Circuit Description

(To follow this description, it will be necessary first to download and print the Multicontroller's circuit diagrams. Click HERE to download the diagrams)

The circuit diagrams of the Multicontroller are in five sheets, with contents as follows:


Sheet 1

  Main circuit board, part 1, Revision A3

Sheet 2

Main circuit board, part 2, Revision A3 (power supply and VHF low pass filter)

Sheet 3

Front panel circuit board, Revision A1

Sheet 4

Rear panel circuit board, Revision A1

Sheet 5

 AM/VLF filter circuit board, Revision A1

The multicontroller circuitry is contained on four printed circuit boards. The main circuit board houses the bulk of the circuitry, while the front and rear panel cicuit boards contain primarily the controls, LED indicators and input and output connectors. An optional AM/VLF high pass filter is contained on the fourth board. The circuit boards plug together as a unit, avoiding point-to-point wiring.

Amplifier Circuits: The heart of the Multicontoller is two Maxim MAX497 integrated circuits. Each MAX497 contains four 275 MHz low-noise amplifier/buffers, primarily intended for use as video amplifiers. In this application, they are used as low noise RF amplifiers, with the bandwidth limited by low-pass and high-pass filters. Each amplifier has an open loop voltage gain Av=2 (6dB), an input resistance of 1.2 MΩ and an output resistance of 0.1 Ω. The output noise is specified as less than 5.6nV/(Hz)^1/2, which corresponds to about 0.16 uV measured in a 1 kHz bandwidth.

As shown on Sheets 1 and 2, the RF input to the Multicontroller is through connector J2, which has has separate inputs for 50 Ω unbalanced antennas and high-impedance antennas, such as long-wires. Signals at the 50 Ω input pass through a 7-element low-pass filter consisting of L1-L3 and C33-C36, which has a cutoff frequency of approximately 75 MHz and is intended to attenuate strong local FM and TV stations. If overload from such stations is not a problem, the component values of the inductors and capacitors can be divided by two, raising the cutoff frequency to 150 MHz and extending the multicontroller’s coverage to the 2 meter amateur band and the commercial FM broadcast band.

After filtering, the signal passes through relay contacts K1b, K4c, K3b,and K2b, and then to seven of the independent inputs of U1 and U2. R18, R17 and R13 set the input resistance of the amplifers at approximately 50 Ω. The value of R18 can be changed if other input resistances are desired, e.g., 75 Ω or 92 Ω. Schottky diodes D3 and D4 protect the inputs of U1 and U2 by limiting peak-to-peak RF voltages to approximately 10.6 V.

The high-impedance input at J2 is routed around the low-pass filter through relays contacts K3b and K2b, and then to the seven inputs of U1 and U2. R2 and C14 dissipate static charge on the high-impedance antenna and provide DC isolation. With high impedance antennas, the input impedance of the amplifiers is set by the parallel combination of R2, R17, and R13 and is approximately 1000 Ω.

The eight amplified outputs of U1 and U2 are coupled through resistors R1 and R3-R9 to the RX OUT connector J1. When operating into a 50 Ω load, half of the amplifiers’ output is dropped across resistors R1 and R3-R9. The 6 dB gain of U1 and U2 compensates for the resulting signal loss.

Overload Circuit: Both 50 Ω and high-impedance antennas are also connected to an eighth input (IN3) on U2. Because this input is always active, it is protected against strong RF voltages by D7-D10, with R11 providing isolation from the other inputs. The buffered output of this amplifer is connected to the Auxiliary Output terminal of J1, and also, via R10, to a half-wave rectifier consisting of Schottky diode D2. This diode has a low “turn-on” threshold, which increases the sensitivity of the overload detector circuit. The rectified output of D2 is filtered by C20 and C21 and applied to the base of Q1. The voltage divider comprised of R12 and R14 determines the threshold voltage at which Q1 conducts and actuates overload relay K2. The attack time of this circuit is very fast (a few milliseconds) while the decay time, determined by C21, R12, and R14, is about 0.25 seconds. When Q1 conducts, K2b disconnects the antenna from the 7 inputs of U1 and U2, while K2c mutes all the connected receivers. An RF voltage in excess of about 1000 mV is required to trip the overload circuit, as shown. Diode D1 is not used in this configuration, but can be used as a voltage doubler with D2 if a 0.1 uF capacitor is added in series with R10. This change increases the sensitivity of the overload circuit, if desired.

Preamplifier: An internal low-noise internal preamplifer can be used with 50 Ω antennas to provide a nominal +12 db gain. The preamplifier is based on U3, a Mini-Circuits GALI-6 low noise integrated amplifier with an intrinsic frequency response of DC – 4 GHz. This amplifier has an impressive third-order intercept point IP3=35.5 dBm and can accommodate input signals as high as +20 dBm without damage.The preamplifier’s low frequency response is set by C22 and C24, which reduce the effective preamplifier gain below 100 kHz. Schottky diodes D5 and D6 and resistor R15 protect U3 from excessive RF signals. Bias current for U3 is set by R16 and is approximately 70 mA. The +12V power for U3 is routed through relay contacts K4b and K3c, which disconnect preamplifier power when the high-impedance antenna is selected or when the preamplifier is bypassed. Diode D15 protects U3 from reverse voltage transients or inductive spikes on the power supply line.

Mute and Control Circuits:The seven receiver outputs have identical mute circuits, which can be configured independently with internal jumpers either as close-to-mute or open-to-mute. One such circuit consists of NAND gate/buffer U4a, in combination with relay K5. Input 1 of U4a is held high by R19, unless it is grounded by the front panel Receiver Enable switch. Similarly, Input 2 of U4a is held high by R26. Note that Input 2 of all the NAND gates are connected together with a common control line, so that grounding this line switches all seven NAND gate outputs high. Input 2 can be grounded by the front panel Mute Enable switch, by relay contacts K2c, or by grounding the rear panel (Sheet 4) Control jack J213. Resistor R200 and capacitor C201 on the rear panel circuit board isolate the control line at J213 from stray RF voltages.

When both inputs of U4a are high, the output of U4a is low, thus energizing relay coil K5a and closing contacts K5b and K5c, When K5a is energized, its corresponding receiver is enabled (unmuted). The normally-open or normally-closed contacts of K5b are selected by jumpers on H1, and K5c lights an LED when its receiver is enabled. Note that grounding the control line at J213 mutes the receivers but does not disable the antenna ports or disconnect the antenna from the receivers. There is no reason to disable these antenna ports, because the overload circuit automatically limits the RF antenna voltages on the ports to safe levels.

AM/VLF Filter: The optional low-frequency filter consists of C300-C309 and inductors L300-L302. This filter is based on a design from the 2000 ARRL Handbook, which has been modified to use off-the-shelf components. The filter response has a slight attenuation of about 3 dB at the lower end of the 160 meter amateur band (1.8 MHz), which increases to about 30 dB at 1.5 MHz. Below 1.5 MHz, the attenuation rises rapidly to 60 dB or greater. The filter is useful in applications where overload from strong AM broadcast band stations is a problem. If it is not needed, the input and output terminals are jumpered on the rear panel circuit board by C200.

Power Supply: The dual 10V windings of power transformer T1 are connected in series and the resulting 20VCT voltage is rectified by diode bridge DB1 and filtered by C37 and C38, to produce unregulated voltages of approximately +12V and -12V. Bleeder resistors R27 and R28 draw about 80 mA from each supply, which helps to stabilize the voltage. Regulators U8 and U9 produce +5V and -5V for powering the amplifiers U1 and U2. If the dual-primary version of T1 is used, the primaries can be jumpered either for 115VAC or 230VAC operation. The single-primary version of the transformer can only be used on 115VAC.