RX filter problem
Posted: Sun May 21, 2017 7:04 pm
Hi
Did someone really measured the IL of HPSDR/Hermes/Angelia/Orion Mk1 filter ?
I’m trying to understand how is working the RX filter of the HPSDR family (and by consequences all the Anan series except the 8000DLE)
HPF are almost perfect, the distribution of all filters from 1.5 to 64 MHz has been choosen with cleverness (the 1.5 MHz and 6.6 are a little bit too high… but it works)
But I really don’t understand the way LPF are build. All “Alex based” filters are using the very same component values, those defined by Graham KE9H and Phil VK6APH. Here are the simulated rolloff frequencies for the LPF branch
2MHz, 4MHz, 7MHz, 14 MHz, 20 MHz, 30 MHz, 64 MHz
- The two first are too close from the highest authorized 160 and 80 m frequencies.
- 7 MHz filter cuts just at the beginning of the 40m band and do not reject the second harmonic of 80m
- 14 MHz filter forbids any use of the 20m band and do not reject F2 of the 40 m band (could be used only for the 30 m)
- The 20 MHz one is usable for 20m and 17m only
- 30 MHz filter is “limit 29.7” (0.5 dB attenuation on a critical band), can be used for 17, 15, 12 & 10m
- The 64 MHz is absolutely perfect at last… far away enough of the end of the 6m band and ate the right place for antialiasing
But simulation is… simulation, a virtual vision of a perfect world. I decided to build 3 sets of filters (the most “critical” one: 4, 7 and 14 MHz using values given by the Alex schematic and all the Anan literature. Results measured with my VNA are, without surprise, strictly those given by the simulation. A little bit worth on 7.2 MHz with a loss of 30 % of power... it's not a filter, it's a bad joke. A tchebytchev would have been less harmfull than the original Cauer.
I’m surprised nobody asked any question about this lpf distribution. People wishing to use high dynamic frontends are generally trying to "hunt for the smallest dB". And Alex is a filter used for more than 9 years.
Or did I missed something ?
73’
Marc f6itu
Did someone really measured the IL of HPSDR/Hermes/Angelia/Orion Mk1 filter ?
I’m trying to understand how is working the RX filter of the HPSDR family (and by consequences all the Anan series except the 8000DLE)
HPF are almost perfect, the distribution of all filters from 1.5 to 64 MHz has been choosen with cleverness (the 1.5 MHz and 6.6 are a little bit too high… but it works)
But I really don’t understand the way LPF are build. All “Alex based” filters are using the very same component values, those defined by Graham KE9H and Phil VK6APH. Here are the simulated rolloff frequencies for the LPF branch
2MHz, 4MHz, 7MHz, 14 MHz, 20 MHz, 30 MHz, 64 MHz
- The two first are too close from the highest authorized 160 and 80 m frequencies.
- 7 MHz filter cuts just at the beginning of the 40m band and do not reject the second harmonic of 80m
- 14 MHz filter forbids any use of the 20m band and do not reject F2 of the 40 m band (could be used only for the 30 m)
- The 20 MHz one is usable for 20m and 17m only
- 30 MHz filter is “limit 29.7” (0.5 dB attenuation on a critical band), can be used for 17, 15, 12 & 10m
- The 64 MHz is absolutely perfect at last… far away enough of the end of the 6m band and ate the right place for antialiasing
But simulation is… simulation, a virtual vision of a perfect world. I decided to build 3 sets of filters (the most “critical” one: 4, 7 and 14 MHz using values given by the Alex schematic and all the Anan literature. Results measured with my VNA are, without surprise, strictly those given by the simulation. A little bit worth on 7.2 MHz with a loss of 30 % of power... it's not a filter, it's a bad joke. A tchebytchev would have been less harmfull than the original Cauer.
I’m surprised nobody asked any question about this lpf distribution. People wishing to use high dynamic frontends are generally trying to "hunt for the smallest dB". And Alex is a filter used for more than 9 years.
Or did I missed something ?
73’
Marc f6itu