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--- filters [2024/10/03 23:23] – created river
+++ filters [2024/10/04 01:57] (current) – river
@@ Line 7: / Line 7: @@
 ===== Bandpass Filters =====
-These reduce signal outside a wanted passband. The main types available to buy for these frequencies are SAW and cavity. I didn't see anyone selling LC type filters - perhaps they are not sharp enough.
+These reduce signal outside a wanted passband. The main types advertised for these frequencies are SAW and cavity. There are also LC filters.
 Cavity filters appear to have excellent specs with very little pass band loss, high power handing, etc, but they are expensive - 200 GBP typical.
-SAW filters are very cheap - a few pounds on aliexpress - have good stop band rejection, but have high passband loss and low power handling.
+SAW filters are very cheap - a few pounds on Aliexpress - have good stop band rejection, but have high passband loss and low power handling.
-===== Aliexpresss 868MHz bandpass =====
+I found, after a bit more searching, some LC filters with fairly good looking specs.
+===== Aliexpress 868MHz bandpass =====
 [[https://www.aliexpress.com/item/1005007539598090.html]]
+It seems to say it is an LC filter.
 The listing claims:
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 I believe I was testing at +5dBm. Device power was set to "-4dB" on low frequency transmitter, and "+5dB" on high frequency one, which a forum, but not the manual, says changes over at 140 MHz. I would like to try on higher power when I get an amplifier.
+Chip marked Gr 2J, or Qr 2J?
+Ohmeter shows open circuit on input, output, and between them.
+The ports are marked in and out. I tried testing the board backwards, and the results were almost the same, but I think there is a small difference in RL.
+Testing one board, at 870 MHz, RL forwards was -17.41dB, backwards -17.04dB. S21 forwards -1.69dB, backwards -1.67dB. I think it probably doesn't matter which way it is connected.
+I noticed the return loss numbers are significantly different from earalier measurements, which as seen in picture above are around -30dB, not -17dB. I don't think I changed anything. Maybe because the LiteVNA was left to warm up? That is a huge difference though. I tried recalibrating and then got numbers around -25dB, closer to what we had before.