12.7 IF Selectivity & Filter Bandwidth
Key Takeaways
- A receiver IF filter bandwidth should be slightly greater than the bandwidth of the received signal
- 2.4 kHz IF selectivity is optimum for SSB voice signals
- 10 kHz IF selectivity is optimum for double-sideband AM
- 15 kHz IF selectivity is desirable for a wideband FM phone receiver
- Too wide an IF filter lets undesired signals reach the audio stage; SAW filters suit micro-miniature circuits
12.7 IF Selectivity & Filter Bandwidth
Quick Answer: IF bandwidth should be slightly greater than the received-signal bandwidth. SSB voice → 2.4 kHz. Double-sideband AM → 10 kHz. Wideband FM phone → 15 kHz. Too wide and undesired signals reach the audio stage. SAW filters suit micro-miniature circuits.
Sub-topic 3-F-045 is three numbers and one principle. Because the numbers map directly onto the modulation types in chapter 14, learning them here pays twice.
The matching principle
How should the filter bandwidth of a receiver IF section compare with the bandwidth of a received signal? Slightly greater than the received-signal bandwidth.
Not equal, not much greater — slightly greater. The reasoning cuts both ways:
| Filter too narrow | Filter too wide |
|---|---|
| Clips the edges of the wanted signal | Undesired signals reach the audio stage |
| Muffled, distorted audio | Adjacent-channel interference audible |
| FM signals distort badly as deviation peaks are cut | Noise floor rises — bandwidth admits noise power |
| Reduced effective sensitivity |
The margin exists because a real signal is never perfectly centred. Transmitter frequency error, receiver oscillator drift, and Doppler shift on a moving vessel all displace the signal slightly within the passband. A filter exactly as wide as the signal would clip whichever edge drifted outward.
What is an undesirable effect of using too wide a filter bandwidth in the IF section of a receiver? Undesired signals will reach the audio stage. That is the pool's phrasing, and it is worth noting the mechanism: a wide filter does not merely add noise, it lets adjacent-channel transmissions through to be demodulated alongside the one you want.
The three numbers
| Signal type | Optimum IF selectivity | Why |
|---|---|---|
| SSB voice | 2.4 kHz | One sideband only, carrying roughly 300–2700 Hz of speech — no carrier, no second sideband |
| Double-sideband AM | 10 kHz | Carrier plus two sidebands: a 5 kHz audio bandwidth becomes 10 kHz of RF |
| Wideband FM phone | 15 kHz | Carson's rule on a narrowband-FM voice channel with ±5 kHz deviation |
These are asked in both directions — "a receiver selectivity of 2.4 kHz is optimum for what type of signals?" as well as "what selectivity is desirable for a wideband FM phone receiver?" — so learn the pairing, not just the list.
Where the numbers come from
SSB — 2.4 kHz. Suppressing the carrier and one sideband is the whole point of SSB (section 14.2). What remains is a single translated copy of the speech band, roughly 2.4 kHz wide. This is why SSB is so spectrum-efficient, and why a 2.4 kHz filter in an SSB receiver is doing exactly the right job.
AM — 10 kHz. In double-sideband AM the transmitted bandwidth is twice the highest audio frequency. Feed 5 kHz of audio in and you occupy 10 kHz of RF, symmetric about the carrier. Note the relationship to SSB: AM needs roughly four times the bandwidth of SSB for the same audio, because it carries two sidebands instead of one.
FM — 15 kHz. For frequency modulation, Carson's rule estimates bandwidth as 2 × (deviation + highest modulating frequency). A marine or land-mobile channel with ±5 kHz deviation and 3 kHz audio gives 2 × (5 + 3) = 16 kHz, which the pool rounds to the standard 15 kHz IF figure. Recall from section 20.3 that ±5 kHz is exactly the maximum allowable deviation for VHF marine radios — the numbers are consistent across the pool.
Filter technologies
Which one of these filters can be used in micro-miniature electronic circuits? Receiver SAW IF filter.
A SAW (Surface Acoustic Wave) filter launches an acoustic wave across a piezoelectric substrate between interdigitated metal transducers. Because acoustic waves travel roughly 100,000 times slower than electromagnetic ones, the structure that would need metres of transmission line becomes millimetres of chip.
| Filter type | Typical use | Size |
|---|---|---|
| SAW | Micro-miniature circuits, IF filtering in handhelds — the pool's answer | Chip-scale |
| Crystal / monolithic crystal | Narrow SSB and CW filters, high Q | Small module |
| Ceramic | Consumer FM/AM IF stages | Small |
| LC | Wideband IF, general purpose | Larger |
| Mechanical | Very sharp HF-band IF filters (older designs) | Bulky |
Practical note: filter characteristics live in the shape factor — the ratio of the −60 dB bandwidth to the −6 dB bandwidth. A shape factor near 1 approaches a brick wall; a value of 3 or more means skirts so gentle that a strong adjacent signal still gets through even though the nominal bandwidth looks correct. Two filters both specified at "2.4 kHz" can perform very differently for this reason.
On the bench
A receiver that sounds muffled and lacks highs on SSB, or that distorts on FM, is a candidate for a too-narrow or misaligned IF. A receiver that is noisy and picks up the channel next door has an IF that is too wide — or a filter that has been bypassed or damaged. Because the IF filter fixes the receiver's fundamental selectivity, no amount of audio filtering downstream can recover what a wrong IF bandwidth has already let through.
A receiver selectivity of 2.4 kHz in the IF circuitry is optimum for what type of signal, and what selectivity is desirable for a wideband FM phone receiver?
How should the IF filter bandwidth compare with the bandwidth of the received signal, and what happens if it is too wide?
A receiver selectivity of 10 kHz in the IF circuitry is optimum for what type of signal, and why is that figure roughly four times the SSB value?
Which filter type is suited to micro-miniature electronic circuits, and what specification distinguishes two filters of the same nominal bandwidth?