Telemetry system have become big business in recent years. In addition to the military applications, many commercial uses are being developed. Today’s telemetry markets range from remote reading of gas and electricity meter to credit card validation, security monitoring, token-free highway toll systems, and remote inventory control.
After completing this article, you should be able to:
- Troubleshooting a radio-telemetry receiver
- Troubleshooting a radio-telemetry transmitter
Let’s start by looking at some possible receiver problems in figure 1. Since this receiver has a low-frequency IF amplifier, an oscilloscope can be used to check signal levels and adjust the tuned circuits. However, a typical oscilloscope probe presents a capacitive load to the circuit you connect it to. The best probe for this work will be a X10 type. These probes divide the signal to the oscilloscope by 10 but only shunt the circuit under test with 8 to 10 pF, whereas a X1 probe would have a shunt capacitance of 100 pF. Sometimes even 10 pF is significant, but it is usually not a problem if the technician is aware of the effect.
No Output: Assume you know that a good transmitted signal is present. Check the easy thing first: battery or power supply and the antenna connections.
- Disconnect the receiver from the decoder. It is possible the decoder is shorted, therefore killing the receiver.
- Check the local oscillator. You can check the LO signal at pin 2 with an oscilloscope by using the low-capacitance probe. If you don’t have one, just us a small capacitor, 10 pF or less in series with your probe. The capacitor forms a voltage divider with the probe capacitance. Assuming the capacitance of the added capacitor is small compared to that of the probe, the loss is approximately the ratio of the capacitances.
If the oscillator is not running, try adjusting the inductor, L1. After the oscilloscope runs, you should be able to find a peak amplitude followed by a point where it quits. Adjust the coil away from the quitting point and just below the output peak.
If adjustment is fruitless, check the crystal or substitute another. Don’t forget the inductor and its tuning capacitor. Integrated circuits are usually more reliable than the rest of the part and harder to solder.
- Check for signal in the IF amplifier stages. Since there are no means od adjusting the oscillator frequency, the IF must be aligned to the transmitter’s frequency. The manufacturer recommends killing the AGC by grounding pin 16. The secondary of T3 is a good test point because it isolates the IF amplifier from the oscilloscope probe. The signal amplitude will be down by a factor of 8 according to the data sheet, but we are only looking for a peak. Adjust T1, T2 and T3 for maximum output. After some signal is obtained, you may find that one coil will not tune properly. Check the inductance and its associated capacitor.
In this case, you will need some means of monitoring the transmitter’s RF output without actually making a physical connection of the transmitter. A spectrum analyzer is nice, but almost any receiver capable of operating on the proper frequency will do.
- No Output. Check for modulation at the input, pin 8; and at the internal modulator’s output, pin 13. The modulator simply turns the oscillator’s power supply on and off. The off-on rate will be in the low audio frequency region. If these are present and these is no RF output, it’s time to check the crystal and the tuned circuits.
- Low Output. This can be caused by low modulator output. Check the modulation voltage peak amplitude at pin 13. For the LM1871, you should have 4.5V. Monitor the signal strength with the receiver or the spectrum analyzer while adjusting the oscillating transformer, T1. Look for a definite peak. If none can be found, check the associated 220-pF and 47-pF capacitors.
The current source is the heart of the both the encoder and the decoder. Both circuits also depend on a high-quality capacitor to integer the current provided by the current source.
First, a short discussion of the current source. The LM329 is an active 6.9-V reference. This voltage is divided by the 6.49- and the 4.02-k-ohm resisters to provide a fixed base-emitter voltage on the 2N2907. Since this voltage is fixed, the voltage across the emitter resistor is fixed. Therefore, the collector current is constant. Now, let’s look at some typical problems.
- Assume the 2N2907 has a base-to-collector short. Now the current is only limited by the emitter resistor. The capacitor will charge rapidly. Therefore, a lower signal voltage will cause the comparator (A1) to trip at a lower voltage, and the gain will appear to be too high. Since the voltage across the capacitor is on longer linear, the system output will also be nonlinear.
- Assume the integrating capacitor has developed a high-resistance leak. Now part of the current is not building charge on the capacitor. The gain will appear to be lower and some nonlinearity may appear. This applies to both the encoder and decoder.
- Assume a failure of the 74LS123 in the decoder. It must provide the proper logic levels to reset the integrator and the trigger the simple-and-hold IC. Incorrect logic levels result in on output.
- Assume the simple-and-hold capacitor (0.01 uFd) had developed a leak. The operational amplifier may have enough drive to charge the capacitor, but voltage will immediately be being to fall. Looking at the output with an oscilloscope, you will see a sawtooth waveform, and the output voltage will be too low.