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· Mentor
39 Posts
Discussion Starter · #1 ·
Okay, ran into an odd scenario with the SWM-8 module and OTA.

Scenario 1:
SWM-8's SWM1 output (with antenna connect to SWM OTA input) ---> PI ---> D-2100LX diplexer -> HR20 SAT1 and OTA inputs.
This scenario works FINE!

Scenario 2:
Same as above, except add an HR21 to the SWM2 output of the SWM-8. I do not yet have the AM21 connected to the HR21 so this is a SAT only leg off the SWM-8, no diplxers or splitters in this path at all. This receiver gets ZERO signal! (The HR20 continues to work perfectly with both SAT and OTA inputs).

Now the odd part. IF and only if I disconnect the OTA input on the HR20 receiver (on SWM1 leg), then the HR21 (on SWM2 leg) gets sat signals perfectly!!!

Even more odd - Reconnect the OTA input on the HR20 (SWM1 leg) but replace the Eagle Aspen D-2100LX diplexer with an RCA 5-2300MHz all ports passive Splitter (not diplexer). This works FINE!

Does this make any sense? I am about ready to just chuck all my Eagle Aspen diplexers and get Channel Master CM4001IFDs or 5-2300MHz high-freq splitters.

· Hall Of Fame
3,937 Posts
AntAltMike said:
The SWM uses a return signal frequency of 2.3 MHz. It has been reported in other threads there that NAS makes a diplexer that passes 2.3 MHz along with the 22KHz lowpass...
AntAltMike said:
The NAS diplexer is deliberately engineered to pass the 2.3 MHz to the correct port. NAS is a company that only supplies to commercial installation companies. Someone could buy one for you from NACE or PDI....
davros74 said:
Actually, I dug around and some of my Eagle Aspen diplexers are different from each other. Some of them are the D-2100LX model, rather than the D-2200 I mentioned earlier. I tried one of the D-2100LX diplexers and it works perfectly!!!

Summary: Eagle Aspen D-2200 diplexer (5-860/950-2300 DC/22kHz pass) does NOT work.
Eagle Aspen D-2100LX (40-850/960-2100 DC/22kHz) does WORK!

Since the biggest difference is the cutoff frequency on the TV side (5MHz vs 40MHz) this seems to confirm my theory that I was getting low-VHF interference on the OTA input possibly corrupting the 2.3MHz control signal.

Both diplexers work with SWM8 with the OTA input disconnected, so both do pass 2.3MHz just fine. The D-2200 must have other issues w.r.t. freq response.
You were given the answer you needed in the previous thread. The NAS diplexer is engineered to function properly when diplexing SWM and off-air or cable. Other diplexers engineered for use with pre-SWM L-band diplexing were not engineered to react to 2.3 MHz signals in any particular way. Not only can one not tell from the published specs for pre-SWM diplexers what their nominal attenuation of the 2.3 MHz signal would be on either port, we also don't know the threshold parameters (receiver's 2.3MHz output level, SWM's minimum input signal requirement) or how much the attenuation may vary from individual unit to unit, so one can't even scientifically assess the suitability of a non-approved diplexer in that application.

Digital signals have avalanche failure characteristics. Typically, as long as they are within some huge window of signal strength, they will be good enough to be processed perfectly, but then, dropping them by another dB or two will put them below the minimum threshold and the circuit will not work. Simply changing any load on a shared system slightly changes signal levels throughout the system and can drop an anemic 2.3MHz return signal below the operating threshold, and similarly, introducing a seemingly minimal amount of noise or interference can devastate a weak but otherwise sufficient return signal, whereas a healthy return signal would easily withstand that additional degradation.

I couldn't find the NAS diplexer on their website, but you can call them at 718-567-8470 ext. 101 or 102 or email [email protected]
Expect to get someone whose English is not too good. You will be asking for a part number and the name of a dealer who may furnish you with one. Most likely, they will give you the part number, but the only distributors they sell to will not sell you one because you are not a dealer. Then come back here with the part number and maybe someone who is a dealer can get you as few.

Alternatively, one reliable forum member here said he got good results with what he described as a "cheap" radio Shack diplexer. Warning: YMMV!

· Mentor
39 Posts
Discussion Starter · #3 ·
Thanks for the help, but the part I am still confused on is why aforementioned diplexer works fine with a single receiver, but causes the OTHER receiver to lose signal only when the OTA input is connected.

I fully plan to replace this diplexer with a Channel Master or Pefect Vision one next since I can get them relatively cheap and fast, and then see if I can get a NAS one if those do not work.

Maybe I don't fully understand what happens inside the diplexer, but as I understand it, the frequency response at each output would be significantly less than using a hi-freq splitter (passing all 5-2300MHz on each leg). No interference problems there with the splitter at all.

I can only think of two things going on here: The diplexer is causing some non-linear distortion of frequencies and interfering with the control signal as a side effect (there is no interference inherent in the signal because the plain splitter works fine passing all freqs). The diplexer does NOT pass 860-950MHz on either leg, but I don't believe that should affect the SWM operation.

Passing 2.3MHz doesn't appear to be the problem, it appears that the OTA leg of my diplexer is introducing interference with the control signal by itself. It is strange that it works on just one receiver, but interferes with a different receiver on a completely different SWM leg. I am using the high-freq splitter for now since it just works and no one has offered up any reasons why such a splitter is any better or worse than a dilplexer in this role, because the frequences in the SWM will never collide with the VHF OTA signals, so why shouldn't I just keep using the splitter since it works? Seems the diplexers are a lot touchier since the specs required to use with SWM are not published, except for the NAS one you mentioned.

Thanks again for all the advice provided in this forum.
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