Difference between SAW filters types | IIDT, DMS, Ladder type
This page compares SAW filter types IIDT filter vs DMS filter vs Ladder filter and mentions difference between IIDT filter, DMS filter and Ladder SAW filter type.
Initially, the SAW filters are used instead of LC filters at IF stage of TVs. Later on SAW filters are employed in radar, CATV, mobile phones, pagers, satellite tuners and so on.
There are three main types of SAW filters.
• IIDT Filter
• DMS filter
• Ladder filter
IIDT SAW Filter
• The figure-1 depicts IIDT (Inter-digitated Inter-Digital Transducer) SAW filter structure.
• It has insertion loss of about 3 to 4 dB.
• It requires external matching circuit.
To overcome the disadvantages of IIDT filter, DMS and ladder type SAW filters are designed and developed later on. These DMS and ladder type of filters have low loss characteristics and are like BAW filters with lower frequency bands.
DMS SAW Filter
• The figure-2 depicts Double Mode SAW (DMS) filter structure.
• It is made by constructing number of IDTs between the gratings.
• This SAW filter type has wideband characteristics due to
combination of various resonant modes.
• It is used in balanced input or output
by having proper arrangement of IDTs.
Hence it is used to connect balanced input amplifier.
• This filter type utilizes two identical resonant modes which are
acoustically coupled in longitudinal direction.
• It offers low loss, lower power durability and good
out of band rejection.
Ladder SAW Filter
• The figure-3 depicts Ladder SAW filter structure.
• As shown, it is made by connecting number of 1-port SAW resonators in ladder like
form.
• It is used for un-balanced input and output.
• It uses two types of 1-port saw resonator one in
series arm and the other in parallel arm. They have different
resonant frequencies and are coupled electrically.
• It offers lower loss, high power durability and
moderate out of band rejection.
Following table summarize comparison between SAW filter types based on one research conducted at Fujitsu Labs. in JAPAN.
| Specifications | IIDT SAW filter | DMS SAW filter | Ladder SAW filter |
|---|---|---|---|
| Insertion Loss | 3 to 4 dB (Maximum) | 2 to 3 dB | 1 to 3 dB |
| Attenuation vs frequency | approx. 25 to 50 dB | approx. 25 to 60 dB | approx. 20 to 40 dB |
| Bandwidth | Variable | Approx. 3.5 % | Approx. 4.0 % (much wider and hence best) |
| High Power durability | Good | Bad | Good |
| IP3 (Third Order Intercept Point) | ~37 dBm | Bad | ~61 dBm (Best) |
| Balanced Type | Possible | Possible | Not possible |
From the table following points can be summarized on diference between SAW filter types.
➨Insertion loss of DMS and ladder types are lower compare to IIDT.
➨Attenuation level of out of band is superior in all three types.
➨Bandwidth of ladder type is wider and best compare to DMS.
➨High power durability of ladder type and IIDT is much better
compare to DMS filter type.
➨The balanced type of design is not possible for ladder type SAW filter.
For this application, lattice type filter design is employed.
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