The long-term goal for NCSIDO is to create a multi-channel Software Defined Radio capable of simultaneously observing several frequencies and transmitters. However, it is important to start acquiring as quickly as possible so that experience can be gained in studying the actual data. Towards that end a single-channel analog receiver was built, in prototype form, and operated for some time. It is described on this page.
An image of the receiver can be seen in the image at the right. Because of the relatively low operating freuqncy of these components, the he schematic can be seen in the image below.
There are three logical sections in this receiver:
- Input Buffer Amplifier
- WWVB Reject Filter
- Station Select Filter
These sections have been labeled on the schematic and will be described in some detail.
The 1kOhm side of the transformer is terminated in a 1kOhm resistor as prescribed elsewhere on this site. Additionally, a pair of back-to-back zener diodes is installed in parallel with the termination resistor for protection. When a zener diode is back biased (negative to cathode) it will not conduct until its zener voltage is reached; when forward biased (positive to cathode) it conducts at 0.6V like a standard rectifiying silicon diode. So this pair will conduct if the peak voltage from the transformer exceeds 10.6V which protects the following op-amp from excessive voltage.
The Input Buffer Amplifier consists of a single op-amp configured as an inverting amplifier. The transformer and terminating resistor are connected directly to the negative input. There are many variants of this circuit which will work successfully; this one represents a minimal parts count circuit.
As noted elsewhere on this site, WWVB represents a very strong signal at the observatory because of its proximity and the receiver must take this signal into account. For this short-term usage receiver it is adequate to filter this frequency out heavily with a relatively broad but easy to impliment filter even though some other frequencies are attenuated significantly.
Each op-amp is built into a two-pole, Sallen-Key, 1dB ripple, low-pass filter with a cut-off frequency of 27kHz. The details of filter design are outside of the scope of this work but can be found in many excellent references. A personal favorite of mine is Donald Lancaster's classic book entitled Active Filter Cookbook (ISBN 075062986X).
The heart of this single channel receiver is the Station Select Filter - a sharply tuned band pass filter which will select a single frequency out of the entire input frequency range and pass it on to the output. It is possible to build a VLF filter from discrete components but doing so would require large inductors, be difficult to adjust, and be subject to considerable drift. Active filters, using multiple op-amps, resistors, and capacitors provide a much more robust solution to this particular design problem. The active filter circuit chosen for this application is called a BiQuad Filter because its transfer function (the mathematical equation which governs the relationship between the filter's input and output) has two quadratic terms in it.
This band pass filter is made up of three op-amps and has independently setable pass band center frequency and pass band width. In contrast to many other active filters the band width (measured in units of Hz) does not change as the center frequency is adjusted. In most other commonly encountered circuits (and many other physical phenomena) the ratio of the center frequency and band width (both measured in units of Hz) is referred to as the circuit Q and is a constant. Not so in the BiQuad - it's Q varies as the center frequency does. While this is remarkable for its variance with (but not violation of) intuitive physical notions it is a great benefit for this application. As the filter's center frequency is tuned across the VLF band the band width continuously matches the modulation bandwidth of the various transmitters.
More on the subject of the BiQuad filter can be found the Active Filter Cookbook and with a Google search on "BiQuad Filter". Try some variants on this name as there is considerable variation in spelling in the literature.
The output of the BiQuad Filter is an AC voltage with a single frequency (selected by the filter), variable amplitude and phase. The amplitude is the characteristic of interest for the science measurement ov the observatory. This output is buffered by an additional op-amp and passed on to the Demodulator described elsewhere on this site.
Once a particular frequency and station has been selected by the Single Channel Receiver the amplitude of the transmitter's carrier must be measured. There are a number of schemes which historically have been used to accomplish this measurement including:
- Thermal Detector
- Simple Diode Rectifier
- Compensated Diode Rectifier
- RMS-DC Convertor
- Demodulating Log Amplifier
The Thermal Detector is a relatively obscure method for measuring RF power but represents a "gold standard". The incoming RF energy is dissipated in a load resistance the the temperature change induced by it is measured. It is an extremely accurate and frequency independent measurement but is relatively slow, consumes significant DC power, is expensive, and operates over only a limited input power (limited dynamic range).
Simple Diode Rectifiers are widely used to generate a DC voltage which is proportional to the power of an incoming RF signal. These circuits are extremely simple, require no DC power and can operate over a wide frequency range. However, they are accurate over a very small range of incoming RF power and there is considerable variance between identical copies of the same circuit. Additionally, they display a strong temperature dependence. They remain popular for many applications where accuracy is not a critical parameter in most part because they are widely described and so easy to impliment.
Compensated Diode Rectifiers are somewhat more complicated circuits which attempt to either increase the measurement accuracy, extend the measurement to a wider dynamic range, or both. There is a wide range of "compensation" which can fit in this category. One example of a commercially available IC is the Linear Circuits LT5507. It is actively biased, temperature compensated, and buffered on-chip. Even so its linearity is quite limited - not much over 20dB. On the other hand it is extremely inexpensive.
The last two categories, RMS-DC Convertors (RDCs) and Demodulating Log Amplifiers (DLAs), share a number of significant characteristics. They are both Integrated Circuits (ICs) which provide complete, single package functionality. The RDC "computes" the root mean square value of the incoming RF signal and emits a DC voltage which is proportional to it or to the logarithm of it. The DLA rectifies the incoming RF signal in a multistage amplifier and emits a voltage proportional to the logarithm of this rectified signal. Both of these circuits provide very large dynamic range, excellent power supply insensitivity and temperature stability, all in contrast to the diode-based methods. They also offer high accuracy, wide frequency response including audio through IF to UHF, small size, low power consumption, and, very importantly, low cost. These ICs cost only a few US dollars each.
The Analog Devices AD8307 is a DLA which is used in this observatory to measure the amplitude of the selected RF signal. The following image shows the schematic of the circuit.
An application note for the AD8307 as a PDF file can be obtained from the Analog Devices web site. It is indispensible for understanding the operation of the IC and hence this sub-system.
The selected RF signal is coupled to the log amplifier through a few capacitors and resistors which tailor the frequency response. For example, the 1nF capacitor across the terminals #1 and #8 together with the two 5kOhm resistors provide a 150kHz low pass filter which limits the high frequency sensitivity of the amplifier. The two 8.2nF capacitors together with the 1.1kOhm input impedance of the amplifier provide a 4kHz high pass filter which limits the low frequency sensitivity of the amplifier. The two variable resistors provide slope and intercept adjustments for calibration purposes. The DC output of the amplifier is passed to a pair of cascaded two-pole, low-pass active filters. This combined four-pole filter provides a corner frequency of 1Hz with an attenuation of -80dB/decade above this frequency and a 1% settling time of 1.5s. This is designed specifically with the 1sample/s digitization rate planned for the observatory.
The log amplifier is specified for an incoming dynamic range of over 80dB which corresponds to eight orders of magnitude in RF power and four orders of magnitude in RF voltage. The DC output is scaled to a value of 100mV/dB which means that the output will vary over a total range of 8V. This voltage is delivered to an ADC which is described separately.
