 dB above the normal losses of a 3 dB hybrid in-phase splitter.

    For 2 bands, you gotta use a broadband 3dB hybrid with a bifilar FT37-43 transformer instead of a powder iron type. Note, the center tap input impedance is about 25 ohms so you might match that with another coil such as an auto-transformer.

    For example, an auto_transformer might go 14 turns on a FT37-43, ground 1 end and connect the hot end to your 50 ohm antenna input. Tap up 10 turns from the grounded end and that is about the 25 ohm point which is directly connected to the center tap of your 3dB hybrid.

    Now you will get about 3 dB loss and a broadband response.

    Also you might still get common mode noise via poor termination of your mixer IF ports. Read about DC offset .

    Also your active parts need good power line filtering and the LO and components should go in a RF tight conductive box.

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  4. Bill,
    The TV splitter performance depends on the individual mfg. but they are 75 ohms. Usually not good at 1.8 MHz but, yes, 3.2 dB (or so)over their stated range.
    Make your own with an FT-37-43 core as in Fig.3.81 of EMRFD. Match the input accordingly.
    The resistive network in best. I use them in most test equipment circuits. Predictable!
    -ND6T

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  5. Mini-Circuits make coaxial power splitters/combines which in addition to having lower loss than resistive dividers have isolation between the ports which can be useful. They often come up used on auction sites. Here is one chosen at random that covers 0.5 to 600 MHz. http://194.75.38.69/pdfs/ZSC-2-1+.pdf You can get other models in small sizes with SMA Connectors which would be more suitable for your purposes.
    I have used these to split an IF output between 2 receivers, the isolation meant that one receiver didn't load the other if it was switched off for example. I remember building one for use as a return loss bridge/ hybrid splitter from a design in the ARRL handbook, I would need to search for the article, but it used a torroid and some resistors. Handy things to have for example if you need to do 2 tone IMD tests with a pair of signal generators and don't want one sig gen to affect the other.Here is a schematic for such a thing http://www.studioadriana.com/vk5fe/wp-content/uploads/2013/07/RLBCircuit.jpg

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  6. I wanted to make a comment regarding your Frankenstein R2 Clock divider, but did not come around to do it until now and fear if I were to put it below the appropriate post, it would be so many pages away nobody sees it. Please forgive me for posting this here if my assumption is wrong. I had a play with two edge-triggered JK - Flip Flops (74HC109 & HC107) and tied the J and K to the appropriate rails to use them as T- Flip Flops. Because of one being positive, the other one being negative edge triggered, this behaves as a divide by 2 IQ clock generator. The HC107 has an inverting clock input, so as with the other design, some kind of inverter is involved. And as Bill has reported, I initially measured the Phase shift on the scope to be off. But while playing around, I realized this was a function of the signal level. I could tune the phase shift by adjusting the signal level of the driving clock! When the clock and power supply levels were almost equal, the phase shift was very close to 90° and pretty stable with frequency (tested with 1-10Mhz). Later I thought about it some more and suspect it might have to do with the exact time the inverter "flips" on different signal levels in relation to supply voltage level. Aside from the exact cause, I believe one could vary the supply voltage of the gates with the same effect on the phase shift as with varying the signal level. I hope my observation helps to somewhat make the advantages of divide by 2 IQ clock generators more accessible.

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