C-band IF filter... 1

[davidjos]

I'm looking for a source to obtain a 70MHz IF bandpass filter for 3.7 to 4.2GHz. Anyone know of one?

Thanks...Dave

 

[Comcokid]

Hummmm - you request has some contradictions.

To request an 70 MHz IF filter would mean you are looking for a IF filter (with a unstated bandwidth) for a application where the center frequency is 70 MHz.

70 MHz is a common center frequency for a second IF and is commonly used for commercial and military receivers. A 70 MHz IF might have a bandpass as wide as 50 MHz.

Now, the difference between 3.7 and 4.2 GHz is 500 MHz and sounds more like the input frequency range of a receiver, or of a downconverter, or the tunable bandwidth of a tuner ahead of the first mixer for a superhet receiver.

Can you provide a little more information on your application?

 

[VEBill]

C-band and 3.7-4.2 GHz indicates OP is refering to Big Ugly Dish satellite TV. These days, most systems would convert down to L-band (normally 950 - 1450MHz) within the LNBF at the focal point. The tuner 'can' in the receiver would then tune across the band (L-band) and convert to the chosen IF. I've seen 70 MHz many times so it is obviously not uncommon for consumer satellite TV receivers.

BTW - typical satellite TV receivers can cover all bands (C, Ku, Ka, whatever...) provided they're connected to suitable LNBFs.

The specs for the filter would be fairly tight for this sort of application (depending on the receiver design). On the other hand, the filters probably sell for pennies in the quantities that are used these days. The entire receiver wholesales for typically US$30 (+/- $20).

 

[davidjos]

VE1BLL's input is my situation. I'd much rather use a filter at the LNB, but have some physical limitations and its a time sensitive (aren't they all?) solution needed. I'm getting clobbered by aeronautical/marine radar and its hosing a news feed downlink for TV. I found a filter thats inserted between the downconverter and receiver but its a week lead before shipping. I do have a filter for the LNB and most likely will attempt making a shroud out of PVC pipe to accomodate it...unless I find an IF-type sooner.

Thanks for both your replies.
-Dave

 

[VEBill]

Assuming the interference is coming in directly at C-band (radar), then the solutions that I've seen before are:

1) Use a feed with low sidelobes (concentric circles)
2) Relocate the dish so that it isn't in view of the interference (but still sees the satellite)
3) Add petals to the edge of the dish so that it improves sidelobes (reduction of edge diffraction).

I'd also make sure that you use high quality 'quad shield' coaxial cable and good terminations. Signals might be leaking in along that path (maybe).

If the interference is at C-band, then filtering it at 70 MHz (effectively the 2nd IF) is probably too late. The strong radar would have already clipped before then and couldn't be filtered out. If it wasn't clipping, then you probably wouldn't be seeing it now...

I've never had a C-band B.U.D. myself (not yet), but there is tons of info on the

http://www on

this topic.

 

[davidjos]

Thanks for the tips. When the weather clears around here, I'll get up to the dish and see where I'm at with adding a filter at the LNB. The pixelation isn't real bad, any pixelation is just bad period. Hoping the filter will cure the problem. Will have to negotiate moving the dish if not.

Regards,
Dave