What's the difference between LPF and BPF in context of bandwidth setting?

Question

I'm used to simple RX/TX bandwidth configurations, on my VHF FM rig it's either narrow, normal, or wide. It seems to be much more complex on my new HF SDR rig, and the instructions don't say anything about what the settings mean.

Below are the options I have when in SSB. For 20m and 40m the UK bandplan says to use 2.7k, but what is the difference between 2.7k LPF and 2.7k BPF ? Both seem to give the same result.

There's also a bunch of strange narrow options. For example the 300Hz 500Hz option sets the bandwidth to 500Hz but moves the tuned frequency 300Hz away from what the dial is tuned to.

I've Googled the question but no luck finding anything.

The SSB bandwidth options available on my radio:

300Hz 500Hz
300Hz 550Hz
300Hz 600Hz
300Hz 650Hz
300Hz 700Hz
300Hz 750Hz
300Hz 800Hz
300Hz 850Hz
300Hz 900Hz
300Hz 950Hz
500Hz 550Hz
500Hz 650Hz
500Hz 850Hz
500Hz 950Hz
500Hz 950Hz
2.7k LPF
2.7k BPF
2.9k LPF
2.9k BPF
3.2k LPF
3.2k BPF
3.4k LPF
3.4k BPF
3.6k LPF
3.6k BPF
3.8k LPF
3.8k BPF
4.0k LPF
4.2k LPF
4.8k LPF
5.0k LPF
5.5k LPF
6.0k LPF
6.5k LPF
7.0k LPF
7.5k LPF
8.0k LPF
8.5k LPF
9.0k LPF
9.5k LPF
10.0k LPF

Answer 1

SSB has a bandwith of roughly 2.7KHz. However, if you are listening to a station, but there is another station (or loud noise) overlapping with it slightly either below it or above it, you can narrow the receive bandwidth to try to clip off the interfering signal. The result will make the fidelity of the station you are listening to slightly worse, but with less noise.

• LPF = Low pass filter, clip a higher frequency station
• HPF = High pass filter, clip a lower frequency station
• BPF = Band pass filter, clip both upper and lower

If you narrow it down to as little as 500Hz, there will be barely enough bandwidth left to distinguish voice. The real use of these very narrow filters is to listen to CW, cutting off lots of surrounding CW stations.

When you are listening to CW, as you turn your dial, the CW station's audible frequency will go up or down. If you tune the dial exactly to the CW station's frequency, it will disappear completely. So typically, you offset to about 300Hz, which puts the audible frequency at about 300Hz.

The wider bandwidth filters are for modes other than SSB. You need 6KHz for AM and 6KHz or more for FM (used on 10m and 6m). You can also use the wider filters for listening to a larger number of stations at once for digital modes like JT65/JT9 and PSK31.

Answer 2

The 300 650 combination is often used for CW (Morse Code) reception. It tunes the SDR receiver’s (Tayloe mixer or FPGA or software synthesized) oscillator(s) 650 Hz below (or above for LSB) the dial frequency so that you will hear a 650 Hz beat frequency tone for RF signals transmitted at the dial frequency, and inserts a 300 Hz bandpass filter that cuts off or reduces interference from signals either or both 150 Hz above 650 Hz audio and 150 Hz below 650 Hz,audio, e.g. a total width of 150+150 = 300 Hz bandwidth. The 300 Hz filtering could either be done on the audio, or around the offset dial frequency in the RF or IF domain, depending on the type of SDR. Or both.

So 650 is the offset, and 300 is a width that doesn’t start down at 0 Hz like an audio lowpass filter, but is centered around a higher 650 Hz audio tone, thus a bandpass..

The offset can be varied from 500 to 950 because CW operators have different preferences for the tone they want to hear when listening to CW, low or high, or from around 2 octaves to almost 4 octaves above Middle C.