Quote:
Originally Posted by G0HZU_JMR
See below for the result with a basic Lmatch on each side. This matching network consists of a series 120pF cap followed by a shunt 2uH inductor across the filter inputs. You can see the return loss is good but the bandwidth where there is a good match is slightly narrower now.
This 10.7MHz ceramic filter is a Toko part and it was used in a VHF/UHF comms receiver I designed at work back in the late 1990s. The receiver had various bandwidths for FM reception.
I think the Toko part number was SK107M1-A0-10X but I've had to look through an old company database to get this number. It is described as having 280kHz bandwidth.
I've also taken VNA data for SK107M3-A0-20X but this is described as having 180kHz bandwidth. The receiver also had an option for SK107M5-A0-10X and this had 110kHz BW according to the old database at work. However, I've not got any of these here to measure.
I also measured a Murata SFE MS3 and this has E10.7S stamped on it. This is a much newer part and was in a bag of samples at work. This is quite a narrow filter with a rounded response and it is a bit lossy with just over 6dB loss when matched.
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Cheers. I know very little about the design of VHF/UHF comms receivers and I thought they are mostly narrow-band FM. In fact I have very little interest in Ham radio or QRP stuff, but I have to force myself reading the relevant technical materials in order to demystify the RF black art. I am more into the the design and evolution of high-end FM stereo tuners.
The standalone measurement of the ceramic filter has limited value and that's why I measured the bandwidth of the two ceramic filters with the CA3053 cascode amp to quantify the bandwidth reduction in the synchronous tuned cascade filters. Also the bandwidth of the IF starts to widen when the amp is limiting with strong signals. So it is difficult to predict how many stages of ceramic filters are required for the best audio sound quality without compromising the selectivity. There are many variables..
Here I attached two random examples of interesting designs:
SAE Mark VI- digital tuner: It uses 10-pole toroid filter in front of the CA3053 IC IF amp followed by 5-pole toroid filter and limiter ICs.
Kenwood KT6040-It is a low-cost tuner, having a 6-gang varactor front end tuning to save costs. The typical Q of the varactor diode tank circuit is about half the value for the air variable gang capacitors. The 6-9 air gang capacitors are the most expensive components in high-end tuners. The diode switching allows the option for either the narrow-band IF with 6 ceramic filters or wide-band with 2 ceramic filters.