Catalog Filters

I was working on a project for the radio club and needed a couple of filters. One for 571 Mhz and the other for 735 Mhz. Previously I made a stripline filter and I have done copper pipe filters. These are rather large and I wanted something quick. Looking in the Digikey catalog I found filters by Toko. If you are buying a million of these filters I am sure Toko would make them exactly to your spec. For ham use in single quantity we have to take what is stocked in the catalog. Of course the catalog selection was made without consulting us hams to what we might need. The chance of finding something in stock value that is useful approches Epsilon. Much to my surprise I found a helical filter for 734 MHz with a 10 MHz bandwidth. Ok, lucked out on that one. For 571 the closest I could get was 550 MHz. Could I tune it up to 571? This little two section filter has tuning screws at the top of the cans. The screws were near the top so it looked like I was already at the top of the frequncy limit. As it turned out I was able to tune up to 571 MHz. For a test I ran the screws down to see how low I could get and still have the same overall filter response. The stock specification was Fo = 550 MHz, 10 MHz bandwidth. I don't recall the published insertion loss but I was looking at about 5DB. That was measured at the dip of the ripple which was approximatly 2 DB. My test jig was not ideal for grounding the cans, terminating the filter, and making connection to the RG188 teflon cable so I am sure I introduced some error in the response. The results:

Minimum Fo = 494.1 MHz
Maximum Fo = 586.0 Mhz

Other specs remained about the same but the insertion loss and bandwidth was increasing as I approched the upper limit.

For this filter I can pull about 8% from center but the catalog frequency was already near the top of the range. This would be unknown to the purchaser until after they examined the product.

If you need a filter check the catalog offerings from Digikey and Mouser. You may luck out and find something that might tune to the desired frequency but I would not try to go beyond 8 to 10 percent.

For reference this part was Toko type 7HW, Part number # 252HXPK-2736F (Digikey #TK3307-ND)

Followup on Cheap Cables

This is a follow up of the cable problem of the previous post.

From the photograph you can see there is minimal copper braid but an inner foil around the center dielectric. Looking into the cut-a-way connector body I found the crimp was applied metal against cable jacket. There were strands of the copper braid sandwiched between the metal connector body and the cable jacket. That was the extent of the ground connection. The foil did not touch the connector body as the inner chamber of the connector widened out past the cable jacket. I suspect the shield connection had been compromised. The bad connection could have been caused a number of factors such as the copper braid being pushed into the jacket, corrosion of the copper wires, or breakage of the wires.



Moral of the story is if you obtain cheap cables they may work fine but keep in mind they may have limited life. Don't be afraid to chuck them at first sign of trouble. I still prefer my homemade cables made from LMR-240 Ultraflex and the correct connectors for that cable type.

Cheap Cables

I picked up a box of 3-foot and 6-foot BNC cables. The cables are marked as RG58. Overall construction of the cables look good as they have molded ends on the BNC connectors. Unfortunately good looks is where it ends. Electrically the cables suck. A good example raised its ugly head today. I was checking a function generator and connected it to my HP 5335 frequency counter. Ran through a few frequencies to make sure the calibration of the generator was reasonable. Higher frequencies no problem. When I went to 1 KHz the counter would not lock onto the signal. I connected the generator to my oscilloscope and observed a nice 1 KHz sine wave. Playing around with the counter I engaged the filter switch and got a correct measurement of 1 KHz. Looks like I am getting noise in the input on lower frequencies that prevents the counter from locking onto the desired signal. I placed a few clamp-on ferrite chokes on the cable and that also took care of the problem. I then replaced the cheap BNC cable with one I made from LMR240 ultra flex. Problem went away without any filtering.

Moral of the story. Buy good cables they will save you headaches in the long run.