(T4B09) SSB Receive Filter

T4B09 from the Technician License Course Section 12.0, Avoiding Interference:
Which of the following is an appropriate receive filter bandwidth to select in order to minimize noise and interference for SSB reception?

A. 500 Hz
B. 1000 Hz
C. 2400 Hz
D. 5000 Hz

A radio receiver is really quite a marvelous device, and we operators ask quite a lot of our receivers. Our environment is awash in radio frequency emissions, some being noise and some intentional transmissions carrying information by various modulation methods such as FM, AM, SSB, CW, or others. The task we expect from our receivers is to be able to cut through the RF cacophony and detect just the singular signal that we desire to hear. It is pretty much analogous to finding the proverbial needle in the haystack.

The ability of a receiver to discriminate between multiple signals is its selectivity. That is, a receiver selects one signal in the band among other signals of frequencies immediately adjacent in the band. The receiver filter bandwidth is a primary factor in determining the receiver selectivity. The receive filter bandwidth determines the range of frequencies that will be allowed to pass on through the demodulation sequence and ultimately drive an audio signal to be heard.

Let’s review a couple of basic concepts about RF signals and then we’ll dive into the heart of this question. First, any RF signal uses a range of frequencies, usually a contiguous range. For instance, the single sideband signal typically uses about 3000 Hz (3 kHz) of frequency range, or bandwidth. With a SSB phone transmission, the 3 kHz of RF bandwidth carries the information of voice audio signals to be received and transformed back into sound by a receiver. We can represent the 3 kHz of RF signals of various strength with a spectrum, like this.

receive band

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The carrier frequency, that value that shows up on your receiver display when you tune, is just a sort of reference point that tells the receiver where to find this 3 kHz band of signals that you wish to demodulate into audio. With SSB, no signal is actually transmitted exactly on the carrier frequency value, but the receiver knows to look across a band 3 kHz higher (upper sideband) or lower (lower sideband) in frequency than the tuned carrier value to find the RF band to receive.

Imagine how the receive spectrum would appear if another SSB signal began to partially interpose itself in the selected receive band. In this image, the signal band represented in blue overlaps with the higher frequency end of the signal band that you actually wish to receive (represented in black). This situation will cause interference in your receive audio, as the lower frequencies of the interfering signal (blue) will be demodulated by the receiver along with the desired band’s higher end frequencies (black). The result is that your receive audio will contain noise that, at least in part, masks the desired audio signal you wish to hear.

interference bands

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Now enters the concept of selectivity and the mighty receive filter bandwidth to save the day! You can select a filter that is somewhat narrower in bandwidth than the desired signal bandwidth and position it to cut out that higher end interference from the blue signal. The red dashed line illustrates the “passed band” by the receive filter, and note how it will attenuate the power of the interfering blue signals. That pesky interfering noise in your audio fades into near nothing!

filtering SSB

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“But,” you now ask, “aren’t my desired signal frequencies also being attenuated by this filter?” Yes, necessarily in this scenario the higher end of the desired receive signal is also attenuated, but plenty of bandwidth is still available to produce an intelligible audio signal. The quality of the audio may be slightly reduced, but the even more quality-killing noise has been eliminated, allowing you to understand the now-slightly-narrower desired signal’s audio.

So, if you have a SSB receiver with multiple receive bandwidth choices, you have an advantage by selecting a bandwidth that matches the bandwidth of the mode (SSB in this case) and that optimizes the ratio of desired signal to undesired noise or interference.  With SSB you will maintain a clearly intelligible audio signal with about 2400 Hz, or 2.4 kHz, of bandwidth. This is a good bandwidth match for SSB reception that will help filter out immediately adjacent or slightly interfering signals on the band.

The answer to Technician Class question T4B09, Which of the following is an appropriate receive filter bandwidth to select in order to minimize noise and interference for SSB reception? ”is “C. 2400 Hz.

Related Questions:  T7A04, T4B08, T4B10