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A Detector Log Amplifier is the receiver section of a radar system which was adapted from the old concept of Morse Code for telegraph pulse communications in the late 19th century.
Basically, a radar system uses a pulse amplitude modulation concept where a microwave switch toggles a microwave signal off and on at the emitter site using any type of digital data signal. The receiver (or DLA) picks up the signal, and the result is pulsed output with the same digital code the emitter has picked up. The only difference is that the pulsed output amplitude of the DLA depends on the input signal strength.
Therefore, the function of the DLA is not only receiving pulse amplitude decoding, but determining the signal strength as well. Also, due to the logarithmic nature of this part, the DLA covers a wide dynamic range. Often, an analog-to-digital converter is used after the DLA to determine the input signal level, as well as for digital decoding.
The following article is a basic theoretical analysis of a Detector Log Amplifier, a single detector approach to the DLA. A more complex DLA, called ERDLA, employs two or more detectors and is based on the same concept.
The DLA0518 is a high-performance single-detector approach Detector Log Amplifier covering over 50dB of dynamic range (-41 to +10 dBm) in a package of 0.21 cubic inches (see Figure 1). The emphasis is put forth in development to build a stand-alone miniaturized package. There are two major factors that make this product happen: electrical and process considerations.
1.0 Electrical Characteristics
This product consists of one microwave detector and analog circuitry. The detector output is in milivolts per miliwatts (mV/mW). The analog section converts it into mV/dB by using a logarithmic function. The detector is within the Square Law Region up to -20 dBm, then becomes semi-linear above -20 dBm. The analog section matched up the characteristics of the detector, which results in a uniform mV/dB up to +10 dBm.
1.1 Detector
The detector consists of an input DC return (a coil), a diode and a capacitor. 95% of DLAs use a Germanium Tunnel Diode as opposed to a Schottky Diode. The fast response time of the tunnel diode and minimal DC drift make it an excellent choice for this application. The diode has an internal resistance, which, with the associated capacitance, determines the response time of the detector and results amplitude demodulator as shown in Figure 2. The speed of the demodulated output may be calculated as follows:
The diode characteristics change as the dynamic range changes. Above -20 dBm Rv changes into the characteristic shown in Figure 3. This characteristic results in a change in input VSWR above -20 dBm and it is an inherent problem of tunnel diodes.
1.2 Analog Section
This section consists of three high speed operational amplifiers, five log cells, and the output stage. The block diagram is shown in Figure 4. After the preamplifiers pick up the signal from the detector, they are fed into the log cells. The effective voltage for each log cell is 18 mV to 95 mV.
1.2.1 Log Cell Characteristics
Each log cell is a differential transistor stage with a unique characteristic (Figure 5). The input is voltage and the output is collector current, which is the summation of all log cells. The voltage to current transfer characteristics of the differential pair is:
The anti-log of the equation above is:
As indicated above, the current I1 is affected by temperature T
. This temperature effect may be canceled by making IE dependent on temperature, but in the opposite direction. The simplified circuit is shown in Figure 6.
The diode voltage change is also dependent on VT.
VD typically changes 2 mV per degree Celsius. The effective input logging voltage for each log cell is from 18 mV to 90 mV, which is logging region. The second log cell starts at 18 mV when the first log cell is at 95 mV (Figure 5). The output of all log cells (I1) are summed together into an output current amplifier. Each log cell covers about 7.5 dB which holds true up to -20 dBm of input level. Above -20 dBm, the detector becomes semi-linear and the equation becomes more complex. The following log cells may cover 7.0 to 11 dB above -20 dBm. Usually it requires some tuning of the log cells above -20 dBm because of the change of diode characteristics for each diode.
1.2.2 Sensitivity
The other improvement in the DLA0518 is in the sensitivity by minimizing the first differential amplifier noise figure using the thin film process, and minimizing common mode problems because the detector is part of the video section and not in a different package.
The TSS of a single detector DLVA (without benefit of pre-amplification) is dependant upon the figure of merit of the detector, the bandwidth and the noise figure of the video amplifier.
The signal power PTSS capable of satisfying the 8 dB TSS criteria can be shown to be:
Where:
The diode figure of merit is a parameter relating to the ratio of open circuit voltage sensitivity K to the square root of the internal resistance of the diode RV. The simplified expression for M is:
The typical range for tunnel diodes is from 40 to 100.
A sample calculation where:
and temperature is +25 °C (298K) indicates PTSS = 6.34 x 10-8 watts, or, in more familiar terms:
1.2.3 DC Drift Consideration
In addition to log cell and DC Drift, a resolution of it is indicated in Section 1.2.1. The other major factor discovered in the late 80's was the drift of the tunnel detector internal resistance (RV) by as much as 5%. This is especially important for DC Coupled DLAs. Therefore, we designed the DLA so that the drift of the detector RV will not affect the video output DC drift. This problem was resolved by minimizing the input bias current of the first differential amplifier to 10 nanoamps. For example, for a 2% RV drift (for RV = 100 Ohm) 1 µamp bias current, the video output drift is 600 mV, but for 10 nanoamps bias current, the video drift is 6 mV, which is far better than the acceptable drift. Of course, there are other parameters which contribute to video drift, such as DC drift of the differential amplifier, but they are not as significant as the input DC bias.
2.0 Process Development
The process we use is based on thin-film technology, adopting from microwave amplifier technology which has been in use for over twenty years. Process development is the most crucial element and often the problems are hidden and they surface during mass production with the environmental testing.
At Advanced Microwave, the DLA is built in a modular form and the video section consists of three thin film substrates, three preamplifiers, log cells and output stage. If one substrate is used, the result will be cracking or separating from the carrier due to thermal expansion.
After many years of development experience in this area, we decided not to use the thick film process for DLAs. The advantage of thin film over the thick film process is the accuracy of the etching of the substrate. The thick film process takes one and a half to double the space for the same video circuitry, requiring a larger substrate and the problems associated with it.
Another problem with the thick film process is that the gold used is a mixture of epoxy and gold as thin film in 99.9% gold. The thick film process deposits gold traces in a rough and uneven substrate which results in poor bonding and inconsistent bonding strength.
3.0 Thermal Considerations
This product uses individual generic components as opposed to a semi-custom IC. A semi-custom IC's power dissipation is concentrated in 0.02 square inch and for 1/2 watt of power dissipation, the junction temperature rise to the case may be as much as 50 °C. On the other hand, we use individual high performance parts spread throughout 0.80 square inch, therefore the junction temperature rise for all individual components will not exceed 10 °C (except that the regulators are 20 °C). It is especially important that the first amplifier not generate too much heat because the input bias current increases exponentially and it is proportional to ek · T/q which results in exponential video output DC drift.
4.0 Typical Applications
The package is designed to be so small that it is ideal for airborne applications and shipboard applications, using it for periscope radar detection finding (RDR). This unit is also perfect for microstrip applications (the connectors are field replaceable).