Delay Lines
Overview
Surface Acoustic Wave (SAW) techniques are extremely general: any linear bandpass filter or delay line may be synthesized, with arbitrary amplitude and phase, limited only by line width and substrate size. The devices are small, rugged, very stable, and capable of high volume low cost production. Delay lines are simply bandpass filters with ~.13in/us additional path length between the transducers to achieve the desired insertion delay. The path can be folded to conserve substrate length.Design:
Adjustment of electrode length and position allows the SAW designer to synthesize any finite impulse response. For delay lines specified in the frequency domain, FIR digital filter design techniques are used to find the initial optimum, i.e. shortest, time response. Delay lines may also be specified directly in the time domain. Delay lines may be specified by defining templates or mathematical functions in the frequency or time domains.
Performance:
| Typical | Limit | |
| Center Frequency fo MHz | 20 – 1500 | 10 – 2500 |
| PassBandWidth B*100/fo for quartz | .5 – 5 | .1 – 10 |
| for lithium tantalite | 5 – 10 | 1 – 40 |
| for lithium niobate | 10 – 30 | 5 – 150 |
| Amplitude ripple in B dB pp | >1 | .2 |
| Phase ripple in B degrees pp | >10 | 2 |
| Min insertion loss IL dB | >25 | 10 |
| Return loss in B dB | none | 14 |
| Delay T us | <10 | 100 |
| Time spurious dB | -40 | -60 |
When appropriate, key system level performance parameters may be specified. They can be verified by computer simulation based on measured frequency data. Simulation software has been prepared for: time response for any input signal, pulse widths and sidelobes, and signal to noise ratio loss.
Key to Abbreviations
Material Code
STQ = 42.75 degree rotated Y cut, X propagating, SiO2, Temp Coeff = 3E-8
##YX-Q = 32 to 43 degree rotated Y cut, X propagating, SiO2, Temp Coeff = 3E-8
YZ-LN = Y cut, X propagating, LiNbO3, Temp Coeff = 94E-6
128YX-LN = 128 degree rotated Y cut, X propagating, LiNbO3, Temp Coeff = 75E-6
X112Y-LT = X cut, 112 degree from Y propagating, LiTaO3, Temp Coeff = 18E-6
Matching Code
external matching elements listed from source to load
series elements upper case, shunt elements lower case
R=resistor, L=inductor, C=capacitor, T=transformer, B=balun, S=saw
Z = internal impedance match
C = connectorized
O = ovenized
A = amplified
S = switched
M = multiplexed
W = weighted amplitude
# = used in module p/n #, or uses component p/n #, with hyperlink to that p/n
Notes:
- A Component is a hermetic SAW product with no DC connections
- A Module contains one or more Components
- A Subsystem contains multiple Modules
- Spurious is in the time domain. The principle spurious are feedthru and triple transit. ‘NA’ is designated in this column for Resonators in the Bandpass Filters section.
- ITAR unrestricted parts in standard packages, indicated by asterisks to the right of the Model numbers below, are available for web sale in small quantities.
Specifying Bandpass Filters
Note: the following specifications are applicable to BOTH bandpass filters and delay lines.
- Do not specify and tolerance the PassBand center frequency and bandwidth, instead define without tolerance the center Fo and width B of the PassBand in which p-p amplitude and phase ripple are specified.
- Specify the minimum insertion loss in the defined PassBand.
- PassBand ripple and return loss may need to be relaxed for realisability at the PassBand edges.
- Do not specify bandwidths unless truly necessary, and then only a max or min, not both. Do not specify amplitude or phase ripple in the transition bands. These specs can drive yields down and costs up. Transition band shape is not easily designed. If max noise band width is important then specify it.
- Do not specify and tolerance a rejection bandwidth, instead define without tolerance the StopBand in which rejection levels are then specified, if necessary with multiple segments. Do not extend the StopBand below Fo/2 or above 2Fo unless truly necessary. Delay line StopBands are usually not specified.
- Test is performed in a 50 ohm system with return loss usually unspecified. If necessary, specify the minimum return loss either at F0 or in B. Return loss will always degrade near the PassBand edges. A 1dB increase in input and output return loss will cause a 1dB increase in insertion loss. Do not specify return loss outside the PassBand, assume it’s zero.
- A polynomial is least mean squared fit to the un-wound measured phase vs freq. Phase is then shown as the deviation from that polynomial, and insertion delay is the derivative of that polynomial at Fo.
- Do not specify group delay ripple, it is highly dependent on insertion delay and far-out time spurious, instead specify p-p phase deviation ripple in B.
- Time domain spurious, eg feedthru and triple-transit, may be specified.
- Specify the operating temperature range with care, an excess can drive yields down and costs up.
- Non-operating temperature range can be -55C to 125C.
- When appropriate, key system level performance parameters may be specified. They can be verified by computer simulation based on measured frequency data. Simulation software has been prepared for: time response for any input signal, pulse widths and sidelobes, inter-symbol interference (ISI), signal to noise ratio loss, bit error rate (BER), TV K-factor, and many others.
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