Time delay unit (TDU)

Updated August 28, 2011

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New for January 2011! A time delay unit is a structure that provides a specific time delay, or programmable time delay, using a multi-path structure. It is similar to a phase shifter, but different. A phase shifter usually provides a fixed insertion phase difference between two states (flat phase over frequency). The two states are only slightly different in time delay, and certainly the two states differ in path length by less than a wavelength. A TDU can provide many, many wavelengths of phase shift, and the phase shift is ideally exactly proportional to frequency such that the group delay difference between the two states is flat over the require bandwidth. Got that?

James Darren and Robert Colbert star in Irwin Allen's Time Tunnel TV show

TDUs are used in phased arrays. Phase shifters at each element steer the beam, but they do not provide true time delay. Without true time delay, the beam will distort (squint) over frequency. TDUs are used at the sub-array level to mitigate (but not eliminate) this problem. Across a one meter phased array, TDUs with several nanoseconds of delay are needed.

Time delay can be accomplished in many ways. The most natural way is to use a length of coax; the rule of thumb one nanosecond per foot applies to air coax, PTFE filled coax (ER=2.2) will provide eight inches. Yes, we jump back and forth between the metric system and English units here at Microwaves101, but get used to it, the United States military uses ridiculous terms like "knots" and "Mach numbers" instead of meters per second, so you need to be able to convert lengths in your head or you will look like a dummy at some meeting in the future.

Another way to provide time delay is using fiber optic delay line. Here you will have to convert microwaves to light and back again. Perhaps an attraction is that the delay line can be made very small, but the TDU will be quite lossy and require amplification. Amplification means the delay line will be non-reciprocal, and you will need separate TDUs for transmit and receive, or some means of turning the amplifiers around.

Delay lines can also be made using microstrip or stripline, however, you will need to be careful in folding them up, as coupling between meandered lines is your enemy and will surely wreck the flat phase response your system needs.

Multi-bit designs

There are many ways to provide switched paths for multi-bit TDUs. The most obvious is to cascade two-state bits, like all commercially available MMIC phase shifters. A pair of SPDTs back-to-back with the delay and reference lines connecting them provides the TDU two states. However, if you consider that SP3T, SP4T, etc. switches can be used, there are many other options.

Specifications for TDUs

The following are some relevant specifications for TDUs:

Bandwidth

Insertion loss

Total time delay (expressed in nanoseconds or picoseconds)

Time delay flatness over frequency (expressed in %)

Number of bits

Amplitude matching over delay states (could be expressed as RMS amplitude in dB)

Amplitude tracking over frequency

P1dB at input

Noise figure (if amplifiers are used to boost the signal)

Problems with TDUs

Different lengths of transmission lines will have losses that increase with length as well as frequency. Here, fixed attenuators can be used to increase losses of short path lengths, and gain equalizers can be used to increase the loss at lower frequencies to straighten out the response.

Reflections that occur at interface to the switches will make the return loss extremely choppy over the frequency band. It is necessary to pay attention to the detail here. Advice: don't use a poorly matched switch!

TDU Example

We'll try to post an example of a time delay unit soon. We'll take a commercially available MMIC SPDT, integrate two of them back to back with coax or microstrip delays and a bypass path with an equalizer.

If you have any specific TDU requirements, pass them along to Microwaves101, we might be able to lead you directly to good source of TDUs!