Re: Separate LPF for a uBitx

Ravi Miranda

Thank you all for the help. Much appreciated.


On Fri, 8 May 2020 at 17:47, Ashhar Farhan <farhanbox@...> wrote:

I used 4 filters to cover 80 to 10 meters. There are two factors at work here.
First, you notice, the driver and the PA are push-pull. This arrangement produces fairly low even harmonics. Hence, a 7 Mhz signal will have 14 mhz signal about -30 db down while the third harmonic at 21 Mhz will be only 15 db down. Thus, a filter cut for 7 and 10 mhz tha shows -20 db attenuation can work for both bands.
Second, a opposed to the usual two section LPFs, we have used three section LPFs which makes them more aggressive.
In effect, there is no free lunch. There are twelve sections in all (4 LPFs of 3 sections each). We could have organized them as 6 LPFs of 2 sections each. They would have led to 4 more relays and used up another 4 Arduino lines that would have needed more digital hardware. We hams are averse to digital ICs on the analog boards. These sre subjective choices.
- f

On Fri 8 May, 2020, 9:50 PM Mark - N7EKU, <@N7EKU> wrote:


However careful filter design is needed when using just four filters.

The obvious reason is that you still need a good cutoff for the lower frequency band.

Not so obvious is that you need to also have a reasonable input impedance, because the input impedance can vary a lot as you move away from the filter design frequency. This can result in lower or higher than expected power output for the lower frequency band.


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